In 2007 Junio wrote
(https://public-inbox.org/git/[email protected]/):
+static int need_to_gc(void)
+{
+ /*
+ * Quickly check if a "gc" is needed, by estimating how
+ * many loose objects there are. Because SHA-1 is evenly
+ * distributed, we can check only one and get a reasonable
+ * estimate.
+ */
+ char path[PATH_MAX];
+ const char *objdir = get_object_directory();
+ DIR *dir;
+ struct dirent *ent;
+ int auto_threshold;
+ int num_loose = 0;
+ int needed = 0;
+
+ if (sizeof(path) <= snprintf(path, sizeof(path), "%s/17", objdir)) {
+ warning("insanely long object directory %.*s", 50, objdir);
+ return 0;
+ }
+ dir = opendir(path);
+ if (!dir)
+ return 0;
+
+ auto_threshold = (gc_auto_threshold + 255) / 256;
+ while ((ent = readdir(dir)) != NULL) {
+ if (strspn(ent->d_name, "0123456789abcdef") != 38 ||
+ ent->d_name[38] != '\0')
+ continue;
+ if (++num_loose > auto_threshold) {
+ needed = 1;
+ break;
+ }
+ }
A couple of questions about this patch, which is in git.git as
2c3c439947 ("Implement git gc --auto", 2007-09-05)
1. We still have this check of objects/17/ in builtin/gc.c today. Why
objects/17/ and not e.g. objects/00/ to go with other 000* magic such
as the 0000000000000000000000000000000000000000 SHA-1? Statistically
it doesn't matter, but 17 seems like an odd thing to pick at random
out of 00..ff, does it have any significance?
2. It seems overly paranoid to be checking that the files in
.git/objects/17/ look like a SHA-1. If we have stuff not generated by
git in .git/objects/??/ we probably have bigger problems than
prematurely triggering auto gc, can this just be removed as
redundant. Was this some check e.g. expecting that this would need to
deal with tempfiles in these directories that we created at the time
(but no longer do?)?
