repack: teach MIDX retention about geometric rollups

When writing an incremental MIDX, existing_packs_retain_midx_packs()
marks packs in the existing MIDX chain as retained. This keeps them from
being deleted by the later existing_packs deletion pass, since retained
MIDX layers may still refer to those packs.

Geometric repacks need a narrower rule. Packs below the split are rolled
up into the newly-written pack, and should remain eligible for deletion
even if the old MIDX chain mentions them. Packs above the split were
marked as retained by the previous commit.

Teach existing_packs_retain_midx_packs() to skip packs which are part of
the geometric rollup. This does not change the current caller's behavior,
since geometric repacks do not yet use the existing_packs deletion path.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/repack.c

@@ -576,7 +576,7 @@ int cmd_repack(int argc,
 
 	if (delete_redundant && pack_everything & ALL_INTO_ONE) {
 		if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL)
-			existing_packs_retain_midx_packs(&existing);
+			existing_packs_retain_midx_packs(&existing, &geometry);
 		existing_packs_mark_for_deletion(&existing, &names);
 	}
 

repack.c

@@ -292,6 +292,39 @@ void existing_packs_mark_for_deletion(struct existing_packs *existing,
 					   &existing->cruft_packs);
 }
 
+static int pack_geometry_contains_pack(struct packed_git **packs,
+				       uint32_t packs_nr,
+				       const char *base)
+{
+	struct strbuf buf = STRBUF_INIT;
+	uint32_t i;
+
+	for (i = 0; i < packs_nr; i++) {
+		strbuf_reset(&buf);
+		strbuf_addstr(&buf, pack_basename(packs[i]));
+		strbuf_strip_suffix(&buf, ".pack");
+
+		if (!strcmp(buf.buf, base)) {
+			strbuf_release(&buf);
+			return 1;
+		}
+	}
+
+	strbuf_release(&buf);
+	return 0;
+}
+
+static int pack_geometry_contains_rollup(const struct pack_geometry *geometry,
+					 const char *base)
+{
+	if (!geometry || !geometry->split_factor)
+		return 0;
+
+	return pack_geometry_contains_pack(geometry->pack, geometry->split, base) ||
+	       pack_geometry_contains_pack(geometry->promisor_pack,
+					   geometry->promisor_split, base);
+}
+
 /*
  * Mark every pack that is referenced by the existing MIDX chain as
  * retained, so that a subsequent call to
@@ -300,9 +333,12 @@ void existing_packs_mark_for_deletion(struct existing_packs *existing,
  * This is used when writing an incremental MIDX layer on top of an
  * existing chain: retained layers continue to reference the same
  * packs on disk, so those packs must not be unlinked even if the
- * freshly-written pack supersedes them.
+ * freshly-written pack supersedes them. When doing a geometric repack,
+ * packs below the split are rewritten into the new MIDX tip and should
+ * remain eligible for deletion.
  */
-void existing_packs_retain_midx_packs(struct existing_packs *existing)
+void existing_packs_retain_midx_packs(struct existing_packs *existing,
+				      const struct pack_geometry *geometry)
 {
 	struct string_list_item *item;
 	struct strbuf buf = STRBUF_INIT;
@@ -315,6 +351,9 @@ void existing_packs_retain_midx_packs(struct existing_packs *existing)
 		strbuf_strip_suffix(&buf, ".pack");
 		strbuf_strip_suffix(&buf, ".idx");
 
+		if (pack_geometry_contains_rollup(geometry, buf.buf))
+			continue;
+
 		found = string_list_lookup(&existing->non_kept_packs, buf.buf);
 		if (found)
 			existing_packs_mark_retained(found);

repack.h

@@ -87,7 +87,8 @@ void existing_packs_retain_from_geometry(struct existing_packs *existing,
 					 const struct pack_geometry *geometry);
 void existing_packs_mark_for_deletion(struct existing_packs *existing,
 				      struct string_list *names);
-void existing_packs_retain_midx_packs(struct existing_packs *existing);
+void existing_packs_retain_midx_packs(struct existing_packs *existing,
+				      const struct pack_geometry *geometry);
 void existing_packs_remove_redundant(struct existing_packs *existing,
 				     const char *packdir,
 				     bool wrote_incremental_midx);