Say where a node's memory goes, and the budget it now keeps (104, 013)

104 measures a node's memory under a receiver that is down and under a
snapshot held for long (021's run.py: resident memory against what was
posted, 30 s a case). A held snapshot grew the node without end (6.4 to
678 MiB, fixed in d1e8f38); a slow receiver grows it by what was sent,
221 B a send, which is state until delivered, so the budget bounds state
and refuses past it.

013 is that budget's design: what it counts (state bytes, deterministic),
what an event over it does, why an answer takes its send's room, how
links and HTTP wait, what it does not count and those bounds, and the
checks. With --memory 4000000 the same load fills it in about 4 s and
the node stays at 9.6-9.8 MiB (45.9 MiB and growing without).

012 records the hole the budget found in the link (the full flag), and
103 that its clock fix fell short (385754c). CHECKPOINT gains the row.

1 files changed, 32 insertions, 0 deletions

diff --git a/research/experiments/021-memory/hold-surface.c b/research/experiments/021-memory/hold-surface.c
new file mode 100644
index 0000000..c1c2d4b
--- /dev/null
+++ b/research/experiments/021-memory/hold-surface.c
@@ -0,0 +1,32 @@
+/* A surface that holds a snapshot until the server stops: a reader that
+ * never lets go, for run.py's held case (research/experiments/021-memory).
+ * It holds the first snapshot whose number view VIEW is at least AT (the
+ * first snapshot at all without them), and lets every other one go.
+ *
+ *   pure-serve ... --surface hold-surface.so [-- VIEW AT] */
+#include "surface.h"
+#include <poll.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int pure_surface_run(const PureSurface *s, int argc, char **argv) {
+    static PureApp reader;
+    int view = argc > 2 ? pure_module_view(s->module, argv[1]) : -1;
+    double at = argc > 2 ? atof(argv[2]) : 0, value = 0;
+    PureSnapshot *held = NULL;
+    while (!s->stopping(s->host)) {
+        struct pollfd wait = {s->changes, POLLIN, 0};
+        if (poll(&wait, 1, 20) > 0 && (wait.revents & POLLIN)) {
+            uint64_t saves;
+            if (read(s->changes, &saves, sizeof(saves)) < 0) { /* cleared already */ }
+        }
+        if (held) continue;
+        PureSnapshot *read_now = s->read(s->host, &reader);
+        if (!read_now) continue;
+        if (view < 0 || (pure_app_view(&reader, (uint32_t)view, &value) && value >= at)) held = read_now;
+        else s->release(s->host, read_now);
+    }
+    if (held) s->release(s->host, held);
+    return 0;
+}