shallow: offer to prune only non-existing entries

The `prune_shallow()` function wants a full reachability check to be
completed before it goes to work, to ensure that all unreachable entries
are removed from the shallow file.

However, in the upcoming patch we do not even want to go that far. We
really only need to remove entries corresponding to pruned commits, i.e.
to commits that no longer exist.

Let's support that use case.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

builtin/prune.c

@@ -161,7 +161,7 @@ int cmd_prune(int argc, const char **argv, const char *prefix)
 	free(s);
 
 	if (is_repository_shallow(the_repository))
-		prune_shallow(show_only);
+		prune_shallow(show_only, 0);
 
 	return 0;
 }

commit.h

@@ -249,7 +249,7 @@ extern void assign_shallow_commits_to_refs(struct shallow_info *info,
 					   uint32_t **used,
 					   int *ref_status);
 extern int delayed_reachability_test(struct shallow_info *si, int c);
-extern void prune_shallow(int show_only);
+extern void prune_shallow(int show_only, int quick_prune);
 extern struct trace_key trace_shallow;
 
 extern int interactive_add(int argc, const char **argv, const char *prefix, int patch);

shallow.c

@@ -247,6 +247,7 @@ static void check_shallow_file_for_update(struct repository *r)
 
 #define SEEN_ONLY 1
 #define VERBOSE   2
+#define QUICK_PRUNE 4
 
 struct write_shallow_data {
 	struct strbuf *out;
@@ -261,7 +262,11 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 	const char *hex = oid_to_hex(&graft->oid);
 	if (graft->nr_parent != -1)
 		return 0;
-	if (data->flags & SEEN_ONLY) {
+	if (data->flags & QUICK_PRUNE) {
+		struct commit *c = lookup_commit(the_repository, &graft->oid);
+		if (!c || parse_commit(c))
+			return 0;
+	} else if (data->flags & SEEN_ONLY) {
 		struct commit *c = lookup_commit(the_repository, &graft->oid);
 		if (!c || !(c->object.flags & SEEN)) {
 			if (data->flags & VERBOSE)
@@ -371,24 +376,31 @@ void advertise_shallow_grafts(int fd)
 
 /*
  * mark_reachable_objects() should have been run prior to this and all
- * reachable commits marked as "SEEN".
+ * reachable commits marked as "SEEN", except when quick_prune is non-zero,
+ * in which case lines are excised from the shallow file if they refer to
+ * commits that do not exist (any longer).
  */
-void prune_shallow(int show_only)
+void prune_shallow(int show_only, int quick_prune)
 {
 	struct lock_file shallow_lock = LOCK_INIT;
 	struct strbuf sb = STRBUF_INIT;
+	unsigned flags = SEEN_ONLY;
 	int fd;
 
+	if (quick_prune)
+		flags |= QUICK_PRUNE;
+
 	if (show_only) {
-		write_shallow_commits_1(&sb, 0, NULL, SEEN_ONLY | VERBOSE);
+		flags |= VERBOSE;
+		write_shallow_commits_1(&sb, 0, NULL, flags);
 		strbuf_release(&sb);
 		return;
 	}
 	fd = hold_lock_file_for_update(&shallow_lock,
 				       git_path_shallow(the_repository),
 				       LOCK_DIE_ON_ERROR);
 	check_shallow_file_for_update(the_repository);
-	if (write_shallow_commits_1(&sb, 0, NULL, SEEN_ONLY)) {
+	if (write_shallow_commits_1(&sb, 0, NULL, flags)) {
 		if (write_in_full(fd, sb.buf, sb.len) < 0)
 			die_errno("failed to write to %s",
 				  get_lock_file_path(&shallow_lock));