Lay Pure out as a core with the parts selected around it

The app model, the tree store and the HTTP host lived in
prototypes/connections among the earlier document runtime, the store was
known only by its format's name, and Vega compiled six Pure sources by path.
Now, as the README's principle has it:

  core/   runtime, flow and the app model (module*.c), with what the core
          asks of storage (state.h, store.h)
  store/  the tree store (PURETRE3, PUREREC1)
  serve/  pure-serve: HTTP host, outbound delivery, JSON in place
  cli/    pure-run, pure-edit-module

A top-level make builds libpure.a (app model and store, for hosts to link)
and the hosts; make check runs the store's and the app model's tests. The
earlier runtime still builds from prototypes/connections against core/ and
store/ (check, check-tree and check-core pass). Design 009 records the rule:
parts by role behind libraries; git repositories split by coupling, so the
store stays here while its contract moves with the core, and Vega stays apart.

1 files changed, 0 insertions, 1856 deletions

diff --git a/prototypes/connections/store-tree.c b/prototypes/connections/store-tree.c
deleted file mode 100644
index 146e919..0000000
--- a/prototypes/connections/store-tree.c
+++ /dev/null
@@ -1,1856 +0,0 @@
-/* Selected storage experiment: every document lives in one append-only file as
- * a persistent B-tree of byte extents. An edit writes only the leaves and the
- * path it changes, so inserting or deleting in a large document costs O(log n)
- * nodes instead of a whole-document copy. Unchanged extents are shared by old
- * and new versions. The document catalog is the same structure holding fixed
- * records sorted by key. A commit appends a record (its head: where its nodes
- * are and their digest) and then its nodes, and syncs once: one contiguous
- * range, which a flash device writes about twice as fast as two. Every few
- * commits the head also goes into the older of two slots at the start, where
- * a reader begins before following the records after it; readers take no
- * lock and map one immutable prefix. */
-#define _POSIX_C_SOURCE 200809L
-#ifdef __APPLE__
-#define _DARWIN_C_SOURCE
-#endif
-#include "store.h"
-#include "state.h"
-#include <errno.h>
-#include <fcntl.h>
-#include <inttypes.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-#include <pthread.h>
-#include <sys/mman.h>
-#include <sys/stat.h>
-#include <unistd.h>
-
-enum { FREE = 0xffff };
-/* Each head slot has a block of its own: a torn write of one never touches
- * the other. */
-enum { SLOT = 128, BLOCK = 4096, DATA = 2 * BLOCK, LEAF_MAX = 4096, LEAF_MIN = 1024,
-       FAN_MAX = 32, FAN_MIN = 8, RECORD = 96, KEY = 64, HEIGHT_MAX = 12,
-       BUFFER = 65536 };
-/* Written zeros kept after the published end (see prepare_tail). */
-enum { TAIL_MIN = 65536, TAIL_STEP = 262144 };
-#ifdef PURE_STORE_TEST
-enum { COMPACT_SLACK = 65536 };
-#else
-enum { COMPACT_SLACK = 1048576 };
-#endif
-#define LEAF_BIT (UINT64_C(1) << 63)
-#define MEMORY_BIT (UINT64_C(1) << 62)
-#define LOCATION (MEMORY_BIT - 1)
-/* Slots of this format, and of the one before tail records (still read). */
-static const unsigned char head_magic[8] = {'P','U','R','E','T','R','E','3'};
-static const unsigned char earlier_magic[8] = {'P','U','R','E','T','R','E','2'};
-static const unsigned char record_magic[8] = {'P','U','R','E','R','E','C','1'};
-/* Commits between two heads written to a slot. */
-enum { CHECKPOINT = 16 };
-static const unsigned char branch_magic[4] = {'P','T','B','1'};
-static const char tree_name[] = "pure.tree";
-/* Threads of one process take turns as the writer: the file lock arbitrates
- * only between processes (it is the process's, and closing any descriptor of
- * the lock file drops it). */
-static pthread_mutex_t writers = PTHREAD_MUTEX_INITIALIZER;
-static uint64_t temporary_sequence;
-
-/* A reference is a file offset, or an index into this edit's node arena.
- * bytes is the size of the whole subtree. Leaves hold raw document bytes. */
-typedef struct { uint64_t ref, bytes; } Entry;
-typedef struct { Entry *items; size_t count, capacity; } List;
-typedef struct {
-    unsigned char *bytes; /* leaf only */
-    Entry *children;      /* branch only */
-    size_t size;          /* leaf bytes, child count, or next free slot + 1 */
-    unsigned height;      /* FREE for a reclaimed slot */
-} Node;
-/* One mapped version of the file, shared by the drafts of a store handle
- * and by whoever still sends extents from it. */
-typedef struct PureMapping { const unsigned char *map; uint64_t end; unsigned refs; } Mapping;
-typedef struct {
-    const unsigned char *map;
-    uint64_t end;
-    Mapping *shared; /* map belongs to this shared mapping, not to the tree */
-    Node *nodes;
-    size_t count, capacity, free_head, free_count; /* free_head is slot + 1 */
-    size_t bytes; /* payload held by arena nodes, including unreachable ones */
-} Tree;
-/* A commit's bytes are start..end (none for a file synced whole before it was
- * published); check is their digest. */
-typedef struct { uint64_t seq, end, catalog, catalog_bytes, live, start, check; } Head;
-/* The xxHash64 construction without a seed: four multiply-rotate lanes over
- * 32-byte stripes, several GB/s. An all-zero Digest is an empty one. */
-typedef struct { uint64_t lane[4], length; unsigned char held[32]; size_t count; } Digest;
-typedef struct {
-    int fd;
-    uint64_t at; /* file offset of the next byte */
-    size_t used;
-    bool failed;
-    Digest digest; /* of every byte written */
-    unsigned char buffer[BUFFER];
-} Writer;
-typedef struct { PureSpan part[3]; size_t index, offset; } Source;
-
-static bool fault(PureStoreStage stage) {
-#ifdef PURE_STORE_TEST
-    if (pure_store_test_fault(stage)) { errno = EIO; return true; }
-#else
-    (void)stage;
-#endif
-    return false;
-}
-
-static bool valid_key(const char *key) {
-    if (!key) return false;
-    size_t length = strlen(key);
-    if (!length || length > KEY) return false;
-    for (size_t i = 0; i < length; ++i) {
-        char b = key[i];
-        if (!((b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') ||
-              (b >= '0' && b <= '9') || b == '-' || b == '_')) return false;
-    }
-    return true;
-}
-
-/* Runtime-owned values (app states) live under '.' followed by a valid key.
- * Document keys never start with '.', so the two never meet. */
-static bool runtime_key(const char *key) {
-    return key && key[0] == '.' && strlen(key) <= KEY && valid_key(key + 1);
-}
-
-static uint64_t decode64(const unsigned char *bytes) {
-    uint64_t value = 0;
-    for (unsigned i = 0; i < 8; ++i) value |= (uint64_t)bytes[i] << (8 * i);
-    return value;
-}
-
-static void encode64(unsigned char *bytes, uint64_t value) {
-    for (unsigned i = 0; i < 8; ++i) bytes[i] = (unsigned char)(value >> (8 * i));
-}
-
-static uint64_t load64(const unsigned char *bytes) {
-    uint64_t value;
-    memcpy(&value, bytes, 8);
-#if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
-    value = __builtin_bswap64(value);
-#endif
-    return value;
-}
-
-/* Digests find torn and missing writes; they are not a defence against a
- * hostile file. */
-static const uint64_t P1 = UINT64_C(0x9E3779B185EBCA87), P2 = UINT64_C(0xC2B2AE3D27D4EB4F),
-                      P3 = UINT64_C(0x165667B19E3779F9), P4 = UINT64_C(0x85EBCA77C2B2AE63),
-                      P5 = UINT64_C(0x27D4EB2F165667C5);
-
-static uint64_t rotate(uint64_t value, unsigned bits) { return value << bits | value >> (64 - bits); }
-static uint64_t mix(uint64_t lane, uint64_t word) { return rotate(lane + word * P2, 31) * P1; }
-
-static void stripes(Digest *digest, const unsigned char *bytes, size_t count) {
-    uint64_t a = digest->lane[0], b = digest->lane[1], c = digest->lane[2], d = digest->lane[3];
-    for (; count; --count, bytes += 32) {
-        a = mix(a, load64(bytes));
-        b = mix(b, load64(bytes + 8));
-        c = mix(c, load64(bytes + 16));
-        d = mix(d, load64(bytes + 24));
-    }
-    digest->lane[0] = a, digest->lane[1] = b, digest->lane[2] = c, digest->lane[3] = d;
-}
-
-static void digest_add(Digest *digest, const unsigned char *bytes, size_t length) {
-    digest->length += length;
-    if (digest->count) {
-        size_t take = 32 - digest->count < length ? 32 - digest->count : length;
-        memcpy(digest->held + digest->count, bytes, take);
-        digest->count += take;
-        bytes += take;
-        length -= take;
-        if (digest->count < 32) return;
-        stripes(digest, digest->held, 1);
-        digest->count = 0;
-    }
-    stripes(digest, bytes, length / 32);
-    digest->count = length % 32;
-    if (digest->count) memcpy(digest->held, bytes + length - digest->count, digest->count);
-}
-
-static uint64_t digest_end(const Digest *digest) {
-    const uint64_t *lane = digest->lane;
-    uint64_t hash = rotate(lane[0], 1) + rotate(lane[1], 7) + rotate(lane[2], 12) + rotate(lane[3], 18);
-    for (unsigned i = 0; i < 4; ++i) hash = (hash ^ mix(0, lane[i])) * P1 + P4;
-    hash += digest->length;
-    size_t i = 0;
-    for (; i + 8 <= digest->count; i += 8) hash = rotate(hash ^ mix(0, load64(digest->held + i)), 27) * P1 + P4;
-    for (; i < digest->count; ++i) hash = rotate(hash ^ digest->held[i] * P5, 11) * P1;
-    hash ^= hash >> 33;
-    hash *= P2;
-    hash ^= hash >> 29;
-    hash *= P3;
-    return hash ^ hash >> 32;
-}
-
-static uint64_t digest_of(const unsigned char *bytes, size_t length) {
-    Digest digest = {0};
-    digest_add(&digest, bytes, length);
-    return digest_end(&digest);
-}
-
-#ifdef PURE_STORE_TEST
-static unsigned long syncs;
-unsigned long pure_tree_test_syncs(void) { return __atomic_load_n(&syncs, __ATOMIC_RELAXED); }
-#define COUNT_SYNC() __atomic_add_fetch(&syncs, 1, __ATOMIC_RELAXED)
-#else
-#define COUNT_SYNC() ((void)0)
-#endif
-
-static bool sync_fd(int fd) {
-    int rc;
-    COUNT_SYNC();
-    do { rc = fsync(fd); } while (rc && errno == EINTR);
-    return rc == 0;
-}
-
-/* Data and the size needed to read it back, without timestamps. */
-static bool sync_data(int fd) {
-#ifdef __APPLE__
-    return sync_fd(fd);
-#else
-    int rc;
-    COUNT_SYNC();
-    do { rc = fdatasync(fd); } while (rc && errno == EINTR);
-    return rc == 0;
-#endif
-}
-
-static bool write_at(int fd, const unsigned char *bytes, size_t length, uint64_t offset) {
-    while (length) {
-        ssize_t n = pwrite(fd, bytes, length, (off_t)offset);
-        if (n < 0 && errno == EINTR) continue;
-        if (n <= 0) return false;
-        bytes += (size_t)n;
-        length -= (size_t)n;
-        offset += (uint64_t)n;
-    }
-    return true;
-}
-
-static bool read_at(int fd, unsigned char *bytes, size_t length, uint64_t offset) {
-    while (length) {
-        ssize_t n = pread(fd, bytes, length, (off_t)offset);
-        if (n < 0 && errno == EINTR) continue;
-        if (n <= 0) return false;
-        bytes += (size_t)n;
-        length -= (size_t)n;
-        offset += (uint64_t)n;
-    }
-    return true;
-}
-
-static void encode_head(unsigned char out[SLOT], const unsigned char magic[8], const Head *head) {
-    memset(out, 0, SLOT);
-    memcpy(out, magic, 8);
-    encode64(out + 8, head->seq);
-    encode64(out + 16, head->end);
-    encode64(out + 24, head->catalog);
-    encode64(out + 32, head->catalog_bytes);
-    encode64(out + 40, head->live);
-    encode64(out + 48, head->start);
-    encode64(out + 56, head->check);
-    encode64(out + SLOT - 8, digest_of(out, SLOT - 8));
-}
-
-/* A slot's head (record false) or a commit record's. */
-static bool decode_head(const unsigned char in[SLOT], bool record, Head *head) {
-    bool magic = record ? !memcmp(in, record_magic, 8) : !memcmp(in, head_magic, 8) || !memcmp(in, earlier_magic, 8);
-    if (!magic || decode64(in + SLOT - 8) != digest_of(in, SLOT - 8)) return false;
-    *head = (Head){decode64(in + 8), decode64(in + 16), decode64(in + 24), decode64(in + 32),
-                   decode64(in + 40), decode64(in + 48), decode64(in + 56)};
-    return head->seq && head->end <= (uint64_t)INT64_MAX && head->end <= SIZE_MAX &&
-           head->start >= DATA && head->start <= head->end && head->catalog_bytes % RECORD == 0 &&
-           !head->catalog == !head->catalog_bytes;
-}
-
-/* ---- Trusted heads --------------------------------------------------------
- * One sync makes a commit's bytes and its head durable together, so a crash
- * during it can leave the head on the device without all of the bytes: a
- * head counts only once its digest matches them. The older slot needs no
- * such luck. A writer builds only on a durable head, so the newer head was
- * written after the older one's commit was complete on the device.
- * The process remembers the heads it published or checked, so it reads a
- * commit's bytes at most once, and which of them it made durable. */
-enum { UNKNOWN, BROKEN, CHECKED, DURABLE, PUBLISHING };
-enum { KNOWN = 32 };
-typedef struct { dev_t device; ino_t inode; uint64_t seq, check; int state; } Known;
-/* The newest head that counts per file, to follow records from. */
-typedef struct { dev_t device; ino_t inode; Head head; } Newest;
-enum { NEWEST = 4 };
-static struct {
-    pthread_mutex_t lock;
-    Known heads[KNOWN];
-    unsigned next;
-    Known publishing; /* written here, not durable yet */
-    Newest newest[NEWEST];
-    unsigned next_newest;
-} known = {.lock = PTHREAD_MUTEX_INITIALIZER};
-
-static bool names(const Known *known_head, const struct stat *file, const Head *head) {
-    return known_head->state && known_head->device == file->st_dev && known_head->inode == file->st_ino &&
-           known_head->seq == head->seq && known_head->check == head->check;
-}
-
-static int recall(const struct stat *file, const Head *head) {
-    int state = UNKNOWN;
-    pthread_mutex_lock(&known.lock);
-    if (names(&known.publishing, file, head)) state = PUBLISHING;
-    for (unsigned i = 0; i < KNOWN && state == UNKNOWN; ++i)
-        if (names(&known.heads[i], file, head)) state = known.heads[i].state;
-    pthread_mutex_unlock(&known.lock);
-    return state;
-}
-
-/* PUBLISHING marks the one head being made durable; any other state ends
- * that mark (UNKNOWN only ends it). */
-static void remember(const struct stat *file, const Head *head, int state) {
-    Known entry = {file->st_dev, file->st_ino, head->seq, head->check, state};
-    pthread_mutex_lock(&known.lock);
-    if (state == PUBLISHING) known.publishing = entry;
-    else if (names(&known.publishing, file, head)) known.publishing.state = UNKNOWN;
-    if (state != PUBLISHING && state != UNKNOWN) {
-        Known *at = NULL;
-        for (unsigned i = 0; i < KNOWN && !at; ++i)
-            if (names(&known.heads[i], file, head)) at = &known.heads[i];
-        if (!at) known.heads[known.next++ % KNOWN] = entry;
-        else if (at->state != DURABLE) *at = entry; /* a durable head stays durable */
-    }
-    if (state == CHECKED || state == DURABLE) {
-        Newest *at = NULL;
-        for (unsigned i = 0; i < NEWEST && !at; ++i)
-            if (known.newest[i].head.seq && known.newest[i].device == file->st_dev &&
-                known.newest[i].inode == file->st_ino) at = &known.newest[i];
-        if (!at) at = &known.newest[known.next_newest++ % NEWEST];
-        else if (at->head.seq >= head->seq) at = NULL;
-        if (at) *at = (Newest){file->st_dev, file->st_ino, *head};
-    }
-    pthread_mutex_unlock(&known.lock);
-}
-
-static bool newest_known(const struct stat *file, Head *out) {
-    bool found = false;
-    pthread_mutex_lock(&known.lock);
-    for (unsigned i = 0; i < NEWEST && !found; ++i)
-        if (known.newest[i].head.seq && known.newest[i].device == file->st_dev && known.newest[i].inode == file->st_ino) {
-            *out = known.newest[i].head;
-            found = true;
-        }
-    pthread_mutex_unlock(&known.lock);
-    return found;
-}
-
-#ifdef PURE_STORE_TEST
-/* Test-only: forget every head, as a new process after a crash would. */
-void pure_tree_test_forget(void) {
-    pthread_mutex_lock(&known.lock);
-    memset(known.heads, 0, sizeof(known.heads));
-    memset(known.newest, 0, sizeof(known.newest));
-    known.publishing.state = UNKNOWN;
-    pthread_mutex_unlock(&known.lock);
-}
-#endif
-
-/* Whether a head's commit has all of its bytes. */
-static bool whole(int fd, const Head *head) {
-    unsigned char buffer[16384];
-    Digest digest = {0};
-    for (uint64_t at = head->start; at < head->end;) {
-        size_t n = head->end - at < sizeof(buffer) ? (size_t)(head->end - at) : sizeof(buffer);
-        if (!read_at(fd, buffer, n, at)) return false;
-        digest_add(&digest, buffer, n);
-        at += n;
-    }
-    return digest_end(&digest) == head->check;
-}
-
-/* The newest head that counts. A reader passes over the head this process is
- * still making durable, so the process never answers with a commit that a
- * crash could take back; a writer never meets one (it is the only writer). */
-static bool pick(int fd, const struct stat *file, const unsigned char *slots[2], bool writer, Head *out,
-                 uint64_t sequences[2]) {
-    Head heads[2];
-    bool valid[2];
-    for (unsigned i = 0; i < 2; ++i) {
-        valid[i] = decode_head(slots[i], false, &heads[i]) && heads[i].end <= (uint64_t)file->st_size;
-        if (sequences) sequences[i] = valid[i] ? heads[i].seq : 0;
-    }
-    unsigned newest = valid[1] && (!valid[0] || heads[1].seq > heads[0].seq);
-    for (unsigned turn = 0; turn < 2; ++turn) {
-        unsigned slot = turn ? !newest : newest;
-        const Head *head = &heads[slot];
-        if (!valid[slot]) continue;
-        int state = recall(file, head);
-        if (state == BROKEN || (state == PUBLISHING && !writer)) continue;
-        if (state == UNKNOWN) {
-            bool counts = whole(fd, head);
-            remember(file, head, counts ? CHECKED : BROKEN);
-            if (!counts) continue;
-        }
-        *out = *head;
-        return true;
-    }
-    return false;
-}
-
-/* The commit whose record follows head's bytes, if its own bytes are whole. */
-static bool follow(int fd, const struct stat *file, bool writer, const Head *head, Head *out) {
-    unsigned char record[SLOT];
-    uint64_t size = (uint64_t)file->st_size;
-    Head next;
-    if (head->end > size || size - head->end < SLOT || !read_at(fd, record, SLOT, head->end) ||
-        !decode_head(record, true, &next) || next.seq != head->seq + 1 || next.start != head->end + SLOT ||
-        next.end > size) return false;
-    int state = recall(file, &next);
-    if (state == BROKEN || (state == PUBLISHING && !writer)) return false;
-    if (state == UNKNOWN) {
-        bool counts = whole(fd, &next);
-        remember(file, &next, counts ? CHECKED : BROKEN);
-        if (!counts) return false;
-    }
-    *out = next;
-    return true;
-}
-
-/* Whether head's record is in the file as it was made. */
-static bool recorded(int fd, const struct stat *file, const Head *head) {
-    unsigned char stored[SLOT], made[SLOT];
-    if (head->start < DATA + SLOT || head->end > (uint64_t)file->st_size) return false;
-    encode_head(made, record_magic, head);
-    return read_at(fd, stored, SLOT, head->start - SLOT) && !memcmp(stored, made, SLOT);
-}
-
-/* The newest commit: from the newer slot's head that counts, or from the
- * newest this process has seen in the file when its record is still there,
- * along the records that follow. sequences gets the slots' (0: none). */
-static bool read_head(int fd, struct stat *file, bool writer, Head *out, uint64_t sequences[2]) {
-    unsigned char slots[BLOCK + SLOT];
-    Head head, seen;
-    if (fstat(fd, file) || !S_ISREG(file->st_mode) || file->st_size < 0 || !read_at(fd, slots, sizeof(slots), 0) ||
-        !pick(fd, file, (const unsigned char *[2]){slots, slots + BLOCK}, writer, &head, sequences)) return false;
-    if (newest_known(file, &seen) && seen.seq > head.seq && recorded(fd, file, &seen)) head = seen;
-    while (follow(fd, file, writer, &head, &head)) {}
-    *out = head;
-    return true;
-}
-
-static bool map_tree(int fd, const struct stat *file, const Head *head, Tree *tree) {
-    *tree = (Tree){0};
-    if ((uint64_t)file->st_size < head->end) return false;
-    void *map = mmap(NULL, (size_t)head->end, PROT_READ, MAP_SHARED, fd, 0);
-    if (map == MAP_FAILED) return false;
-    *tree = (Tree){.map = map, .end = head->end};
-    return true;
-}
-
-/* A lent mapping may be released on another thread (a host that answers on
- * its connection threads): its count is atomic. */
-static void release_mapping(Mapping *mapping) {
-    if (!mapping || __atomic_sub_fetch(&mapping->refs, 1, __ATOMIC_ACQ_REL)) return;
-    munmap((void *)mapping->map, (size_t)mapping->end);
-    free(mapping);
-}
-
-static void release_tree(Tree *tree) {
-    if (tree->shared) release_mapping(tree->shared);
-    else if (tree->map) munmap((void *)tree->map, (size_t)tree->end);
-    for (size_t i = 0; i < tree->count; ++i) {
-        free(tree->nodes[i].bytes);
-        free(tree->nodes[i].children);
-    }
-    free(tree->nodes);
-    *tree = (Tree){0};
-}
-
-static const unsigned char *leaf_bytes(const Tree *tree, Entry entry) {
-    if (!(entry.ref & LEAF_BIT) || !entry.bytes || entry.bytes > LEAF_MAX) return NULL;
-    uint64_t at = entry.ref & LOCATION;
-    if (entry.ref & MEMORY_BIT) {
-        if (at >= tree->count) return NULL;
-        const Node *node = &tree->nodes[at];
-        return node->bytes && node->size == entry.bytes ? node->bytes : NULL;
-    }
-    if (at < DATA || at > tree->end || entry.bytes > tree->end - at) return NULL;
-    return tree->map + at;
-}
-
-/* Returns the child count of a checked branch at height, or 0. */
-static size_t children(const Tree *tree, Entry entry, unsigned height, Entry out[FAN_MAX]) {
-    if ((entry.ref & LEAF_BIT) || !height) return 0;
-    uint64_t at = entry.ref & LOCATION;
-    size_t count;
-    if (entry.ref & MEMORY_BIT) {
-        if (at >= tree->count) return 0;
-        const Node *node = &tree->nodes[at];
-        if (node->bytes || node->height != height || !node->size || node->size > FAN_MAX) return 0;
-        count = node->size;
-        memcpy(out, node->children, count * sizeof(*out));
-    } else {
-        if (at < DATA || at % 8 || at > tree->end || tree->end - at < 8) return 0;
-        const unsigned char *p = tree->map + at;
-        count = (size_t)p[4] | (size_t)p[5] << 8;
-        unsigned stored = (unsigned)p[6] | (unsigned)p[7] << 8;
-        if (memcmp(p, branch_magic, sizeof(branch_magic)) || stored != height || !count ||
-            count > FAN_MAX || (tree->end - at - 8) / 16 < count) return 0;
-        for (size_t i = 0; i < count; ++i)
-            out[i] = (Entry){decode64(p + 8 + 16 * i), decode64(p + 16 + 16 * i)};
-    }
-    uint64_t sum = 0;
-    for (size_t i = 0; i < count; ++i) {
-        bool leaf = (out[i].ref & LEAF_BIT) != 0;
-        if (!out[i].bytes || out[i].bytes > UINT64_MAX - sum || leaf != (height == 1)) return 0;
-        sum += out[i].bytes;
-    }
-    return sum == entry.bytes ? count : 0;
-}
-
-static bool root_height(const Tree *tree, Entry root, unsigned *height) {
-    *height = 0;
-    if (!root.bytes) return !root.ref;
-    if (root.ref & LEAF_BIT) return true;
-    uint64_t at = root.ref & LOCATION;
-    if (root.ref & MEMORY_BIT) {
-        if (at >= tree->count || tree->nodes[at].bytes) return false;
-        *height = tree->nodes[at].height;
-    } else {
-        if (at < DATA || at > tree->end || tree->end - at < 8) return false;
-        *height = (unsigned)tree->map[at + 6] | (unsigned)tree->map[at + 7] << 8;
-    }
-    return *height >= 1 && *height <= HEIGHT_MAX;
-}
-
-/* Visit a range as borrowed extents: 1 done, 0 damaged, -1 stopped by visit. */
-static int visit_range(const Tree *tree, Entry entry, unsigned height, uint64_t offset,
-                       uint64_t length, bool (*visit)(void *, PureSpan), void *context) {
-    if (!height) {
-        const unsigned char *bytes = leaf_bytes(tree, entry);
-        if (!bytes || offset > entry.bytes || length > entry.bytes - offset) return 0;
-        return visit(context, (PureSpan){bytes + offset, (size_t)length}) ? 1 : -1;
-    }
-    Entry kids[FAN_MAX];
-    size_t count = children(tree, entry, height, kids);
-    if (!count) return 0;
-    uint64_t start = 0;
-    for (size_t i = 0; i < count && length; ++i) {
-        uint64_t end = start + kids[i].bytes;
-        if (offset < end) {
-            uint64_t inside = offset - start, take = kids[i].bytes - inside;
-            if (take > length) take = length;
-            int result = visit_range(tree, kids[i], height - 1, inside, take, visit, context);
-            if (result != 1) return result;
-            offset += take;
-            length -= take;
-        }
-        start = end;
-    }
-    return length ? 0 : 1;
-}
-
-static bool copy_span(void *context, PureSpan span) {
-    unsigned char **out = context;
-    memcpy(*out, span.data, span.size);
-    *out += span.size;
-    return true;
-}
-
-static bool copy_range(const Tree *tree, Entry entry, unsigned height, uint64_t offset,
-                       uint64_t length, unsigned char *out) {
-    return !length || visit_range(tree, entry, height, offset, length, copy_span, &out) == 1;
-}
-
-/* The bytes of a range held by one leaf, or NULL when it spans leaves. */
-static const unsigned char *single_leaf(const Tree *tree, Entry entry, unsigned height,
-                                        uint64_t offset, uint64_t length) {
-    while (height) {
-        Entry kids[FAN_MAX];
-        size_t count = children(tree, entry, height, kids), i = 0;
-        uint64_t start = 0;
-        if (!count) return NULL;
-        while (i + 1 < count && offset >= start + kids[i].bytes) start += kids[i++].bytes;
-        if (offset - start > kids[i].bytes || length > kids[i].bytes - (offset - start)) return NULL;
-        entry = kids[i];
-        offset -= start;
-        --height;
-    }
-    const unsigned char *bytes = leaf_bytes(tree, entry);
-    return bytes && offset <= entry.bytes && length <= entry.bytes - offset ? bytes + offset : NULL;
-}
-
-static bool push(List *list, Entry entry) {
-    if (list->count == list->capacity) {
-        size_t capacity = list->capacity ? 2 * list->capacity : 8;
-        Entry *items = realloc(list->items, capacity * sizeof(*items));
-        if (!items) return false;
-        list->items = items;
-        list->capacity = capacity;
-    }
-    list->items[list->count++] = entry;
-    return true;
-}
-
-static size_t node_bytes(const Node *node) {
-    return node->bytes ? node->size : node->size * sizeof(Entry);
-}
-
-static bool add_node(Tree *tree, Node node, uint64_t *index) {
-    tree->bytes += node_bytes(&node);
-    if (tree->free_head) {
-        *index = tree->free_head - 1;
-        tree->free_head = tree->nodes[*index].size;
-        --tree->free_count;
-        tree->nodes[*index] = node;
-        return true;
-    }
-    if (tree->count == tree->capacity) {
-        size_t capacity = tree->capacity ? 2 * tree->capacity : 64;
-        Node *nodes = realloc(tree->nodes, capacity * sizeof(*nodes));
-        if (!nodes) { tree->bytes -= node_bytes(&node); return false; }
-        tree->nodes = nodes;
-        tree->capacity = capacity;
-    }
-    tree->nodes[tree->count] = node;
-    *index = tree->count++;
-    return true;
-}
-
-static void take(Source *source, unsigned char *out, size_t length) {
-    while (length) {
-        PureSpan part = source->part[source->index];
-        size_t left = part.size - source->offset;
-        if (!left) { ++source->index; source->offset = 0; continue; }
-        size_t n = left < length ? left : length;
-        memcpy(out, part.data + source->offset, n);
-        out += n;
-        length -= n;
-        source->offset += n;
-    }
-}
-
-/* Balanced leaves: every leaf of a multi-leaf result holds >= LEAF_MAX / 2. */
-static bool chunk_bytes(Tree *tree, Source *source, uint64_t total, List *out) {
-    uint64_t pieces = (total + LEAF_MAX - 1) / LEAF_MAX;
-    for (uint64_t i = 0; i < pieces; ++i) {
-        size_t size = (size_t)(total / pieces + (i < total % pieces));
-        unsigned char *bytes = malloc(size);
-        uint64_t index;
-        if (!bytes) return false;
-        take(source, bytes, size);
-        if (!add_node(tree, (Node){.bytes = bytes, .size = size}, &index)) { free(bytes); return false; }
-        if (!push(out, (Entry){LEAF_BIT | MEMORY_BIT | index, size})) return false;
-    }
-    return true;
-}
-
-static bool chunk_entries(Tree *tree, const Entry *items, size_t count, unsigned height, List *out) {
-    size_t pieces = (count + FAN_MAX - 1) / FAN_MAX;
-    for (size_t i = 0, at = 0; i < pieces; ++i) {
-        size_t size = count / pieces + (i < count % pieces);
-        Entry *kids = malloc(size * sizeof(*kids));
-        uint64_t sum = 0, index;
-        if (!kids) return false;
-        memcpy(kids, items + at, size * sizeof(*kids));
-        for (size_t k = 0; k < size; ++k) sum += kids[k].bytes;
-        if (!add_node(tree, (Node){.children = kids, .size = size, .height = height}, &index)) {
-            free(kids);
-            return false;
-        }
-        if (!push(out, (Entry){MEMORY_BIT | index, sum})) return false;
-        at += size;
-    }
-    return true;
-}
-
-/* Only nodes created by this edit can be underfull; stored siblings are left as they are. */
-static bool underfull(const Tree *tree, Entry entry, unsigned height) {
-    if (!(entry.ref & MEMORY_BIT)) return false;
-    return height ? tree->nodes[entry.ref & LOCATION].size < FAN_MIN : entry.bytes < LEAF_MIN;
-}
-
-static bool replace(List *list, size_t at, size_t removed, const List *with) {
-    size_t count = list->count - removed + with->count;
-    if (count > list->capacity) {
-        Entry *items = realloc(list->items, count * sizeof(*items));
-        if (!items) return false;
-        list->items = items;
-        list->capacity = count;
-    }
-    memmove(list->items + at + with->count, list->items + at + removed,
-            (list->count - at - removed) * sizeof(*list->items));
-    memcpy(list->items + at, with->items, with->count * sizeof(*list->items));
-    list->count = count;
-    return true;
-}
-
-static bool merge(Tree *tree, Entry a, Entry b, unsigned height, List *out);
-
-/* Merge each new underfull node with a neighbour. A merge removes a node or
- * yields two nodes of at least half capacity, so the loop terminates. */
-static bool repair(Tree *tree, List *all, unsigned height) {
-    bool ok = true;
-    for (size_t i = 0; ok && all->count > 1 && i < all->count;) {
-        if (!underfull(tree, all->items[i], height)) { ++i; continue; }
-        size_t at = i + 1 < all->count ? i : i - 1;
-        List merged = {0};
-        ok = merge(tree, all->items[at], all->items[at + 1], height, &merged) &&
-             replace(all, at, 2, &merged);
-        free(merged.items);
-        i = at;
-    }
-    return ok;
-}
-
-static bool merge(Tree *tree, Entry a, Entry b, unsigned height, List *out) {
-    if (!height) {
-        const unsigned char *x = leaf_bytes(tree, a), *y = leaf_bytes(tree, b);
-        if (!x || !y) return false;
-        Source source = {.part = {{x, (size_t)a.bytes}, {y, (size_t)b.bytes}}};
-        return chunk_bytes(tree, &source, a.bytes + b.bytes, out);
-    }
-    Entry kids[FAN_MAX];
-    List all = {0};
-    size_t count = children(tree, a, height, kids);
-    bool ok = count != 0;
-    for (size_t i = 0; ok && i < count; ++i) ok = push(&all, kids[i]);
-    count = ok ? children(tree, b, height, kids) : 0;
-    ok = count != 0;
-    for (size_t i = 0; ok && i < count; ++i) ok = push(&all, kids[i]);
-    /* A small child carried in from a one-child parent is repaired here too. */
-    ok = ok && repair(tree, &all, height - 1) && chunk_entries(tree, all.items, all.count, height, out);
-    free(all.items);
-    return ok;
-}
-
-/* Replace [offset, offset + remove) of entry with insert. Appends the
- * replacement nodes, all at entry's height, to out (possibly none). */
-static bool edit(Tree *tree, Entry entry, unsigned height, uint64_t offset, uint64_t remove,
-                 PureSpan insert, List *out) {
-    if (offset > entry.bytes || remove > entry.bytes - offset) return false;
-    if (!height) {
-        const unsigned char *bytes = leaf_bytes(tree, entry);
-        if (!bytes) return false;
-        Source source = {.part = {{bytes, (size_t)offset}, insert,
-            {bytes + offset + remove, (size_t)(entry.bytes - offset - remove)}}};
-        return chunk_bytes(tree, &source, entry.bytes - remove + insert.size, out);
-    }
-    Entry kids[FAN_MAX];
-    size_t count = children(tree, entry, height, kids), first = count, final = count;
-    if (!count) return false;
-    uint64_t last = remove ? offset + remove - 1 : offset, start = 0, first_start = 0, final_start = 0;
-    for (size_t i = 0; i < count; ++i) {
-        uint64_t end = start + kids[i].bytes;
-        bool tail = i + 1 == count;
-        if (first == count && (offset < end || tail)) { first = i; first_start = start; }
-        if (first != count && (last < end || tail)) { final = i; final_start = start; break; }
-        start = end;
-    }
-    List middle = {0}, all = {0};
-    bool ok = first == final ?
-        edit(tree, kids[first], height - 1, offset - first_start, remove, insert, &middle) :
-        edit(tree, kids[first], height - 1, offset - first_start,
-             first_start + kids[first].bytes - offset, insert, &middle) &&
-        edit(tree, kids[final], height - 1, 0, offset + remove - final_start, (PureSpan){0}, &middle);
-    for (size_t i = 0; ok && i < first; ++i) ok = push(&all, kids[i]);
-    for (size_t i = 0; ok && i < middle.count; ++i) ok = push(&all, middle.items[i]);
-    for (size_t i = final + 1; ok && i < count; ++i) ok = push(&all, kids[i]);
-    free(middle.items);
-    ok = ok && repair(tree, &all, height - 1) && chunk_entries(tree, all.items, all.count, height, out);
-    free(all.items);
-    return ok;
-}
-
-/* The new root of root with [offset, offset + remove) replaced by insert. */
-static bool rebuild(Tree *tree, Entry root, uint64_t offset, uint64_t remove,
-                    PureSpan insert, Entry *out) {
-    unsigned height;
-    List list = {0};
-    bool ok = root_height(tree, root, &height);
-    if (ok && !root.bytes) {
-        Source source = {.part = {insert}};
-        ok = !offset && !remove && chunk_bytes(tree, &source, insert.size, &list);
-    } else if (ok) ok = edit(tree, root, height, offset, remove, insert, &list);
-    while (ok && list.count > 1) {
-        List up = {0};
-        ok = height < HEIGHT_MAX && chunk_entries(tree, list.items, list.count, ++height, &up);
-        free(list.items);
-        list = up;
-    }
-    Entry result = ok && list.count ? list.items[0] : (Entry){0};
-    free(list.items);
-    while (ok && height && result.bytes) {
-        Entry kids[FAN_MAX];
-        size_t count = children(tree, result, height, kids);
-        if (count != 1) { ok = count > 1; break; }
-        result = kids[0];
-        --height;
-    }
-    *out = result;
-    return ok;
-}
-
-static bool flush(Writer *writer) {
-    if (writer->used && !writer->failed) {
-        digest_add(&writer->digest, writer->buffer, writer->used);
-        if (!write_at(writer->fd, writer->buffer, writer->used, writer->at - writer->used)) writer->failed = true;
-    }
-    writer->used = 0;
-    return !writer->failed;
-}
-
-/* Zeros from..to; a failure only costs speed. */
-static void zero_fill(int fd, uint64_t from, uint64_t to) {
-    static const unsigned char zeros[BUFFER];
-    while (from < to) {
-        size_t n = to - from < BUFFER ? (size_t)(to - from) : BUFFER;
-        if (!write_at(fd, zeros, n, from)) return;
-        from += n;
-    }
-}
-
-/* Where the written zeros after a published end should reach. */
-static uint64_t tail_end(uint64_t end) {
-    return end + ((end - DATA) / 8 > TAIL_STEP ? (end - DATA) / 8 : TAIL_STEP);
-}
-
-static void put_bytes(Writer *writer, const unsigned char *bytes, size_t length) {
-    while (length && !writer->failed) {
-        if (writer->used == BUFFER && !flush(writer)) return;
-        size_t n = BUFFER - writer->used < length ? BUFFER - writer->used : length;
-        memcpy(writer->buffer + writer->used, bytes, n);
-        writer->used += n;
-        writer->at += n;
-        bytes += n;
-        length -= n;
-    }
-}
-
-static uint64_t put_branch(Writer *writer, const Entry *kids, size_t count, unsigned height) {
-    static const unsigned char zero[8];
-    put_bytes(writer, zero, (size_t)((8 - writer->at % 8) % 8));
-    uint64_t at = writer->at;
-    unsigned char header[8];
-    memcpy(header, branch_magic, sizeof(branch_magic));
-    header[4] = (unsigned char)count;
-    header[5] = (unsigned char)(count >> 8);
-    header[6] = (unsigned char)height;
-    header[7] = (unsigned char)(height >> 8);
-    put_bytes(writer, header, sizeof(header));
-    for (size_t i = 0; i < count; ++i) {
-        unsigned char pair[16];
-        encode64(pair, kids[i].ref);
-        encode64(pair + 8, kids[i].bytes);
-        put_bytes(writer, pair, sizeof(pair));
-    }
-    return at;
-}
-
-/* Append a subtree's arena nodes, children first, and set entry to the
- * stored reference. The arena is not changed, so a failed commit leaves a
- * draft intact. With relocate, stored nodes are copied too (compaction, or a
- * draft whose base file was replaced by compaction). */
-static bool write_subtree(const Tree *tree, Writer *writer, Entry *entry, unsigned height,
-                          bool relocate) {
-    if (!entry->bytes || (!(entry->ref & MEMORY_BIT) && !relocate)) return true;
-    if (!height) {
-        const unsigned char *bytes = leaf_bytes(tree, *entry);
-        if (!bytes) return false;
-        entry->ref = LEAF_BIT | writer->at;
-        put_bytes(writer, bytes, (size_t)entry->bytes);
-        return !writer->failed;
-    }
-    Entry kids[FAN_MAX];
-    size_t count = children(tree, *entry, height, kids);
-    if (!count) return false;
-    for (size_t i = 0; i < count; ++i)
-        if (!write_subtree(tree, writer, &kids[i], height - 1, relocate)) return false;
-    entry->ref = put_branch(writer, kids, count, height);
-    return !writer->failed;
-}
-
-static bool write_root(const Tree *tree, Writer *writer, Entry *root, bool relocate) {
-    unsigned height;
-    return root_height(tree, *root, &height) && write_subtree(tree, writer, root, height, relocate);
-}
-
-static bool read_record(const Tree *tree, Entry catalog, uint64_t index,
-                        unsigned char record[RECORD], Entry *document, int64_t *revision) {
-    unsigned height;
-    if (!root_height(tree, catalog, &height) ||
-        !copy_range(tree, catalog, height, index * RECORD, RECORD, record)) return false;
-    *document = (Entry){decode64(record + KEY), decode64(record + KEY + 8)};
-    uint64_t stored = decode64(record + KEY + 16);
-    *revision = (int64_t)stored;
-    return stored && stored <= (uint64_t)INT64_MAX && !document->ref == !document->bytes &&
-           document->bytes <= PURE_OBJECT_LIMIT;
-}
-
-/* Binary search over records ordered by their zero-padded key bytes. */
-static bool find(const Tree *tree, Entry catalog, const char *key, uint64_t *index,
-                 bool *found, Entry *document, int64_t *revision) {
-    unsigned char wanted[KEY] = {0}, record[RECORD];
-    memcpy(wanted, key, strlen(key));
-    uint64_t low = 0, high = catalog.bytes / RECORD;
-    *found = false;
-    *document = (Entry){0};
-    *revision = 0;
-    while (low < high) {
-        uint64_t middle = low + (high - low) / 2;
-        if (!read_record(tree, catalog, middle, record, document, revision)) return false;
-        int order = memcmp(wanted, record, KEY);
-        if (!order) { *index = middle; *found = true; return true; }
-        if (order < 0) high = middle; else low = middle + 1;
-    }
-    *document = (Entry){0};
-    *revision = 0;
-    *index = low;
-    return true;
-}
-
-static int create_temporary(int directory, char name[96]) {
-    for (unsigned attempt = 0; attempt < 64; ++attempt) {
-        if (temporary_sequence == UINT64_MAX) break;
-        snprintf(name, 96, ".pending-%ld-%" PRIu64, (long)getpid(), ++temporary_sequence);
-        int fd = openat(directory, name, O_RDWR | O_CREAT | O_EXCL | O_NOFOLLOW | O_CLOEXEC, 0600);
-        if (fd >= 0) return fd;
-        if (errno != EEXIST) break;
-    }
-    name[0] = 0; /* Never remove an existing file on a name collision. */
-    return -1;
-}
-
-/* Publish a fresh file with both slots pointing at head. It is synced whole
- * before it is renamed into place, so its head names no commit bytes. */
-static bool install(int directory, char temporary[96], int fd, Head *head) {
-    unsigned char slot[SLOT];
-    struct stat file;
-    head->start = head->end;
-    head->check = digest_end(&(Digest){0});
-    encode_head(slot, head_magic, head);
-    zero_fill(fd, head->end, tail_end(head->end));
-    bool ok = write_at(fd, slot, SLOT, 0) && write_at(fd, slot, SLOT, BLOCK) && sync_fd(fd) && !fstat(fd, &file);
-    if (close(fd)) ok = false;
-    ok = ok && !renameat(directory, temporary, directory, tree_name);
-    if (ok) temporary[0] = 0;
-    ok = ok && sync_fd(directory);
-    if (ok) remember(&file, head, DURABLE);
-    return ok;
-}
-
-/* Rewrite only reachable nodes. Open readers keep their mapping of the
- * previous file. A failure leaves the published file untouched. */
-static void compact(int directory, int source, const struct stat *file, const Head *head) {
-    Tree tree;
-    if (!map_tree(source, file, head, &tree)) return;
-    char temporary[96];
-    int fd = create_temporary(directory, temporary);
-    Writer *writer = fd >= 0 ? malloc(sizeof(*writer)) : NULL;
-    uint64_t count = head->catalog_bytes / RECORD;
-    unsigned char *records = count ? malloc((size_t)(count * RECORD)) : NULL;
-    Entry catalog = {head->catalog, head->catalog_bytes};
-    unsigned height;
-    bool ok = writer && (!count || records) && root_height(&tree, catalog, &height) &&
-              (!count || copy_range(&tree, catalog, height, 0, count * RECORD, records));
-    if (writer) *writer = (Writer){.fd = fd, .at = DATA};
-    for (uint64_t i = 0; ok && i < count; ++i) {
-        unsigned char *record = records + i * RECORD;
-        Entry document = {decode64(record + KEY), decode64(record + KEY + 8)};
-        unsigned document_height;
-        ok = root_height(&tree, document, &document_height) &&
-             write_subtree(&tree, writer, &document, document_height, true);
-        encode64(record + KEY, document.ref);
-    }
-    Entry fresh = {0};
-    ok = ok && rebuild(&tree, (Entry){0}, 0, 0, (PureSpan){records, (size_t)(count * RECORD)}, &fresh) &&
-         write_root(&tree, writer, &fresh, false) && flush(writer);
-    if (ok) {
-        Head compacted = {head->seq + 1, writer->at, fresh.ref, fresh.bytes, writer->at - DATA, 0, 0};
-        ok = install(directory, temporary, fd, &compacted);
-        fd = -1;
-    }
-    if (fd >= 0) close(fd);
-    if (*temporary) unlinkat(directory, temporary, 0);
-    free(records);
-    free(writer);
-    release_tree(&tree);
-}
-
-static bool lock_writer(int lock, bool wait) {
-    struct flock hold = {.l_type = F_WRLCK, .l_whence = SEEK_SET, .l_start = 0, .l_len = 0};
-    int rc;
-    do { rc = fcntl(lock, wait ? F_SETLKW : F_SETLK, &hold); } while (rc && wait && errno == EINTR);
-    return rc == 0;
-}
-
-PureResult pure_store_init(const char *path) {
-    if (!path || !*path) return PURE_STORE_ERROR;
-    bool created = mkdir(path, 0700) == 0;
-    if (!created && errno != EEXIST) return PURE_STORE_ERROR;
-    int directory = open(path, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
-    if (directory < 0) return PURE_STORE_ERROR; /* Never reinterpret an old DB file. */
-    pthread_mutex_lock(&writers);
-    int lock = openat(directory, ".writer.lock", O_RDWR | O_CREAT | O_NOFOLLOW | O_CLOEXEC, 0600);
-    struct stat info;
-    bool ok = lock >= 0 && fstat(lock, &info) == 0 && S_ISREG(info.st_mode) && lock_writer(lock, true);
-    if (ok) {
-        Head head;
-        struct stat file;
-        int fd = openat(directory, tree_name, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
-        if (fd >= 0) {
-            ok = read_head(fd, &file, true, &head, NULL); /* An unreadable file is never replaced. */
-            close(fd);
-        } else if (errno != ENOENT) ok = false;
-        else {
-            char temporary[96];
-            int created_fd = create_temporary(directory, temporary);
-            head = (Head){.seq = 1, .end = DATA};
-            ok = created_fd >= 0 && install(directory, temporary, created_fd, &head);
-            if (*temporary) unlinkat(directory, temporary, 0);
-        }
-    }
-    ok = ok && sync_fd(lock) && sync_fd(directory);
-    if (lock >= 0) close(lock);
-    pthread_mutex_unlock(&writers);
-    close(directory);
-    if (ok && created) {
-        char *parent = strdup(path);
-        if (!parent) return PURE_STORE_ERROR;
-        size_t length = strlen(parent);
-        while (length > 1 && parent[length - 1] == '/') parent[--length] = 0;
-        char *slash = strrchr(parent, '/');
-        if (!slash) strcpy(parent, ".");
-        else if (slash == parent) parent[1] = 0;
-        else *slash = 0;
-        int fd = open(parent, O_RDONLY | O_DIRECTORY | O_CLOEXEC);
-        free(parent);
-        ok = fd >= 0 && sync_fd(fd);
-        if (fd >= 0) close(fd);
-    }
-    return ok ? PURE_OK : PURE_STORE_ERROR;
-}
-
-void pure_store_release(PureStoredView *view) {
-    if (view->mapped_size) munmap(view->mapping, view->mapped_size);
-    *view = (PureStoredView){0};
-}
-
-PureResult pure_store_select(const char *path, const char *key, size_t offset,
-                             size_t length, bool whole, size_t minimum,
-                             size_t available, PureStoredView *out) {
-    *out = (PureStoredView){0};
-    if (!valid_key(key)) return PURE_STORE_ERROR;
-    int directory = open(path, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
-    if (directory < 0) return PURE_STORE_ERROR;
-    int fd = openat(directory, tree_name, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
-    close(directory);
-    if (fd < 0) return PURE_STORE_ERROR;
-    Tree tree = {0};
-    Head head;
-    struct stat file;
-    Entry document;
-    uint64_t index;
-    int64_t revision;
-    bool found;
-    unsigned height;
-    PureResult result = PURE_STORE_ERROR;
-    if (!read_head(fd, &file, false, &head, NULL) || !map_tree(fd, &file, &head, &tree) ||
-        !find(&tree, (Entry){head.catalog, head.catalog_bytes}, key, &index, &found,
-              &document, &revision)) goto finish;
-    if (!found) { result = PURE_NOT_FOUND; goto finish; }
-    size_t size = (size_t)document.bytes;
-    if (size < minimum || offset > size) { result = PURE_LIMIT; goto finish; }
-    if (whole) length = size - offset;
-    if (length > size - offset) { result = PURE_LIMIT; goto finish; }
-    if (length > available) { result = PURE_BUDGET; goto finish; }
-    if (length) {
-        if (!root_height(&tree, document, &height)) goto finish;
-        const unsigned char *direct = single_leaf(&tree, document, height, offset, length);
-        void *mapping;
-        size_t mapped_size, prefix = 0;
-        if (direct) {
-            /* One extent: map it in place, as the flat store does. */
-            long page = sysconf(_SC_PAGESIZE);
-            if (page <= 0) goto finish;
-            size_t start = (size_t)(direct - tree.map), aligned = start - start % (size_t)page;
-            prefix = start - aligned;
-            mapped_size = prefix + length;
-            mapping = mmap(NULL, mapped_size, PROT_READ, MAP_PRIVATE, fd, (off_t)aligned);
-            if (mapping == MAP_FAILED) goto finish;
-        } else {
-            /* A contiguous view of several extents is assembled for this API. */
-            mapped_size = length;
-            mapping = mmap(NULL, mapped_size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
-            if (mapping == MAP_FAILED) goto finish;
-            if (!copy_range(&tree, document, height, offset, length, mapping) ||
-                mprotect(mapping, mapped_size, PROT_READ)) {
-                munmap(mapping, mapped_size);
-                goto finish;
-            }
-        }
-        out->mapping = mapping;
-        out->mapped_size = mapped_size;
-        out->body = (PureSpan){(const unsigned char *)mapping + prefix, length};
-    }
-    out->revision = revision;
-    out->object_size = size;
-    result = PURE_OK;
-finish:
-    release_tree(&tree);
-    close(fd);
-    return result;
-}
-
-PureResult pure_store_get(const char *path, const char *key, size_t available, PureStoredView *out) {
-    return pure_store_select(path, key, 0, 0, true, 0, available, out);
-}
-
-PureResult pure_store_get_range(const char *path, const char *key, size_t offset,
-                                size_t length, size_t available, PureStoredView *out) {
-    return pure_store_select(path, key, offset, length, false, 0, available, out);
-}
-
-/* A locked writer over the current published file. */
-typedef struct {
-    int directory, lock, fd;
-    Head head;
-    uint64_t slots[2]; /* the sequences of the slots' heads */
-    Tree tree;
-    struct stat file;
-    bool turn; /* holds this process's writer turn */
-} Session;
-
-/* Keep written zeros after the published end, where commits and their
- * unpublished leftovers go. A sync of blocks the file already has writes
- * only them; a new size or new blocks add a journal commit (on an SD card
- * 2.4 ms against 6 ms per commit). The zeros become durable with the next
- * commit's sync. */
-static void prepare_tail(Session *session) {
-    uint64_t size = (uint64_t)session->file.st_size, end = session->head.end;
-    if (size < end + TAIL_MIN) zero_fill(session->fd, size > end ? size : end, tail_end(end));
-}
-
-static void close_session(Session *session) {
-    release_tree(&session->tree);
-    if (session->fd >= 0) close(session->fd);
-    if (session->lock >= 0) close(session->lock);
-    if (session->directory >= 0) close(session->directory);
-    if (session->turn) pthread_mutex_unlock(&writers);
-    session->turn = false;
-}
-
-static PureResult open_session(const char *path, Session *session) {
-    *session = (Session){.directory = -1, .lock = -1, .fd = -1};
-    pthread_mutex_lock(&writers); /* another thread of this process waits, not fails */
-    session->turn = true;
-    session->directory = open(path, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
-    if (session->directory < 0) return PURE_STORE_ERROR;
-    session->lock = openat(session->directory, ".writer.lock", O_RDWR | O_NOFOLLOW | O_CLOEXEC);
-    struct stat info;
-    if (session->lock < 0 || fstat(session->lock, &info) || !S_ISREG(info.st_mode)) return PURE_STORE_ERROR;
-    if (!lock_writer(session->lock, false))
-        return errno == EACCES || errno == EAGAIN ? PURE_BUSY : PURE_STORE_ERROR;
-    session->fd = openat(session->directory, tree_name, O_RDWR | O_NOFOLLOW | O_CLOEXEC);
-    if (session->fd < 0 || !read_head(session->fd, &session->file, true, &session->head, session->slots))
-        return PURE_STORE_ERROR;
-    /* A head this process did not make durable may be the last write of a
-     * writer that died before its sync: make it durable before building on it. */
-    if (recall(&session->file, &session->head) != DURABLE) {
-        if (!sync_data(session->fd)) return PURE_STORE_ERROR;
-        remember(&session->file, &session->head, DURABLE);
-    }
-    prepare_tail(session);
-    return map_tree(session->fd, &session->file, &session->head, &session->tree) ? PURE_OK : PURE_STORE_ERROR;
-}
-
-/* A commit's bytes start after room for its record at the published end. */
-static uint64_t commit_start(const Session *session) { return session->head.end + SLOT; }
-
-/* The documents' new nodes are already written at writer. Record each in
- * the catalog (revisions come in as the base and leave as the new one), then
- * write the head as the record before them and sync once: every document
- * becomes visible at once, or none does. Readers of this process see the
- * head only once it is durable. */
-#ifdef PURE_STORE_TEST
-static unsigned long publications;
-unsigned long pure_tree_test_publications(void) { return publications; }
-#endif
-
-static PureResult publish(Session *session, Writer *writer, size_t count, const char *const *keys,
-                          const Entry *documents, int64_t *revisions) {
-#ifdef PURE_STORE_TEST
-    ++publications;
-#endif
-    Entry catalog = {session->head.catalog, session->head.catalog_bytes};
-    bool written = true;
-    for (size_t i = 0; written && i < count; ++i) {
-        unsigned char record[RECORD] = {0};
-        uint64_t index;
-        bool found;
-        Entry current, updated;
-        int64_t stored;
-        memcpy(record, keys[i], strlen(keys[i]));
-        encode64(record + KEY, documents[i].ref);
-        encode64(record + KEY + 8, documents[i].bytes);
-        encode64(record + KEY + 16, (uint64_t)revisions[i] + 1);
-        written = find(&session->tree, catalog, keys[i], &index, &found, &current, &stored) &&
-                  rebuild(&session->tree, catalog, index * RECORD, found ? RECORD : 0,
-                          (PureSpan){record, RECORD}, &updated);
-        catalog = updated;
-    }
-    written = written && write_root(&session->tree, writer, &catalog, false) && flush(writer);
-    /* Unpublished bytes stay in the tail; the next commit overwrites them. */
-    if (!written || fault(PURE_AFTER_HEADER) || fault(PURE_BEFORE_FILE_SYNC) || fault(PURE_BEFORE_RENAME))
-        return PURE_STORE_ERROR;
-    Head published = {session->head.seq + 1, writer->at, catalog.ref, catalog.bytes, session->head.live,
-                      commit_start(session), digest_end(&writer->digest)};
-    unsigned char record[SLOT], slot[SLOT];
-    encode_head(record, record_magic, &published);
-    encode_head(slot, head_magic, &published);
-    /* Every few commits the head goes into the older slot as well: one more
-     * range for that sync, and fewer records for a reader to follow. */
-    uint64_t newer = session->slots[0] > session->slots[1] ? session->slots[0] : session->slots[1];
-    unsigned older = session->slots[0] > session->slots[1];
-    bool checkpoint = published.seq - newer >= CHECKPOINT;
-    for (size_t i = 0; i < count; ++i) ++revisions[i]; /* Past this point the head may already be visible. */
-    remember(&session->file, &published, PUBLISHING);
-    bool durable = write_at(session->fd, record, SLOT, published.start - SLOT) &&
-                   (!checkpoint || write_at(session->fd, slot, SLOT, older * BLOCK)) &&
-                   !fault(PURE_BEFORE_DIR_SYNC) && sync_data(session->fd);
-    remember(&session->file, &published, durable ? DURABLE : CHECKED);
-    if (!durable) return PURE_UNCERTAIN;
-    if (published.end - DATA > 4 * published.live + COMPACT_SLACK && !fstat(session->fd, &session->file))
-        compact(session->directory, session->fd, &session->file, &published);
-    return PURE_OK;
-}
-
-static PureResult mutate(const char *path, const char *key, int64_t expected,
-                         PureSpan body, bool splice, size_t offset, size_t remove,
-                         size_t max_size, int64_t *revision) {
-    *revision = 0;
-    if (!valid_key(key) || (body.size && !body.data)) return PURE_STORE_ERROR;
-    if (body.size > PURE_OBJECT_LIMIT || max_size > PURE_OBJECT_LIMIT || expected < 0) return PURE_LIMIT;
-    Session session;
-    Writer *writer = NULL;
-    Entry document, next;
-    uint64_t index;
-    bool found;
-    PureResult result = open_session(path, &session);
-    if (result != PURE_OK) goto finish;
-    result = PURE_STORE_ERROR;
-    if (!find(&session.tree, (Entry){session.head.catalog, session.head.catalog_bytes}, key,
-              &index, &found, &document, revision)) goto finish;
-    if (*revision != expected) { result = PURE_CONFLICT; goto finish; }
-    if (*revision == INT64_MAX) { result = PURE_LIMIT; goto finish; }
-    uint64_t old_size = document.bytes;
-    if (splice && (offset > old_size || remove > old_size - offset ||
-                   old_size - remove > max_size ||
-                   body.size > max_size - (old_size - remove))) { result = PURE_LIMIT; goto finish; }
-    writer = malloc(sizeof(*writer));
-    if (!writer || !rebuild(&session.tree, splice ? document : (Entry){0}, splice ? offset : 0,
-                            splice ? remove : 0, body, &next)) goto finish;
-    *writer = (Writer){.fd = session.fd, .at = commit_start(&session)};
-    if (!write_root(&session.tree, writer, &next, false)) goto finish;
-    result = publish(&session, writer, 1, &key, &next, revision);
-finish:
-    free(writer);
-    close_session(&session);
-    return result;
-}
-
-/* Keys come from the catalog in ascending byte order. Runtime values are not
- * documents and are left out. */
-PureResult pure_store_keys(const char *path, bool (*visit)(void *, const char *key),
-                           void *context) {
-    int directory = open(path, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
-    if (directory < 0) return PURE_STORE_ERROR;
-    int fd = openat(directory, tree_name, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
-    close(directory);
-    if (fd < 0) return PURE_STORE_ERROR;
-    Tree tree = {0};
-    Head head;
-    struct stat file;
-    PureResult result = PURE_STORE_ERROR;
-    if (read_head(fd, &file, false, &head, NULL) && map_tree(fd, &file, &head, &tree)) {
-        Entry catalog = {head.catalog, head.catalog_bytes}, document;
-        unsigned char record[RECORD];
-        char key[KEY + 1] = {0};
-        int64_t revision;
-        result = PURE_OK;
-        for (uint64_t i = 0; i < catalog.bytes / RECORD; ++i) {
-            if (!read_record(&tree, catalog, i, record, &document, &revision)) { result = PURE_STORE_ERROR; break; }
-            memcpy(key, record, KEY);
-            if (runtime_key(key)) continue;
-            if (!valid_key(key)) { result = PURE_STORE_ERROR; break; }
-            if (!visit(context, key)) break;
-        }
-    }
-    release_tree(&tree);
-    close(fd);
-    return result;
-}
-
-/* A store kept open by one owner thread: its current file and a mapping of
- * its newest version, shared by the drafts opened through it. */
-struct PureStore {
-    char *path;
-    int directory, fd;
-    dev_t device;
-    ino_t inode;
-    Head head;
-    Mapping *mapping;
-    bool pinned; /* drafts open at the version of the last check */
-};
-
-struct PureDraft {
-    PureStore *store; /* the handle it was opened through, or NULL */
-    char *path;       /* owned unless opened through a store handle */
-    char key[KEY + 1];
-    Tree tree;        /* the base file's mapping plus this draft's node arena */
-    dev_t device;     /* identity of the base file; compaction replaces it */
-    ino_t inode;
-    Entry root;
-    int64_t revision;
-    size_t live;      /* arena bytes reachable at the last collection */
-    bool held;        /* a transaction still reads earlier versions */
-};
-
-/* Rebase the draft on the committed version of its key. */
-static PureResult load_draft(PureDraft *draft) {
-    int directory = open(draft->path, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
-    if (directory < 0) return PURE_STORE_ERROR;
-    int fd = openat(directory, tree_name, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
-    close(directory);
-    if (fd < 0) return PURE_STORE_ERROR;
-    Tree tree = {0};
-    Head head;
-    struct stat info;
-    Entry document;
-    uint64_t index;
-    int64_t revision;
-    bool found;
-    bool ok = read_head(fd, &info, false, &head, NULL) && map_tree(fd, &info, &head, &tree) &&
-              find(&tree, (Entry){head.catalog, head.catalog_bytes}, draft->key, &index, &found,
-                   &document, &revision);
-    close(fd);
-    if (!ok) { release_tree(&tree); return PURE_STORE_ERROR; }
-    release_tree(&draft->tree);
-    draft->tree = tree;
-    draft->device = info.st_dev;
-    draft->inode = info.st_ino;
-    draft->root = document;
-    draft->revision = revision;
-    draft->live = 0;
-    return PURE_OK;
-}
-
-PureResult pure_draft_open(const char *path, const char *key, PureDraft **out) {
-    *out = NULL;
-    if (!path || !(valid_key(key) || runtime_key(key))) return PURE_INVALID;
-    PureDraft *draft = calloc(1, sizeof(*draft));
-    if (!draft || !(draft->path = strdup(path))) { free(draft); return PURE_ALLOCATION; }
-    memcpy(draft->key, key, strlen(key) + 1);
-    PureResult result = load_draft(draft);
-    if (result != PURE_OK) { pure_draft_close(draft); return result; }
-    *out = draft;
-    return PURE_OK;
-}
-
-PureResult pure_state_open(const char *path, const char *name, PureDraft **out) {
-    char key[KEY + 2];
-    *out = NULL;
-    if (!valid_key(name) || strlen(name) >= KEY) return PURE_INVALID;
-    snprintf(key, sizeof(key), ".%s", name);
-    return pure_draft_open(path, key, out);
-}
-
-PureDraft *pure_draft_new(void) {
-    return calloc(1, sizeof(PureDraft)); /* no path: nothing mapped, nothing to commit */
-}
-
-PureDraft *pure_draft_freeze(const PureDraft *draft) {
-    if (!draft->tree.shared || (draft->root.ref & MEMORY_BIT)) return NULL;
-    PureDraft *frozen = calloc(1, sizeof(*frozen));
-    if (!frozen) return NULL;
-    Mapping *mapping = draft->tree.shared;
-    __atomic_add_fetch(&mapping->refs, 1, __ATOMIC_RELAXED);
-    *frozen = (PureDraft){.tree = {.map = mapping->map, .end = mapping->end, .shared = mapping},
-                          .device = draft->device, .inode = draft->inode, .root = draft->root,
-                          .revision = draft->revision};
-    memcpy(frozen->key, draft->key, sizeof(frozen->key));
-    return frozen;
-}
-
-void pure_draft_close(PureDraft *draft) {
-    if (!draft) return;
-    release_tree(&draft->tree);
-    if (!draft->store) free(draft->path);
-    free(draft);
-}
-
-/* Bring the handle to the newest published version: an unchanged store costs
- * one system call (the mapping shows the head slots); a new commit maps
- * again, a compaction reopens. */
-static PureResult refresh(PureStore *store) {
-    struct stat info;
-    if (fstatat(store->directory, tree_name, &info, AT_SYMLINK_NOFOLLOW) || !S_ISREG(info.st_mode))
-        return PURE_STORE_ERROR;
-    if (store->fd < 0 || info.st_dev != store->device || info.st_ino != store->inode) {
-        int fd = openat(store->directory, tree_name, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
-        if (fd < 0 || fstat(fd, &info)) { if (fd >= 0) close(fd); return PURE_STORE_ERROR; }
-        if (store->fd >= 0) close(store->fd);
-        store->fd = fd;
-        store->device = info.st_dev;
-        store->inode = info.st_ino;
-        store->head = (Head){0};
-    }
-    if (store->head.seq) {
-        /* The next commit's record follows the head's bytes; a slot shows a
-         * newer head only for a commit of the earlier format. */
-        const unsigned char *map = store->mapping->map;
-        unsigned char next[16];
-        bool newer = decode64(map + 8) > store->head.seq || decode64(map + BLOCK + 8) > store->head.seq ||
-                     (read_at(store->fd, next, sizeof(next), store->head.end) && !memcmp(next, record_magic, 8) &&
-                      decode64(next + 8) == store->head.seq + 1);
-        if (!newer) return PURE_OK;
-    }
-    Head head;
-    if (!read_head(store->fd, &info, false, &head, NULL)) return PURE_STORE_ERROR;
-    if (store->mapping && head.seq == store->head.seq && head.check == store->head.check) return PURE_OK;
-    Tree tree;
-    Mapping *mapping = malloc(sizeof(*mapping));
-    if (!mapping) return PURE_ALLOCATION;
-    if (!map_tree(store->fd, &info, &head, &tree)) { free(mapping); return PURE_STORE_ERROR; }
-    *mapping = (Mapping){tree.map, tree.end, 1};
-    release_mapping(store->mapping);
-    store->mapping = mapping;
-    store->head = head;
-    return PURE_OK;
-}
-
-/* Base a draft on the handle's newest version of its key, sharing the mapping.
- * On failure the draft is unchanged. */
-static PureResult rebase(PureDraft *draft) {
-    PureStore *store = draft->store;
-    Mapping *mapping = store->mapping;
-    Tree tree = {.map = mapping->map, .end = mapping->end, .shared = mapping};
-    Entry document;
-    uint64_t index;
-    int64_t revision;
-    bool found;
-    if (!find(&tree, (Entry){store->head.catalog, store->head.catalog_bytes}, draft->key, &index, &found, &document,
-              &revision)) return PURE_STORE_ERROR;
-    __atomic_add_fetch(&mapping->refs, 1, __ATOMIC_RELAXED);
-    release_tree(&draft->tree);
-    draft->tree = tree;
-    draft->device = store->device;
-    draft->inode = store->inode;
-    draft->root = document;
-    draft->revision = revision;
-    draft->live = 0;
-    return PURE_OK;
-}
-
-PureResult pure_store_open(const char *path, PureStore **out) {
-    *out = NULL;
-    PureStore *store = calloc(1, sizeof(*store));
-    if (!store || !path || !(store->path = strdup(path))) { free(store); return PURE_ALLOCATION; }
-    store->fd = -1;
-    store->directory = open(path, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
-    PureResult result = store->directory < 0 ? PURE_STORE_ERROR : refresh(store);
-    if (result != PURE_OK) { pure_store_close(store); return result; }
-    *out = store;
-    return PURE_OK;
-}
-
-void pure_store_close(PureStore *store) {
-    if (!store) return;
-    release_mapping(store->mapping);
-    if (store->fd >= 0) close(store->fd);
-    if (store->directory >= 0) close(store->directory);
-    free(store->path);
-    free(store);
-}
-
-PureMapping *pure_draft_mapping(const PureDraft *draft) {
-    Mapping *mapping = draft->tree.shared;
-    if (mapping) __atomic_add_fetch(&mapping->refs, 1, __ATOMIC_RELAXED);
-    return mapping;
-}
-
-bool pure_mapping_holds(const PureMapping *mapping, PureSpan span) {
-    uintptr_t base = (uintptr_t)mapping->map, at = (uintptr_t)span.data;
-    return !span.size || (span.data && at >= base && at - base <= mapping->end && span.size <= mapping->end - (at - base));
-}
-
-void pure_mapping_release(PureMapping *mapping) { release_mapping(mapping); }
-
-PureResult pure_store_pin(PureStore *store, bool pinned) {
-    store->pinned = false;
-    PureResult result = pinned ? refresh(store) : PURE_OK;
-    store->pinned = pinned && result == PURE_OK;
-    return result;
-}
-
-static PureResult open_through(PureStore *store, const char *key, PureDraft **out) {
-    *out = NULL;
-    PureResult result = store->pinned ? PURE_OK : refresh(store);
-    if (result != PURE_OK) return result;
-    PureDraft *draft = calloc(1, sizeof(*draft));
-    if (!draft) return PURE_ALLOCATION;
-    *draft = (PureDraft){.store = store, .path = store->path};
-    memcpy(draft->key, key, strlen(key) + 1);
-    result = rebase(draft);
-    if (result != PURE_OK) { free(draft); return result; }
-    *out = draft;
-    return PURE_OK;
-}
-
-PureResult pure_store_draft(PureStore *store, const char *key, PureDraft **out) {
-    *out = NULL;
-    return store && valid_key(key) ? open_through(store, key, out) : PURE_INVALID;
-}
-
-PureResult pure_store_state(PureStore *store, const char *name, PureDraft **out) {
-    char key[KEY + 2];
-    *out = NULL;
-    if (!store || !valid_key(name) || strlen(name) >= KEY) return PURE_INVALID;
-    snprintf(key, sizeof(key), ".%s", name);
-    return open_through(store, key, out);
-}
-
-size_t pure_draft_size(const PureDraft *draft) { return (size_t)draft->root.bytes; }
-
-int64_t pure_draft_revision(const PureDraft *draft) { return draft->revision; }
-
-static size_t mark(const Tree *tree, Entry entry, unsigned char *marks) {
-    if (!(entry.ref & MEMORY_BIT)) return 0;
-    size_t at = (size_t)(entry.ref & LOCATION);
-    if (marks[at]) return 0;
-    marks[at] = 1;
-    const Node *node = &tree->nodes[at];
-    size_t bytes = node_bytes(node);
-    if (!node->bytes)
-        for (size_t i = 0; i < node->size; ++i) bytes += mark(tree, node->children[i], marks);
-    return bytes;
-}
-
-/* Every edit copies a leaf and path, so reclaim arena nodes the root no
- * longer reaches once they outweigh the reachable ones (and 32 KiB). */
-static void collect(PureDraft *draft) {
-    Tree *tree = &draft->tree;
-    if (draft->held) return;
-    size_t allowance = draft->live > 32768 ? draft->live : 32768;
-    if (tree->bytes <= draft->live || tree->bytes - draft->live <= allowance) return;
-    unsigned char *marks = calloc(tree->count, 1);
-    if (!marks) return;
-    draft->live = mark(tree, draft->root, marks);
-    for (size_t i = 0; i < tree->count; ++i) {
-        Node *node = &tree->nodes[i];
-        if (marks[i] || node->height == FREE) continue;
-        tree->bytes -= node_bytes(node);
-        free(node->bytes);
-        free(node->children);
-        *node = (Node){.size = tree->free_head, .height = FREE};
-        tree->free_head = i + 1;
-        ++tree->free_count;
-    }
-    free(marks);
-}
-
-PureResult pure_draft_splice(PureDraft *draft, size_t offset, size_t remove, PureSpan insert) {
-    uint64_t size = draft->root.bytes;
-    if (insert.size && !insert.data) return PURE_INVALID;
-    if (offset > size || remove > size - offset || insert.size > PURE_OBJECT_LIMIT - (size - remove))
-        return PURE_LIMIT;
-    if (!remove && !insert.size) return PURE_OK;
-    Entry next;
-    bool ok = rebuild(&draft->tree, draft->root, offset, remove, insert, &next);
-    if (ok) draft->root = next;
-    collect(draft);
-    return ok ? PURE_OK : PURE_STORE_ERROR;
-}
-
-PureVersion pure_draft_version(const PureDraft *draft) {
-    return (PureVersion){draft->root.ref, draft->root.bytes};
-}
-
-PureResult pure_draft_read_version(const PureDraft *draft, PureVersion version, size_t offset,
-                                   size_t length, bool (*visit)(void *, PureSpan), void *context) {
-    Entry root = {version.ref, version.bytes};
-    unsigned height;
-    if (offset > root.bytes || length > root.bytes - offset) return PURE_LIMIT;
-    if (!length) return PURE_OK;
-    if (!root_height(&draft->tree, root, &height)) return PURE_STORE_ERROR;
-    return visit_range(&draft->tree, root, height, offset, length, visit, context) ?
-           PURE_OK : PURE_STORE_ERROR;
-}
-
-PureResult pure_draft_read(const PureDraft *draft, size_t offset, size_t length,
-                           bool (*visit)(void *, PureSpan), void *context) {
-    return pure_draft_read_version(draft, pure_draft_version(draft), offset, length, visit, context);
-}
-
-void pure_draft_restore(PureDraft *draft, PureVersion version) {
-    draft->root = (Entry){version.ref, version.bytes};
-}
-
-void pure_draft_hold(PureDraft *draft, bool held) {
-    draft->held = held;
-    if (!held) collect(draft);
-}
-
-/* ---- Group commits ------------------------------------------------------
- * Threads that commit at the same time share one commit: they queue, the
- * first becomes the leader and publishes every queued request of the same
- * store with one head (one pair of syncs); the others wait for its result.
- * Each request stays all or nothing on its own: a request with a stale draft
- * is left out, and so is one whose key an earlier request of the same group
- * already changes. The leader touches the waiters' drafts and handles only
- * while they wait. */
-typedef struct Waiter {
-    PureDraft *const *drafts;
-    size_t count;
-    int64_t *revisions;
-    PureResult result;
-    bool accepted, done;
-    struct Waiter *next;
-} Waiter;
-
-static struct {
-    pthread_mutex_t lock;
-    pthread_cond_t finished, joined;
-    Waiter *head, *tail;
-    size_t queued, expected; /* expected: how many requests groups have held lately */
-    bool leading;
-    unsigned long groups, requests;
-} group = {PTHREAD_MUTEX_INITIALIZER, PTHREAD_COND_INITIALIZER, PTHREAD_COND_INITIALIZER, NULL, NULL, 0, 1, false, 0, 0};
-
-void pure_store_group_counts(unsigned long *groups, unsigned long *requests) {
-    pthread_mutex_lock(&group.lock);
-    *groups = group.groups;
-    *requests = group.requests;
-    pthread_mutex_unlock(&group.lock);
-}
-
-enum { GATHER_NS = 2000000 };
-
-/* Publish the queued requests of the store at path in one session. */
-static void commit_group(const char *path, Waiter *first) {
-    size_t total = 0;
-    for (Waiter *w = first; w; w = w->next)
-        if (!strcmp(w->drafts[0]->path, path)) { total += w->count; w->accepted = false; }
-    Session session;
-    Writer *writer = NULL;
-    Entry *roots = malloc(total * sizeof(*roots));
-    const char **keys = malloc(total * sizeof(*keys));
-    int64_t *revisions = malloc(total * sizeof(*revisions));
-    PureDraft **drafts = malloc(total * sizeof(*drafts));
-    bool allocated = roots && keys && revisions && drafts, checked = false;
-    PureResult result = allocated ? open_session(path, &session) : PURE_ALLOCATION;
-    size_t count = 0;
-    if (result == PURE_OK) {
-        /* A request holds when each of its keys is at its draft's base
-         * revision and no earlier request of this group changes it. */
-        for (Waiter *w = first; w; w = w->next) {
-            if (strcmp(w->drafts[0]->path, path)) continue;
-            w->result = PURE_OK;
-            for (size_t i = 0; i < w->count && w->result == PURE_OK; ++i) {
-                PureDraft *d = w->drafts[i];
-                Entry current;
-                uint64_t index;
-                int64_t stored;
-                bool found;
-                if (!find(&session.tree, (Entry){session.head.catalog, session.head.catalog_bytes}, d->key, &index,
-                          &found, &current, &stored)) { w->result = PURE_STORE_ERROR; break; }
-                for (size_t k = 0; k < count; ++k)
-                    if (!strcmp(keys[k], d->key)) { ++stored; break; } /* it moves with the earlier request */
-                if (stored != d->revision) { w->revisions[i] = stored; w->result = PURE_CONFLICT; }
-                else if (stored == INT64_MAX) w->result = PURE_LIMIT;
-            }
-            if (w->result != PURE_OK) continue;
-            w->accepted = true;
-            for (size_t i = 0; i < w->count; ++i) {
-                drafts[count] = w->drafts[i];
-                roots[count] = w->drafts[i]->root;
-                keys[count] = w->drafts[i]->key;
-                revisions[count++] = w->drafts[i]->revision;
-            }
-        }
-        checked = true;
-        if (count) {
-            result = PURE_ALLOCATION;
-            if ((writer = malloc(sizeof(*writer)))) {
-                *writer = (Writer){.fd = session.fd, .at = commit_start(&session)};
-                bool written = true;
-                for (size_t i = 0; i < count && written; ++i) {
-                    /* After compaction a draft's stored references name the old file. */
-                    bool relocate = session.file.st_dev != drafts[i]->device || session.file.st_ino != drafts[i]->inode;
-                    written = write_root(&drafts[i]->tree, writer, &roots[i], relocate);
-                }
-                result = written ? publish(&session, writer, count, keys, roots, revisions) : PURE_STORE_ERROR;
-            }
-        }
-        /* Rebase while still holding the writer lock; the arenas are no longer
-         * needed. Drafts of a store handle share one new mapping. If remapping
-         * fails a draft keeps equal content at its new revision. */
-        PureStore *refreshed[8] = {NULL};
-        size_t fresh = 0;
-        for (size_t i = 0; result == PURE_OK && i < count; ++i) {
-            PureStore *store = drafts[i]->store;
-            bool rebased;
-            if (store) {
-                bool seen = false;
-                for (size_t k = 0; k < fresh && !seen; ++k) seen = refreshed[k] == store;
-                if (!seen && refresh(store) == PURE_OK) {
-                    seen = true;
-                    if (fresh < 8) refreshed[fresh++] = store;
-                }
-                rebased = seen && rebase(drafts[i]) == PURE_OK;
-            } else rebased = load_draft(drafts[i]) == PURE_OK;
-            if (!rebased) drafts[i]->revision = revisions[i];
-        }
-    }
-    /* Before the checks every request shares the failure; after them an
-     * accepted request takes the publication's result and new revisions, and
-     * a refused one keeps its own result. */
-    size_t at = 0;
-    for (Waiter *w = first; w; w = w->next) {
-        if (strcmp(w->drafts[0]->path, path)) continue;
-        if (!checked) { w->result = result; continue; }
-        if (!w->accepted) continue;
-        for (size_t i = 0; i < w->count; ++i) w->revisions[i] = revisions[at++];
-        w->result = result;
-    }
-    free(writer);
-    free(roots);
-    free((void *)keys);
-    free(revisions);
-    free(drafts);
-    if (allocated) close_session(&session);
-}
-
-PureResult pure_draft_commit_all(PureDraft *const *drafts, size_t count, int64_t *revisions) {
-    for (size_t i = 0; i < count; ++i) {
-        revisions[i] = drafts[i]->revision;
-        if (!drafts[i]->path || strcmp(drafts[i]->path, drafts[0]->path)) return PURE_INVALID;
-        for (size_t k = 0; k < i; ++k) if (!strcmp(drafts[k]->key, drafts[i]->key)) return PURE_INVALID;
-    }
-    if (!count) return PURE_OK;
-    Waiter me = {.drafts = drafts, .count = count, .revisions = revisions};
-    pthread_mutex_lock(&group.lock);
-    if (group.tail) group.tail->next = &me; else group.head = &me;
-    group.tail = &me;
-    ++group.queued;
-    pthread_cond_signal(&group.joined);
-    while (!me.done) {
-        if (group.leading) { pthread_cond_wait(&group.finished, &group.lock); continue; }
-        group.leading = true;
-        /* When several threads have been committing together, give the others
-         * a moment (at most 2 ms) to join, or they would miss this group and
-         * wait for the next one: a sync costs far more. One thread never waits. */
-        if (group.expected > 1 && group.queued < group.expected) {
-            struct timespec deadline;
-            clock_gettime(CLOCK_REALTIME, &deadline);
-            deadline.tv_nsec += GATHER_NS;
-            if (deadline.tv_nsec >= 1000000000) { ++deadline.tv_sec; deadline.tv_nsec -= 1000000000; }
-            while (group.queued < group.expected &&
-                   pthread_cond_timedwait(&group.joined, &group.lock, &deadline) == 0) {}
-        }
-        Waiter *batch = group.head;
-        size_t size = group.queued;
-        group.head = group.tail = NULL;
-        group.queued = 0;
-        ++group.groups;
-        group.requests += size;
-        /* Remember the largest recent group, forgetting one member per group. */
-        group.expected = size > group.expected - 1 ? size : group.expected - 1;
-        if (!group.expected) group.expected = 1;
-        pthread_mutex_unlock(&group.lock);
-        /* One session per store among the queued requests. */
-        for (Waiter *w = batch; w; w = w->next) {
-            bool earlier = false;
-            for (Waiter *v = batch; v != w && !earlier; v = v->next) earlier = !strcmp(v->drafts[0]->path, w->drafts[0]->path);
-            if (!earlier) commit_group(w->drafts[0]->path, batch);
-        }
-        pthread_mutex_lock(&group.lock);
-        for (Waiter *w = batch, *next; w; w = next) { next = w->next; w->done = true; }
-        group.leading = false;
-        pthread_cond_broadcast(&group.finished);
-    }
-    pthread_mutex_unlock(&group.lock);
-    return me.result;
-}
-
-PureResult pure_draft_commit(PureDraft *draft, int64_t *revision) {
-    return pure_draft_commit_all(&draft, 1, revision);
-}
-
-PureResult pure_store_put(const char *path, const char *key, int64_t expected,
-                          PureSpan body, int64_t *revision) {
-    return mutate(path, key, expected, body, false, 0, 0, PURE_OBJECT_LIMIT, revision);
-}
-
-PureResult pure_store_splice(const char *path, const char *key, int64_t expected,
-                             size_t offset, size_t remove, PureSpan insert,
-                             size_t max_size, int64_t *revision) {
-    return mutate(path, key, expected, insert, true, offset, remove, max_size, revision);
-}
-
-#ifdef PURE_STORE_TEST
-static bool shape(const Tree *tree, Entry entry, unsigned height, bool root,
-                  uint64_t *leaves, uint64_t *small) {
-    if (!height) {
-        ++*leaves;
-        if (!root && entry.bytes < LEAF_MIN) ++*small;
-        return leaf_bytes(tree, entry) != NULL;
-    }
-    Entry kids[FAN_MAX];
-    size_t count = children(tree, entry, height, kids);
-    if (!count || (root && count < 2)) return false;
-    if (!root && count < FAN_MIN) ++*small;
-    for (size_t i = 0; i < count; ++i)
-        if (!shape(tree, kids[i], height - 1, false, leaves, small)) return false;
-    return true;
-}
-
-/* Test-only: live arena nodes and their payload bytes. */
-void pure_draft_test_arena(const PureDraft *draft, size_t *nodes, size_t *bytes) {
-    *nodes = *bytes = 0;
-    for (size_t i = 0; i < draft->tree.count; ++i) {
-        const Node *node = &draft->tree.nodes[i];
-        if (node->height == FREE) continue;
-        ++*nodes;
-        *bytes += node->bytes ? node->size : node->size * sizeof(Entry);
-    }
-}
-
-/* Test-only: the stored height, leaf count and number of underfull nodes. */
-bool pure_tree_test_shape(const char *path, const char *key, unsigned *height,
-                          uint64_t *leaves, uint64_t *small, uint64_t *file_end) {
-    int directory = open(path, O_RDONLY | O_DIRECTORY | O_CLOEXEC);
-    int fd = directory >= 0 ? openat(directory, tree_name, O_RDONLY | O_CLOEXEC) : -1;
-    if (directory >= 0) close(directory);
-    Tree tree = {0};
-    Head head;
-    struct stat file;
-    Entry document;
-    uint64_t index;
-    int64_t revision;
-    bool found = false;
-    *leaves = *small = 0;
-    bool ok = fd >= 0 && read_head(fd, &file, false, &head, NULL) && map_tree(fd, &file, &head, &tree) &&
-              find(&tree, (Entry){head.catalog, head.catalog_bytes}, key, &index, &found,
-                   &document, &revision) && found && root_height(&tree, document, height) &&
-              (!document.bytes || shape(&tree, document, *height, true, leaves, small));
-    if (ok) *file_end = head.end;
-    release_tree(&tree);
-    if (fd >= 0) close(fd);
-    return ok;
-}
-#endif