odb: use odb_source_files_try() in source-chain iterations

Convert all source-chain iteration sites that access
files-specific internals (pack store, loose cache, MIDX) to use
odb_source_files_try() instead of odb_source_files_downcast().

These loops iterate the full source chain, which may include
alternates. When a non-files backend is added to the chain (as an
alternate or additional source), these sites must skip it rather
than abort. The _try() helper returns NULL for non-files sources,
and each site checks for NULL before accessing files-specific
members.

Call sites that access odb->sources directly (the primary source,
which is always a files backend) or that are inside files-backend
vtable callbacks continue to use odb_source_files_downcast() with
its BUG() safety guarantee.

Sites converted (22 across 11 files):

  - loose.c: loose object map loading and oid lookup
  - midx.c: multi-pack-index access and cleanup
  - object-file.c: loose cache and reprepare
  - object-name.c: short object filename search
  - packfile.c: window/fd management, pack entry lookup
  - packfile.h: repo_for_each_pack iterator macros
  - commit-graph.c: packed object enumeration
  - builtin/cat-file.c: batch object iteration
  - builtin/fast-import.c: pack lookup in store/stream
  - builtin/grep.c: pack preparation
  - builtin/pack-objects.c: cruft/kept pack enumeration

Add a unit test verifying odb_source_files_try() returns the
correct backend for ODB_SOURCE_FILES and NULL for other types.

Note: repo_approximate_object_count() and has_object_pack() will
not account for objects in non-files sources. These are
display and optimization functions respectively, and a follow-up
series can add vtable callbacks to address this.

Signed-off-by: Aaron Paterson <apaterson@pm.me>

Author: MayCXC

Makefile

@@ -1544,6 +1544,7 @@ CLAR_TEST_SUITES += u-strvec
 CLAR_TEST_SUITES += u-trailer
 CLAR_TEST_SUITES += u-urlmatch-normalization
 CLAR_TEST_SUITES += u-utf8-width
+CLAR_TEST_SUITES += u-odb-source
 CLAR_TEST_PROG = $(UNIT_TEST_BIN)/unit-tests$(X)
 CLAR_TEST_OBJS = $(patsubst %,$(UNIT_TEST_DIR)/%.o,$(CLAR_TEST_SUITES))
 CLAR_TEST_OBJS += $(UNIT_TEST_DIR)/clar/clar.o

builtin/cat-file.c

@@ -882,9 +882,12 @@ static void batch_each_object(struct batch_options *opt,
 		struct object_info oi = { 0 };
 
 		for (source = the_repository->objects->sources; source; source = source->next) {
-			struct odb_source_files *files = odb_source_files_downcast(source);
-			int ret = packfile_store_for_each_object(files->packed, &oi,
-								 batch_one_object_oi, &payload, flags);
+			struct odb_source_files *files = odb_source_files_try(source);
+			int ret;
+			if (!files)
+				continue;
+			ret = packfile_store_for_each_object(files->packed, &oi,
+							     batch_one_object_oi, &payload, flags);
 			if (ret)
 				break;
 		}

builtin/fast-import.c

@@ -982,7 +982,9 @@ static int store_object(
 	}
 
 	for (source = the_repository->objects->sources; source; source = source->next) {
-		struct odb_source_files *files = odb_source_files_downcast(source);
+		struct odb_source_files *files = odb_source_files_try(source);
+		if (!files)
+			continue;
 
 		if (!packfile_list_find_oid(packfile_store_get_packs(files->packed), &oid))
 			continue;
@@ -1189,7 +1191,9 @@ static void stream_blob(uintmax_t len, struct object_id *oidout, uintmax_t mark)
 	}
 
 	for (source = the_repository->objects->sources; source; source = source->next) {
-		struct odb_source_files *files = odb_source_files_downcast(source);
+		struct odb_source_files *files = odb_source_files_try(source);
+		if (!files)
+			continue;
 
 		if (!packfile_list_find_oid(packfile_store_get_packs(files->packed), &oid))
 			continue;

builtin/grep.c

@@ -1219,7 +1219,9 @@ int cmd_grep(int argc,
 
 			odb_prepare_alternates(the_repository->objects);
 			for (source = the_repository->objects->sources; source; source = source->next) {
-				struct odb_source_files *files = odb_source_files_downcast(source);
+				struct odb_source_files *files = odb_source_files_try(source);
+				if (!files)
+					continue;
 				packfile_store_prepare(files->packed);
 			}
 		}

builtin/pack-objects.c

@@ -1531,8 +1531,11 @@ static int want_cruft_object_mtime(struct repository *r,
 	struct odb_source *source;
 
 	for (source = r->objects->sources; source; source = source->next) {
-		struct odb_source_files *files = odb_source_files_downcast(source);
-		struct packed_git **cache = packfile_store_get_kept_pack_cache(files->packed, flags);
+		struct odb_source_files *files = odb_source_files_try(source);
+		struct packed_git **cache;
+		if (!files)
+			continue;
+		cache = packfile_store_get_kept_pack_cache(files->packed, flags);
 
 		for (; *cache; cache++) {
 			struct packed_git *p = *cache;
@@ -1754,7 +1757,9 @@ static int want_object_in_pack_mtime(const struct object_id *oid,
 	}
 
 	for (source = the_repository->objects->sources; source; source = source->next) {
-		struct odb_source_files *files = odb_source_files_downcast(source);
+		struct odb_source_files *files = odb_source_files_try(source);
+		if (!files)
+			continue;
 
 		for (e = files->packed->packs.head; e; e = e->next) {
 			struct packed_git *p = e->pack;
@@ -4350,7 +4355,9 @@ static void add_objects_in_unpacked_packs(void)
 
 	odb_prepare_alternates(to_pack.repo->objects);
 	for (source = to_pack.repo->objects->sources; source; source = source->next) {
-		struct odb_source_files *files = odb_source_files_downcast(source);
+		struct odb_source_files *files = odb_source_files_try(source);
+		if (!files)
+			continue;
 
 		if (!source->local)
 			continue;

commit-graph.c

@@ -1981,7 +1981,9 @@ static void fill_oids_from_all_packs(struct write_commit_graph_context *ctx)
 
 	odb_prepare_alternates(ctx->r->objects);
 	for (source = ctx->r->objects->sources; source; source = source->next) {
-		struct odb_source_files *files = odb_source_files_downcast(source);
+		struct odb_source_files *files = odb_source_files_try(source);
+		if (!files)
+			continue;
 		packfile_store_for_each_object(files->packed, &oi, add_packed_commits_oi,
 					       ctx, ODB_FOR_EACH_OBJECT_PACK_ORDER);
 	}

loose.c

@@ -64,10 +64,13 @@ static int insert_loose_map(struct odb_source *source,
 
 static int load_one_loose_object_map(struct repository *repo, struct odb_source *source)
 {
-	struct odb_source_files *files = odb_source_files_downcast(source);
+	struct odb_source_files *files = odb_source_files_try(source);
 	struct strbuf buf = STRBUF_INIT, path = STRBUF_INIT;
 	FILE *fp;
 
+	if (!files)
+		return 0;
+
 	if (!files->loose->map)
 		loose_object_map_init(&files->loose->map);
 	if (!files->loose->cache) {
@@ -235,8 +238,11 @@ int repo_loose_object_map_oid(struct repository *repo,
 	khiter_t pos;
 
 	for (source = repo->objects->sources; source; source = source->next) {
-		struct odb_source_files *files = odb_source_files_downcast(source);
-		struct loose_object_map *loose_map = files->loose->map;
+		struct odb_source_files *files = odb_source_files_try(source);
+		struct loose_object_map *loose_map;
+		if (!files)
+			continue;
+		loose_map = files->loose->map;
 		if (!loose_map)
 			continue;
 		map = (to == repo->compat_hash_algo) ?

midx.c

@@ -95,7 +95,9 @@ static int midx_read_object_offsets(const unsigned char *chunk_start,
 
 struct multi_pack_index *get_multi_pack_index(struct odb_source *source)
 {
-	struct odb_source_files *files = odb_source_files_downcast(source);
+	struct odb_source_files *files = odb_source_files_try(source);
+	if (!files)
+		return NULL;
 	packfile_store_prepare(files->packed);
 	return files->packed->midx;
 }
@@ -806,7 +808,9 @@ void clear_midx_file(struct repository *r)
 		struct odb_source *source;
 
 		for (source = r->objects->sources; source; source = source->next) {
-			struct odb_source_files *files = odb_source_files_downcast(source);
+			struct odb_source_files *files = odb_source_files_try(source);
+			if (!files)
+				continue;
 			if (files->packed->midx)
 				close_midx(files->packed->midx);
 			files->packed->midx = NULL;

object-file.c

@@ -1879,14 +1879,20 @@ static int append_loose_object(const struct object_id *oid,
 struct oidtree *odb_source_loose_cache(struct odb_source *source,
 				       const struct object_id *oid)
 {
-	struct odb_source_files *files = odb_source_files_downcast(source);
-	int subdir_nr = oid->hash[0];
+	struct odb_source_files *files = odb_source_files_try(source);
+	int subdir_nr;
 	struct strbuf buf = STRBUF_INIT;
-	size_t word_bits = bitsizeof(files->loose->subdir_seen[0]);
-	size_t word_index = subdir_nr / word_bits;
-	size_t mask = (size_t)1u << (subdir_nr % word_bits);
+	size_t word_bits, word_index, mask;
 	uint32_t *bitmap;
 
+	if (!files)
+		return NULL;
+
+	subdir_nr = oid->hash[0];
+	word_bits = bitsizeof(files->loose->subdir_seen[0]);
+	word_index = subdir_nr / word_bits;
+	mask = (size_t)1u << (subdir_nr % word_bits);
+
 	if (subdir_nr < 0 ||
 	    (size_t) subdir_nr >= bitsizeof(files->loose->subdir_seen))
 		BUG("subdir_nr out of range");
@@ -1919,7 +1925,9 @@ static void odb_source_loose_clear_cache(struct odb_source_loose *loose)
 
 void odb_source_loose_reprepare(struct odb_source *source)
 {
-	struct odb_source_files *files = odb_source_files_downcast(source);
+	struct odb_source_files *files = odb_source_files_try(source);
+	if (!files)
+		return;
 	odb_source_loose_clear_cache(files->loose);
 }
 

object-name.c

@@ -115,9 +115,11 @@ static void find_short_object_filename(struct disambiguate_state *ds)
 {
 	struct odb_source *source;
 
-	for (source = ds->repo->objects->sources; source && !ds->ambiguous; source = source->next)
-		oidtree_each(odb_source_loose_cache(source, &ds->bin_pfx),
-				&ds->bin_pfx, ds->len, match_prefix, ds);
+	for (source = ds->repo->objects->sources; source && !ds->ambiguous; source = source->next) {
+		struct oidtree *tree = odb_source_loose_cache(source, &ds->bin_pfx);
+		if (tree)
+			oidtree_each(tree, &ds->bin_pfx, ds->len, match_prefix, ds);
+	}
 }
 
 static int match_hash(unsigned len, const unsigned char *a, const unsigned char *b)