Add aup watch: build again when what a build read changes

A build records every path it opened or looked for and a digest of the
bytes there, so a rebuild happens exactly when one of them reads
differently: the source, a library (the one with the error too), or a
library appearing where one was searched. A change is built once it reads
the same twice in a row, so a file caught halfway through being written
is never built; the module is replaced whole through a rename.

1 files changed, 114 insertions, 22 deletions

diff --git a/src/main.c b/src/main.c
index 2797d93..b117df4 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1,17 +1,21 @@
 #define _POSIX_C_SOURCE 200809L
 #include "aup.h"
+#include <signal.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <time.h>
+#include <unistd.h>
 
 static int usage(void) {
-    fputs("aup check FILE\naup build FILE -o OUT.pmod\n"
+    fputs("aup check FILE\naup build FILE -o OUT.pmod\naup watch FILE -o OUT.pmod\n"
           "A library `use name` is name.aup beside FILE or in a directory of AUP_PATH (':' separated).\n", stderr);
     return 2;
 }
 
-/* Read a whole source; NULL if it cannot be read. Longer than the limit is
- * left for the compiler to report. */
+/* Read a whole source (up to one byte past the limit, which the compiler
+ * reports); NULL if it cannot be read. */
 static char *read_source(const char *path, size_t *length) {
     FILE *file = fopen(path, "rb");
     if (!file) return NULL;
@@ -23,21 +27,51 @@ static char *read_source(const char *path, size_t *length) {
     return source;
 }
 
-typedef struct { char directory[AUP_PATH]; char *sources[16]; unsigned count; } Libraries;
+/* Everything a build read: each path it opened or looked for, with a digest
+ * of what it found there (0: nothing). A build is a function of these bytes
+ * and AUP_PATH alone, so a path that reads differently is a reason to build
+ * again and nothing else is. */
+enum { READS = 64 };
+typedef struct {
+    char directory[AUP_PATH], paths[READS][AUP_PATH];
+    uint64_t digests[READS];
+    unsigned reads;
+    bool overflowed; /* it read more paths than READS */
+    char *sources[16];
+    unsigned count;
+} Inputs;
 
-static bool try_library(Libraries *l, const char *directory, size_t directory_length, const char *name,
+static uint64_t digest(const char *bytes, size_t length) {
+    uint64_t h = UINT64_C(0xcbf29ce484222325) ^ length;
+    for (size_t i = 0; i < length; ++i) h = (h ^ (unsigned char)bytes[i]) * UINT64_C(0x100000001b3);
+    return h ? h : 1;
+}
+
+/* read_source, noting in inputs the path and what it held. */
+static char *read_input(Inputs *in, const char *path, size_t *length) {
+    char *text = read_source(path, length);
+    if (in->reads == READS) {
+        in->overflowed = true;
+    } else {
+        snprintf(in->paths[in->reads], AUP_PATH, "%s", path);
+        in->digests[in->reads++] = text ? digest(text, *length) : 0;
+    }
+    return text;
+}
+
+static bool try_library(Inputs *in, const char *directory, size_t directory_length, const char *name,
                         const char **source, size_t *length, char path[AUP_PATH]) {
     int written = snprintf(path, AUP_PATH, "%.*s/%s.aup", (int)directory_length, directory, name);
-    if (written < 0 || written >= AUP_PATH || l->count == sizeof(l->sources) / sizeof(*l->sources)) return false;
-    char *text = read_source(path, length);
+    if (written < 0 || written >= AUP_PATH || in->count == sizeof(in->sources) / sizeof(*in->sources)) return false;
+    char *text = read_input(in, path, length);
     if (!text) return false;
-    *source = l->sources[l->count++] = text;
+    *source = in->sources[in->count++] = text;
     return true;
 }
 
 /* Beside the main source first, then each directory of AUP_PATH in order. */
 static bool load_library(void *context, const char *name, const char **source, size_t *length, char path[AUP_PATH]) {
-    Libraries *l = context;
+    Inputs *l = context;
     if (try_library(l, l->directory, strlen(l->directory), name, source, length, path)) return true;
     for (const char *at = getenv("AUP_PATH"); at && *at;) {
         size_t span = strcspn(at, ":");
@@ -47,32 +81,90 @@ static bool load_library(void *context, const char *name, const char **source, s
     return false;
 }
 
-int main(int argc, char **argv) {
-    bool build = argc == 5 && !strcmp(argv[1], "build") && !strcmp(argv[3], "-o");
-    if (!build && !(argc == 3 && !strcmp(argv[1], "check"))) return usage();
-    const char *path = argv[2];
+/* Compile path; write the module to out (unless NULL) through a temporary
+ * file renamed over it, so a reader never sees half of one. What it read is
+ * left in inputs. 0 on success, 1 on a diagnostic, 2 when a file failed. */
+static int compile(const char *path, const char *out, Inputs *inputs) {
+    *inputs = (Inputs){.count = 0};
     size_t length;
-    char *source = read_source(path, &length);
+    char *source = read_input(inputs, path, &length);
     if (!source) { perror(path); return 2; }
-    Libraries libraries = {.count = 0};
     const char *slash = strrchr(path, '/');
-    snprintf(libraries.directory, sizeof(libraries.directory), "%.*s", slash ? (int)(slash - path) : 1, slash ? path : ".");
+    snprintf(inputs->directory, sizeof(inputs->directory), "%.*s", slash ? (int)(slash - path) : 1, slash ? path : ".");
     unsigned char *module;
     size_t size;
     AupDiagnostic error;
-    bool compiled = aup_compile(source, length, load_library, &libraries, &module, &size, &error);
-    for (unsigned i = 0; i < libraries.count; ++i) free(libraries.sources[i]);
+    bool compiled = aup_compile(source, length, load_library, inputs, &module, &size, &error);
+    for (unsigned i = 0; i < inputs->count; ++i) free(inputs->sources[i]);
     free(source);
     if (!compiled) {
         fprintf(stderr, "%s:%d:%d: %s\n", *error.path ? error.path : path, error.line, error.column, error.message);
         return 1;
     }
     int status = 0;
-    if (build) {
-        FILE *out = fopen(argv[4], "wb");
-        if (!out || fwrite(module, 1, size, out) != size) { perror(argv[4]); status = 2; }
-        if (out && fclose(out)) { perror(argv[4]); status = 2; }
+    if (out) {
+        char temporary[AUP_PATH + 32];
+        snprintf(temporary, sizeof(temporary), "%s.%ld.tmp", out, (long)getpid());
+        FILE *file = fopen(temporary, "wb");
+        if (!file || fwrite(module, 1, size, file) != size) { perror(temporary); status = 2; }
+        if (file && fclose(file)) { perror(temporary); status = 2; }
+        if (!status && rename(temporary, out)) { perror(out); status = 2; }
+        if (status) remove(temporary);
     }
     free(module);
     return status;
 }
+
+static volatile sig_atomic_t stopping;
+static void on_stop(int signal_number) { (void)signal_number; stopping = 1; }
+
+/* One digest of what the paths of the last build hold: as it read them, or
+ * as they read now. Comparing bytes, not file times, no write is lost
+ * between a build and its bookkeeping, and saving the same bytes is no change. */
+static uint64_t contents(const Inputs *in, bool now) {
+    uint64_t h = 0;
+    for (unsigned i = 0; i < in->reads; ++i) {
+        uint64_t d = in->digests[i];
+        if (now) {
+            size_t length;
+            char *text = read_source(in->paths[i], &length);
+            d = text ? digest(text, length) : 0;
+            free(text);
+        }
+        h = (h ^ d) * UINT64_C(0x100000001b3);
+    }
+    return h;
+}
+
+/* Build now and again whenever what the last build read reads differently:
+ * the source, a library (the one with the error in it too), or a library
+ * appearing where one was looked for. A change is built once it reads the
+ * same twice in a row, so a file caught halfway through being written is not
+ * (a running app swapped to it would lose its states). A failed build keeps
+ * the last module. Each outcome is one line on stderr. */
+static int watch(const char *path, const char *out) {
+    signal(SIGINT, on_stop);
+    signal(SIGTERM, on_stop);
+    Inputs inputs;
+    uint64_t built = 0, seen = 0;
+    for (bool first = true; !stopping; first = false) {
+        uint64_t now = first ? 0 : contents(&inputs, true);
+        if (first || (now != built && now == seen)) {
+            if (!compile(path, out, &inputs)) fprintf(stderr, "aup: built %s\n", out);
+            if (inputs.overflowed) fprintf(stderr, "aup: watching only the first %d paths this build read\n", READS);
+            built = seen = contents(&inputs, false);
+            continue;
+        }
+        seen = now;
+        nanosleep(&(struct timespec){0, 50000000}, NULL);
+    }
+    return 0;
+}
+
+int main(int argc, char **argv) {
+    Inputs inputs;
+    if (argc == 5 && !strcmp(argv[3], "-o") && !strcmp(argv[1], "build")) return compile(argv[2], argv[4], &inputs);
+    if (argc == 5 && !strcmp(argv[3], "-o") && !strcmp(argv[1], "watch")) return watch(argv[2], argv[4]);
+    if (argc == 3 && !strcmp(argv[1], "check")) return compile(argv[2], NULL, &inputs);
+    return usage();
+}