Implement git gc --auto

This implements a new option "git gc --auto".  When gc.auto is
set to a positive value, and the object database has accumulated
roughly that many number of loose objects, this runs a
lightweight version of "git gc".  The primary difference from
the full "git gc" is that it does not pass "-a" option to "git
repack", which means we do not try to repack _everything_, but
only repack incrementally.  We still do "git prune-packed".  The
default threshold is arbitrarily set by yours truly to:

 - not trigger it for fully unpacked git v0.99 history;

 - do trigger it for fully unpacked git v1.0.0 history;

 - not trigger it for incremental update to git v1.0.0 starting
   from fully packed git v0.99 history.

This patch does not add invocation of the "auto repacking".  It
is left to key Porcelain commands that could produce tons of
loose objects to add a call to "git gc --auto" after they are
done their work.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-gc.c

@@ -20,6 +20,7 @@ static const char builtin_gc_usage[] = "git-gc [--prune] [--aggressive]";
 
 static int pack_refs = 1;
 static int aggressive_window = -1;
+static int gc_auto_threshold = 6700;
 
 #define MAX_ADD 10
 static const char *argv_pack_refs[] = {"pack-refs", "--all", "--prune", NULL};
@@ -28,6 +29,8 @@ static const char *argv_repack[MAX_ADD] = {"repack", "-a", "-d", "-l", NULL};
 static const char *argv_prune[] = {"prune", NULL};
 static const char *argv_rerere[] = {"rerere", "gc", NULL};
 
+static const char *argv_repack_auto[] = {"repack", "-d", "-l", NULL};
+
 static int gc_config(const char *var, const char *value)
 {
 	if (!strcmp(var, "gc.packrefs")) {
@@ -41,6 +44,10 @@ static int gc_config(const char *var, const char *value)
 		aggressive_window = git_config_int(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "gc.auto")) {
+		gc_auto_threshold = git_config_int(var, value);
+		return 0;
+	}
 	return git_default_config(var, value);
 }
 
@@ -57,10 +64,49 @@ static void append_option(const char **cmd, const char *opt, int max_length)
 	cmd[i] = NULL;
 }
 
+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;
+		}
+	}
+	closedir(dir);
+	return needed;
+}
+
 int cmd_gc(int argc, const char **argv, const char *prefix)
 {
 	int i;
 	int prune = 0;
+	int auto_gc = 0;
 	char buf[80];
 
 	git_config(gc_config);
@@ -82,12 +128,28 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 			}
 			continue;
 		}
-		/* perhaps other parameters later... */
+		if (!strcmp(arg, "--auto")) {
+			if (gc_auto_threshold <= 0)
+				return 0;
+			auto_gc = 1;
+			continue;
+		}
 		break;
 	}
 	if (i != argc)
 		usage(builtin_gc_usage);
 
+	if (auto_gc) {
+		/*
+		 * Auto-gc should be least intrusive as possible.
+		 */
+		prune = 0;
+		for (i = 0; i < ARRAY_SIZE(argv_repack_auto); i++)
+			argv_repack[i] = argv_repack_auto[i];
+		if (!need_to_gc())
+			return 0;
+	}
+
 	if (pack_refs && run_command_v_opt(argv_pack_refs, RUN_GIT_CMD))
 		return error(FAILED_RUN, argv_pack_refs[0]);