Objects treated as missing despite being present, due to race with geometric repacking

builtin/mktree.c

@@ -125,7 +125,6 @@ static void mktree_line(struct repository *repo, char *buf, int nul_term_line, i
 	oi.typep = &obj_type;
 	if (odb_read_object_info_extended(repo->objects, &oid, &oi,
 					  OBJECT_INFO_LOOKUP_REPLACE |
-					  OBJECT_INFO_QUICK |
 					  OBJECT_INFO_SKIP_FETCH_OBJECT) < 0)
 		obj_type = -1;
 
@@ -200,8 +199,11 @@ int cmd_mktree(int ac,
 			puts(oid_to_hex(&oid));
 			fflush(stdout);
 		}
+		for (int i = 0; i < used; i++)
+			free(entries[i]);
 		used=0; /* reset tree entry buffer for re-use in batch mode */
 	}
+	free(entries);
 	strbuf_release(&sb);
 
 	return 0;

builtin/pack-objects.c

@@ -1786,7 +1786,7 @@ static int want_object_in_pack_mtime(const struct object_id *oid,
 		struct multi_pack_index *m = get_multi_pack_index(files->packed);
 		struct pack_entry e;
 
-		if (m && fill_midx_entry(m, oid, &e, NULL)) {
+		if (m && midx_fill_entry(m, oid, &e, NULL) == MIDX_FILL_HIT) {
 			want = want_object_in_pack_one(e.p, oid, exclude, found_pack, found_offset, found_mtime);
 			if (want != -1)
 				return want;

midx.c

@@ -589,23 +589,23 @@ uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos)
 					       (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH);
 }
 
-int fill_midx_entry(struct multi_pack_index *m,
-		    const struct object_id *oid,
-		    struct pack_entry *e,
-		    struct packed_git **bad_pack)
+enum midx_fill_result midx_fill_entry(struct multi_pack_index *m,
+				      const struct object_id *oid,
+				      struct pack_entry *e,
+				      struct packed_git **bad_pack)
 {
 	uint32_t pos;
 	uint32_t pack_int_id;
 	struct packed_git *p;
 
 	if (!bsearch_midx(oid, m, &pos))
-		return 0;
+		return MIDX_FILL_MISS;
 
 	midx_for_object(&m, pos);
 	pack_int_id = nth_midxed_pack_int_id(m, pos);
 
 	if (prepare_midx_pack(m, pack_int_id))
-		return 0;
+		return MIDX_FILL_OWNER_UNAVAILABLE;
 	p = m->packs[pack_int_id - m->num_packs_in_base];
 
 	/*
@@ -616,19 +616,19 @@ int fill_midx_entry(struct multi_pack_index *m,
 	* loaded!
 	*/
 	if (!is_pack_valid(p))
-		return 0;
+		return MIDX_FILL_OWNER_UNAVAILABLE;
 
 	if (oidset_size(&p->bad_objects) &&
 	    oidset_contains(&p->bad_objects, oid)) {
 		if (bad_pack && !*bad_pack)
 			*bad_pack = p;
-		return 0;
+		return MIDX_FILL_MISS;
 	}
 
 	e->offset = nth_midxed_offset(m, pos);
 	e->p = p;
 
-	return 1;
+	return MIDX_FILL_HIT;
 }
 
 /* Match "foo.idx" against either "foo.pack" _or_ "foo.idx". */
@@ -1032,7 +1032,7 @@ int verify_midx_file(struct odb_source_packed *source, unsigned flags)
 
 		nth_midxed_object_oid(&oid, m, pairs[i].pos);
 
-		if (!fill_midx_entry(m, &oid, &e, NULL)) {
+		if (midx_fill_entry(m, &oid, &e, NULL) != MIDX_FILL_HIT) {
 			midx_report(_("failed to load pack entry for oid[%d] = %s"),
 				    pairs[i].pos, oid_to_hex(&oid));
 			continue;

midx.h

@@ -117,8 +117,25 @@ uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos);
 struct object_id *nth_midxed_object_oid(struct object_id *oid,
 					struct multi_pack_index *m,
 					uint32_t n);
-int fill_midx_entry(struct multi_pack_index *m, const struct object_id *oid,
-		    struct pack_entry *e, struct packed_git **bad_pack);
+/*
+ * Result of looking an object up in a multi-pack-index.  MIDX_FILL_HIT means
+ * "e was filled in"; the two miss variants distinguish an object the midx does
+ * not know about (MIDX_FILL_MISS) from one it does know about but whose owning
+ * pack we can no longer open (MIDX_FILL_OWNER_UNAVAILABLE -- the signature of a
+ * concurrent repack having removed that pack).  A known-bad (corrupt) object
+ * reports MIDX_FILL_MISS but also sets *bad_pack, if provided, to the owning
+ * pack so the caller can tell "corrupt" apart from "absent".
+ */
+enum midx_fill_result {
+	MIDX_FILL_MISS = 0,
+	MIDX_FILL_HIT,
+	MIDX_FILL_OWNER_UNAVAILABLE,
+};
+
+enum midx_fill_result midx_fill_entry(struct multi_pack_index *m,
+				      const struct object_id *oid,
+				      struct pack_entry *e,
+				      struct packed_git **bad_pack);
 int midx_contains_pack(struct multi_pack_index *m,
 		       const char *idx_or_pack_name);
 int midx_layer_contains_pack(struct multi_pack_index *m,

odb/source-packed.c

@@ -17,13 +17,18 @@
 static int find_pack_entry(struct odb_source_packed *store,
 			   const struct object_id *oid,
 			   struct pack_entry *e,
+			   enum object_info_flags flags,
 			   struct packed_git **bad_pack)
 {
 	struct packfile_list_entry *l;
+	enum midx_fill_result midx_result = MIDX_FILL_MISS;
 
 	odb_source_prepare(&store->base, 0);
-	if (store->midx && fill_midx_entry(store->midx, oid, e, bad_pack))
-		return 1;
+	if (store->midx) {
+		midx_result = midx_fill_entry(store->midx, oid, e, bad_pack);
+		if (midx_result == MIDX_FILL_HIT)
+			return 1;
+	}
 
 	for (l = store->packs.head; l; l = l->next) {
 		struct packed_git *p = l->pack;
@@ -35,6 +40,33 @@ static int find_pack_entry(struct odb_source_packed *store,
 		}
 	}
 
+	/*
+	 * Recovery for a concurrent-repack race: a stale MIDX may still name a
+	 * vanished owning pack even though the object survives in another pack
+	 * the same MIDX covers.  The regular fallback above skips MIDX-covered
+	 * packs, and repreparing the on-disk pack set does not reload the
+	 * borrowed, cached MIDX, so scan its packs directly for the survivor.
+	 *
+	 * Do this only on the second read, by which point repreparing packs has
+	 * already had a chance to find an object merely relocated into a new,
+	 * uncovered pack; only a genuine hidden duplicate reaches here.
+	 */
+	if (midx_result == MIDX_FILL_OWNER_UNAVAILABLE &&
+	    (flags & OBJECT_INFO_SECOND_READ)) {
+		struct multi_pack_index *m = store->midx;
+		uint32_t i;
+
+		for (i = 0; i < m->num_packs + m->num_packs_in_base; i++) {
+			struct packed_git *p;
+
+			if (prepare_midx_pack(m, i))
+				continue;
+			p = nth_midxed_pack(m, i);
+			if (p && packfile_fill_entry(p, oid, e, bad_pack))
+				return 1;
+		}
+	}
+
 	return 0;
 }
 
@@ -57,7 +89,7 @@ static enum odb_read_status odb_source_packed_read_object_info(struct odb_source
 	if (flags & OBJECT_INFO_SECOND_READ)
 		odb_source_prepare(source, ODB_PREPARE_FLUSH_CACHES);
 
-	if (!find_pack_entry(packed, oid, &e, &bad_pack)) {
+	if (!find_pack_entry(packed, oid, &e, flags, &bad_pack)) {
 		/*
 		 * The lookup may have failed because the object is known to be
 		 * corrupt in one of the packfiles. Report the object as
@@ -105,7 +137,7 @@ static int odb_source_packed_read_object_stream(struct odb_read_stream **out,
 	struct odb_source_packed *packed = odb_source_packed_downcast(source);
 	struct pack_entry e;
 
-	if (!find_pack_entry(packed, oid, &e, NULL))
+	if (!find_pack_entry(packed, oid, &e, 0, NULL))
 		return -1;
 
 	return packfile_read_object_stream(out, oid, e.p, e.offset);
@@ -611,7 +643,7 @@ static int odb_source_packed_freshen_object(struct odb_source *source,
 		timesp = &times;
 	}
 
-	if (!find_pack_entry(packed, oid, &e, NULL))
+	if (!find_pack_entry(packed, oid, &e, 0, NULL))
 		return 0;
 	if (e.p->is_cruft)
 		return 0;

replay.c

@@ -327,6 +327,13 @@ static struct commit *pick_regular_commit(struct repository *repo,
 	merge_opt->ancestor = NULL;
 	merge_opt->branch2 = NULL;
 
+	if (result->clean < 0) {
+		error(_("merge of %s onto %s failed"),
+		      oid_to_hex(&pickme->object.oid),
+		      oid_to_hex(&replayed_base->object.oid));
+		return NULL;
+	}
+
 	if (!result->clean)
 		return NULL;
 

t/helper/test-read-midx.c

@@ -82,7 +82,7 @@ static int read_midx_file(const char *object_dir, const char *checksum,
 		for (i = 0; i < m->num_objects; i++) {
 			nth_midxed_object_oid(&oid, m,
 					      i + m->num_objects_in_base);
-			fill_midx_entry(m, &oid, &e, NULL);
+			midx_fill_entry(m, &oid, &e, NULL);
 
 			printf("%s %"PRIu64"\t%s\n",
 			       oid_to_hex(&oid), e.offset, e.p->pack_name);

t/t1010-mktree.sh

@@ -69,4 +69,52 @@ test_expect_success 'mktree refuses to read ls-tree -r output (2)' '
 	test_must_fail git mktree <all.withsub
 '
 
+test_expect_success PIPE 'mktree --batch survives a concurrent repack retiring a pack' '
+	test_when_finished "rm -fr race" &&
+	git init race &&
+	(
+		cd race &&
+		test_commit seed &&
+		a=$(echo A | git hash-object -w --stdin) &&
+		b=$(echo B | git hash-object -w --stdin) &&
+		echo "$a" | git pack-objects .git/objects/pack/pack >pack-a &&
+		echo "$b" | git pack-objects .git/objects/pack/pack >pack-b &&
+
+		# Drop the loose copies so the blobs resolve only through the
+		# packs the multi-pack-index names.
+		git prune-packed &&
+		git multi-pack-index write &&
+		printf "100644 blob %s\ta\n" "$a" >tree-a &&
+		printf "100644 blob %s\tb\n" "$b" >tree-b &&
+
+		victim=".git/objects/pack/pack-$(cat pack-b)" &&
+		mkfifo in out &&
+
+		# mktree --batch stays resident, so its pack view predates the
+		# repack below; feed it one tree at a time over a fifo.  The
+		# subshell exit closes the fifos, letting mktree see EOF and quit.
+		(git mktree --batch <in >out 2>err &) &&
+		exec 9>in &&
+		exec 8<out &&
+
+		# The first tree makes the reader cache its (soon stale) view.
+		cat tree-a >&9 && echo >&9 && read tree_a <&8 &&
+
+		# Mimic a concurrent repack: a replacement pack holds every
+		# object, and the pack for b loses its .idx (its .pack lingers),
+		# matching the order in which unlink_pack_path() removes files.
+		git cat-file --batch-all-objects --batch-check="%(objectname)" >oids &&
+		git pack-objects .git/objects/pack/pack <oids >/dev/null &&
+		rm -f "$victim.idx" &&
+
+		# Resolving b used to fail, as its QUICK lookup accepted the
+		# miss; without QUICK the reader repreps and finds b in the
+		# replacement pack.
+		cat tree-b >&9 && echo >&9 && read tree_b <&8 &&
+		exec 9>&- &&
+
+		test -n "$tree_b"
+	)
+'
+
 test_done

t/t3650-replay-basics.sh

@@ -565,4 +565,38 @@ test_expect_success '--onto with --ref rejects multiple revision ranges' '
 	test_grep "cannot be used with multiple revision ranges" err
 '
 
+test_expect_success 'replay fails without segfault when objects are missing' '
+	test_when_finished "rm -fr unreadable" &&
+	git init unreadable &&
+	(
+		cd unreadable &&
+
+		test_write_lines l1 l2 l3 l4 l5 l6 l7 l8 >f &&
+		git add f &&
+		git commit -m base &&
+		git branch base &&
+
+		test_write_lines l1 l2 l3 l4 l5 l6 l7 CHANGED >f &&
+		git commit -am side &&
+		git branch side &&
+
+		git switch -c onto base &&
+		test_write_lines CHANGED l2 l3 l4 l5 l6 l7 l8 >f &&
+		git commit -am onto &&
+
+		# The replay works while every object is readable.
+		git replay --onto onto base..side &&
+
+		# Removing the onto tree makes parse_tree() fail during the
+		# incore merge, driving clean < 0 with a NULL result tree.
+		onto_tree=$(git rev-parse onto^{tree}) &&
+		obj=$(test_oid_to_path "$onto_tree") &&
+		mv .git/objects/${obj} saved-tree &&
+
+		# Ensure replay gracefully handles the missing object
+		test_must_fail git replay --onto onto base..side 2>err &&
+		test_grep -e "Could not read" -e "collecting merge info failed" err
+	)
+'
+
 test_done

t/t5319-multi-pack-index.sh

@@ -1393,4 +1393,44 @@ test_expect_success 'pack.preferBitmapTips interprets patterns as hierarchy' '
 	)
 '
 
+test_expect_success 'lookup recovers object whose midx-owning pack was removed' '
+	test_when_finished "rm -fr repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		# "keep" ends up only in the big pack; "dup" is deliberately
+		# placed in two packs so the midx has to choose an owner.
+		test_commit keep &&
+		echo duplicated-content >dup &&
+		git add dup &&
+		git commit -m dup &&
+		dup_oid=$(git rev-parse HEAD:dup) &&
+
+		# Roll every object, including dup, into a single big pack.
+		git repack -adq &&
+
+		# Build a second, "moderate" pack that also contains dup, so dup
+		# now lives in two packs that the midx will cover.
+		moderate=$(echo "$dup_oid" |
+			git pack-objects --quiet $objdir/pack/pack) &&
+
+		# Attribute dup to the moderate pack in the midx.
+		git multi-pack-index write \
+			--preferred-pack="pack-$moderate.idx" &&
+
+		# Simulate a concurrent "git repack" retiring the moderate pack:
+		# its files disappear, but the now-stale midx still names it as
+		# the owner of dup.  A valid copy of dup survives in the big pack.
+		rm -f $objdir/pack/pack-$moderate.* &&
+
+		# The midx routes the lookup to the deleted pack, and the regular
+		# pack fallback skips midx-covered packs, so without recovery dup
+		# would appear missing even though it is physically present.
+		echo blob >expect &&
+		git cat-file -t "$dup_oid" >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done