[PATCH v3 2/3] shallow: offer to prune only non-existing entries

["Johannes Schindelin via GitGitGadget" <gitgitgadget@xxxxxxxxx>]

From: Johannes Schindelin <johannes.schindelin@xxxxxx>

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 <johannes.schindelin@xxxxxx>
---
 builtin/prune.c |  2 +-
 commit.h        |  2 +-
 shallow.c       | 22 +++++++++++++++++-----
 3 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/builtin/prune.c b/builtin/prune.c
index 41230f821..6d6ab6cf1 100644
--- a/builtin/prune.c
+++ b/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;
 }
diff --git a/commit.h b/commit.h
index 1d260d62f..ff34447ab 100644
--- a/commit.h
+++ b/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);
diff --git a/shallow.c b/shallow.c
index 732e18d54..0a2671bc2 100644
--- a/shallow.c
+++ b/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)
@@ -371,16 +376,23 @@ 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;
 	}
@@ -388,7 +400,7 @@ void prune_shallow(int show_only)
 				       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));
-- 
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