odb: embed base source in the "files" backend

The "files" backend is implemented as a pointer in the `struct
odb_source`. This contradicts our typical pattern for pluggable backends
like we use it for example in the ref store or for object database
streams, where we typically embed the generic base structure in the
specialized implementation. This pattern has a couple of small benefits:

  - We avoid an extra allocation.

  - We hide implementation details in the generic structure.

  - We can easily downcast from a generic backend to the specialized
    structure and vice versa because the offsets are known at compile
    time.

  - It becomes trivial to identify locations where we depend on backend
    specific logic because the cast needs to be explicit.

Refactor our "files" object database source to do the same and embed the
`struct odb_source` in the `struct odb_source_files`.

There are still a bunch of sites in our code base where we do have to
access internals of the "files" backend. The intent is that those will
go away over time, but this will certainly take a while. Meanwhile,
provide a `odb_source_files_downcast()` function that can convert a
generic source into a "files" source.

As we only have a single source the downcast succeeds unconditionally
for now. Eventually though the intent is to make the cast `BUG()` in
case the caller requests to downcast a non-"files" backend to a "files"
backend.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/cat-file.c

@@ -882,7 +882,8 @@ static void batch_each_object(struct batch_options *opt,
 		struct object_info oi = { 0 };
 
 		for (source = the_repository->objects->sources; source; source = source->next) {
-			int ret = packfile_store_for_each_object(source->files->packed, &oi,
+			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);
 			if (ret)
 				break;

builtin/fast-import.c

@@ -875,6 +875,7 @@ static void end_packfile(void)
 	running = 1;
 	clear_delta_base_cache();
 	if (object_count) {
+		struct odb_source_files *files = odb_source_files_downcast(pack_data->repo->objects->sources);
 		struct packed_git *new_p;
 		struct object_id cur_pack_oid;
 		char *idx_name;
@@ -900,8 +901,7 @@ static void end_packfile(void)
 		idx_name = keep_pack(create_index());
 
 		/* Register the packfile with core git's machinery. */
-		new_p = packfile_store_load_pack(pack_data->repo->objects->sources->files->packed,
-						 idx_name, 1);
+		new_p = packfile_store_load_pack(files->packed, idx_name, 1);
 		if (!new_p)
 			die(_("core Git rejected index %s"), idx_name);
 		all_packs[pack_id] = new_p;
@@ -982,7 +982,9 @@ static int store_object(
 	}
 
 	for (source = the_repository->objects->sources; source; source = source->next) {
-		if (!packfile_list_find_oid(packfile_store_get_packs(source->files->packed), &oid))
+		struct odb_source_files *files = odb_source_files_downcast(source);
+
+		if (!packfile_list_find_oid(packfile_store_get_packs(files->packed), &oid))
 			continue;
 		e->type = type;
 		e->pack_id = MAX_PACK_ID;
@@ -1187,7 +1189,9 @@ static void stream_blob(uintmax_t len, struct object_id *oidout, uintmax_t mark)
 	}
 
 	for (source = the_repository->objects->sources; source; source = source->next) {
-		if (!packfile_list_find_oid(packfile_store_get_packs(source->files->packed), &oid))
+		struct odb_source_files *files = odb_source_files_downcast(source);
+
+		if (!packfile_list_find_oid(packfile_store_get_packs(files->packed), &oid))
 			continue;
 		e->type = OBJ_BLOB;
 		e->pack_id = MAX_PACK_ID;

builtin/grep.c

@@ -1218,8 +1218,10 @@ int cmd_grep(int argc,
 			struct odb_source *source;
 
 			odb_prepare_alternates(the_repository->objects);
-			for (source = the_repository->objects->sources; source; source = source->next)
-				packfile_store_prepare(source->files->packed);
+			for (source = the_repository->objects->sources; source; source = source->next) {
+				struct odb_source_files *files = odb_source_files_downcast(source);
+				packfile_store_prepare(files->packed);
+			}
 		}
 
 		start_threads(&opt);

builtin/index-pack.c

@@ -1637,9 +1637,11 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
 	rename_tmp_packfile(&final_index_name, curr_index_name, &index_name,
 			    hash, "idx", 1);
 
-	if (do_fsck_object && startup_info->have_repository)
-		packfile_store_load_pack(the_repository->objects->sources->files->packed,
-					 final_index_name, 0);
+	if (do_fsck_object && startup_info->have_repository) {
+		struct odb_source_files *files =
+			odb_source_files_downcast(the_repository->objects->sources);
+		packfile_store_load_pack(files->packed, final_index_name, 0);
+	}
 
 	if (!from_stdin) {
 		printf("%s\n", hash_to_hex(hash));

builtin/pack-objects.c

@@ -1531,7 +1531,8 @@ static int want_cruft_object_mtime(struct repository *r,
 	struct odb_source *source;
 
 	for (source = r->objects->sources; source; source = source->next) {
-		struct packed_git **cache = packfile_store_get_kept_pack_cache(source->files->packed, flags);
+		struct odb_source_files *files = odb_source_files_downcast(source);
+		struct packed_git **cache = packfile_store_get_kept_pack_cache(files->packed, flags);
 
 		for (; *cache; cache++) {
 			struct packed_git *p = *cache;
@@ -1753,11 +1754,13 @@ static int want_object_in_pack_mtime(const struct object_id *oid,
 	}
 
 	for (source = the_repository->objects->sources; source; source = source->next) {
-		for (e = source->files->packed->packs.head; e; e = e->next) {
+		struct odb_source_files *files = odb_source_files_downcast(source);
+
+		for (e = files->packed->packs.head; e; e = e->next) {
 			struct packed_git *p = e->pack;
 			want = want_object_in_pack_one(p, oid, exclude, found_pack, found_offset, found_mtime);
 			if (!exclude && want > 0)
-				packfile_list_prepend(&source->files->packed->packs, p);
+				packfile_list_prepend(&files->packed->packs, p);
 			if (want != -1)
 				return want;
 		}
@@ -4337,10 +4340,12 @@ 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);
+
 		if (!source->local)
 			continue;
 
-		if (packfile_store_for_each_object(source->files->packed, &oi,
+		if (packfile_store_for_each_object(files->packed, &oi,
 						   add_object_in_unpacked_pack, NULL,
 						   ODB_FOR_EACH_OBJECT_PACK_ORDER |
 						   ODB_FOR_EACH_OBJECT_LOCAL_ONLY |

commit-graph.c

@@ -1980,9 +1980,11 @@ static void fill_oids_from_all_packs(struct write_commit_graph_context *ctx)
 			ctx->approx_nr_objects);
 
 	odb_prepare_alternates(ctx->r->objects);
-	for (source = ctx->r->objects->sources; source; source = source->next)
-		packfile_store_for_each_object(source->files->packed, &oi, add_packed_commits_oi,
+	for (source = ctx->r->objects->sources; source; source = source->next) {
+		struct odb_source_files *files = odb_source_files_downcast(source);
+		packfile_store_for_each_object(files->packed, &oi, add_packed_commits_oi,
 					       ctx, ODB_FOR_EACH_OBJECT_PACK_ORDER);
+	}
 
 	if (ctx->progress_done < ctx->approx_nr_objects)
 		display_progress(ctx->progress, ctx->approx_nr_objects);

http.c

@@ -2543,8 +2543,9 @@ cleanup:
 void http_install_packfile(struct packed_git *p,
 			   struct packfile_list *list_to_remove_from)
 {
+	struct odb_source_files *files = odb_source_files_downcast(the_repository->objects->sources);
 	packfile_list_remove(list_to_remove_from, p);
-	packfile_store_add_pack(the_repository->objects->sources->files->packed, p);
+	packfile_store_add_pack(files->packed, p);
 }
 
 struct http_pack_request *new_http_pack_request(

loose.c

@@ -3,6 +3,7 @@
 #include "path.h"
 #include "object-file.h"
 #include "odb.h"
+#include "odb/source-files.h"
 #include "hex.h"
 #include "repository.h"
 #include "wrapper.h"
@@ -49,27 +50,29 @@ static int insert_loose_map(struct odb_source *source,
 			    const struct object_id *oid,
 			    const struct object_id *compat_oid)
 {
-	struct loose_object_map *map = source->files->loose->map;
+	struct odb_source_files *files = odb_source_files_downcast(source);
+	struct loose_object_map *map = files->loose->map;
 	int inserted = 0;
 
 	inserted |= insert_oid_pair(map->to_compat, oid, compat_oid);
 	inserted |= insert_oid_pair(map->to_storage, compat_oid, oid);
 	if (inserted)
-		oidtree_insert(source->files->loose->cache, compat_oid);
+		oidtree_insert(files->loose->cache, compat_oid);
 
 	return inserted;
 }
 
 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 strbuf buf = STRBUF_INIT, path = STRBUF_INIT;
 	FILE *fp;
 
-	if (!source->files->loose->map)
-		loose_object_map_init(&source->files->loose->map);
-	if (!source->files->loose->cache) {
-		ALLOC_ARRAY(source->files->loose->cache, 1);
-		oidtree_init(source->files->loose->cache);
+	if (!files->loose->map)
+		loose_object_map_init(&files->loose->map);
+	if (!files->loose->cache) {
+		ALLOC_ARRAY(files->loose->cache, 1);
+		oidtree_init(files->loose->cache);
 	}
 
 	insert_loose_map(source, repo->hash_algo->empty_tree, repo->compat_hash_algo->empty_tree);
@@ -125,7 +128,8 @@ int repo_read_loose_object_map(struct repository *repo)
 
 int repo_write_loose_object_map(struct repository *repo)
 {
-	kh_oid_map_t *map = repo->objects->sources->files->loose->map->to_compat;
+	struct odb_source_files *files = odb_source_files_downcast(repo->objects->sources);
+	kh_oid_map_t *map = files->loose->map->to_compat;
 	struct lock_file lock;
 	int fd;
 	khiter_t iter;
@@ -231,7 +235,8 @@ int repo_loose_object_map_oid(struct repository *repo,
 	khiter_t pos;
 
 	for (source = repo->objects->sources; source; source = source->next) {
-		struct loose_object_map *loose_map = source->files->loose->map;
+		struct odb_source_files *files = odb_source_files_downcast(source);
+		struct loose_object_map *loose_map = files->loose->map;
 		if (!loose_map)
 			continue;
 		map = (to == repo->compat_hash_algo) ?

midx.c

@@ -95,8 +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)
 {
-	packfile_store_prepare(source->files->packed);
-	return source->files->packed->midx;
+	struct odb_source_files *files = odb_source_files_downcast(source);
+	packfile_store_prepare(files->packed);
+	return files->packed->midx;
 }
 
 static struct multi_pack_index *load_multi_pack_index_one(struct odb_source *source,
@@ -447,6 +448,7 @@ static uint32_t midx_for_pack(struct multi_pack_index **_m,
 int prepare_midx_pack(struct multi_pack_index *m,
 		      uint32_t pack_int_id)
 {
+	struct odb_source_files *files = odb_source_files_downcast(m->source);
 	struct strbuf pack_name = STRBUF_INIT;
 	struct packed_git *p;
 
@@ -457,10 +459,10 @@ int prepare_midx_pack(struct multi_pack_index *m,
 	if (m->packs[pack_int_id])
 		return 0;
 
-	strbuf_addf(&pack_name, "%s/pack/%s", m->source->path,
+	strbuf_addf(&pack_name, "%s/pack/%s", files->base.path,
 		    m->pack_names[pack_int_id]);
-	p = packfile_store_load_pack(m->source->files->packed,
-				     pack_name.buf, m->source->local);
+	p = packfile_store_load_pack(files->packed,
+				     pack_name.buf, files->base.local);
 	strbuf_release(&pack_name);
 
 	if (!p) {
@@ -703,18 +705,19 @@ int midx_preferred_pack(struct multi_pack_index *m, uint32_t *pack_int_id)
 
 int prepare_multi_pack_index_one(struct odb_source *source)
 {
+	struct odb_source_files *files = odb_source_files_downcast(source);
 	struct repository *r = source->odb->repo;
 
 	prepare_repo_settings(r);
 	if (!r->settings.core_multi_pack_index)
 		return 0;
 
-	if (source->files->packed->midx)
+	if (files->packed->midx)
 		return 1;
 
-	source->files->packed->midx = load_multi_pack_index(source);
+	files->packed->midx = load_multi_pack_index(source);
 
-	return !!source->files->packed->midx;
+	return !!files->packed->midx;
 }
 
 int midx_checksum_valid(struct multi_pack_index *m)
@@ -803,9 +806,10 @@ void clear_midx_file(struct repository *r)
 		struct odb_source *source;
 
 		for (source = r->objects->sources; source; source = source->next) {
-			if (source->files->packed->midx)
-				close_midx(source->files->packed->midx);
-			source->files->packed->midx = NULL;
+			struct odb_source_files *files = odb_source_files_downcast(source);
+			if (files->packed->midx)
+				close_midx(files->packed->midx);
+			files->packed->midx = NULL;
 		}
 	}
 

object-file.c

@@ -219,8 +219,9 @@ static void *odb_source_loose_map_object(struct odb_source *source,
 					 const struct object_id *oid,
 					 unsigned long *size)
 {
+	struct odb_source_files *files = odb_source_files_downcast(source);
 	const char *p;
-	int fd = open_loose_object(source->files->loose, oid, &p);
+	int fd = open_loose_object(files->loose, oid, &p);
 
 	if (fd < 0)
 		return NULL;
@@ -401,6 +402,7 @@ static int read_object_info_from_path(struct odb_source *source,
 				      struct object_info *oi,
 				      enum object_info_flags flags)
 {
+	struct odb_source_files *files = odb_source_files_downcast(source);
 	int ret;
 	int fd;
 	unsigned long mapsize;
@@ -423,7 +425,7 @@ static int read_object_info_from_path(struct odb_source *source,
 		struct stat st;
 
 		if ((!oi || (!oi->disk_sizep && !oi->mtimep)) && (flags & OBJECT_INFO_QUICK)) {
-			ret = quick_has_loose(source->files->loose, oid) ? 0 : -1;
+			ret = quick_has_loose(files->loose, oid) ? 0 : -1;
 			goto out;
 		}
 
@@ -1866,33 +1868,34 @@ 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 strbuf buf = STRBUF_INIT;
-	size_t word_bits = bitsizeof(source->files->loose->subdir_seen[0]);
+	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);
 	uint32_t *bitmap;
 
 	if (subdir_nr < 0 ||
-	    (size_t) subdir_nr >= bitsizeof(source->files->loose->subdir_seen))
+	    (size_t) subdir_nr >= bitsizeof(files->loose->subdir_seen))
 		BUG("subdir_nr out of range");
 
-	bitmap = &source->files->loose->subdir_seen[word_index];
+	bitmap = &files->loose->subdir_seen[word_index];
 	if (*bitmap & mask)
-		return source->files->loose->cache;
-	if (!source->files->loose->cache) {
-		ALLOC_ARRAY(source->files->loose->cache, 1);
-		oidtree_init(source->files->loose->cache);
+		return files->loose->cache;
+	if (!files->loose->cache) {
+		ALLOC_ARRAY(files->loose->cache, 1);
+		oidtree_init(files->loose->cache);
 	}
 	strbuf_addstr(&buf, source->path);
 	for_each_file_in_obj_subdir(subdir_nr, &buf,
 				    source->odb->repo->hash_algo,
 				    append_loose_object,
 				    NULL, NULL,
-				    source->files->loose->cache);
+				    files->loose->cache);
 	*bitmap |= mask;
 	strbuf_release(&buf);
-	return source->files->loose->cache;
+	return files->loose->cache;
 }
 
 static void odb_source_loose_clear_cache(struct odb_source_loose *loose)
@@ -1905,7 +1908,8 @@ static void odb_source_loose_clear_cache(struct odb_source_loose *loose)
 
 void odb_source_loose_reprepare(struct odb_source *source)
 {
-	odb_source_loose_clear_cache(source->files->loose);
+	struct odb_source_files *files = odb_source_files_downcast(source);
+	odb_source_loose_clear_cache(files->loose);
 }
 
 static int check_stream_oid(git_zstream *stream,

odb.c

@@ -691,7 +691,8 @@ static int do_oid_object_info_extended(struct object_database *odb,
 
 		/* Most likely it's a loose object. */
 		for (source = odb->sources; source; source = source->next) {
-			if (!packfile_store_read_object_info(source->files->packed, real, oi, flags) ||
+			struct odb_source_files *files = odb_source_files_downcast(source);
+			if (!packfile_store_read_object_info(files->packed, real, oi, flags) ||
 			    !odb_source_loose_read_object_info(source, real, oi, flags))
 				return 0;
 		}
@@ -699,9 +700,11 @@ static int do_oid_object_info_extended(struct object_database *odb,
 		/* Not a loose object; someone else may have just packed it. */
 		if (!(flags & OBJECT_INFO_QUICK)) {
 			odb_reprepare(odb->repo->objects);
-			for (source = odb->sources; source; source = source->next)
-				if (!packfile_store_read_object_info(source->files->packed, real, oi, flags))
+			for (source = odb->sources; source; source = source->next) {
+				struct odb_source_files *files = odb_source_files_downcast(source);
+				if (!packfile_store_read_object_info(files->packed, real, oi, flags))
 					return 0;
+			}
 		}
 
 		/*
@@ -962,7 +965,9 @@ int odb_freshen_object(struct object_database *odb,
 
 	odb_prepare_alternates(odb);
 	for (source = odb->sources; source; source = source->next) {
-		if (packfile_store_freshen_object(source->files->packed, oid))
+		struct odb_source_files *files = odb_source_files_downcast(source);
+
+		if (packfile_store_freshen_object(files->packed, oid))
 			return 1;
 
 		if (odb_source_loose_freshen_object(source, oid))
@@ -982,6 +987,8 @@ int odb_for_each_object(struct object_database *odb,
 
 	odb_prepare_alternates(odb);
 	for (struct odb_source *source = odb->sources; source; source = source->next) {
+		struct odb_source_files *files = odb_source_files_downcast(source);
+
 		if (flags & ODB_FOR_EACH_OBJECT_LOCAL_ONLY && !source->local)
 			continue;
 
@@ -992,7 +999,7 @@ int odb_for_each_object(struct object_database *odb,
 				return ret;
 		}
 
-		ret = packfile_store_for_each_object(source->files->packed, request,
+		ret = packfile_store_for_each_object(files->packed, request,
 						     cb, cb_data, flags);
 		if (ret)
 			return ret;
@@ -1090,8 +1097,10 @@ struct object_database *odb_new(struct repository *repo,
 void odb_close(struct object_database *o)
 {
 	struct odb_source *source;
-	for (source = o->sources; source; source = source->next)
-		packfile_store_close(source->files->packed);
+	for (source = o->sources; source; source = source->next) {
+		struct odb_source_files *files = odb_source_files_downcast(source);
+		packfile_store_close(files->packed);
+	}
 	close_commit_graph(o);
 }
 
@@ -1148,8 +1157,9 @@ void odb_reprepare(struct object_database *o)
 	odb_prepare_alternates(o);
 
 	for (source = o->sources; source; source = source->next) {
+		struct odb_source_files *files = odb_source_files_downcast(source);
 		odb_source_loose_reprepare(source);
-		packfile_store_reprepare(source->files->packed);
+		packfile_store_reprepare(files->packed);
 	}
 
 	o->approximate_object_count_valid = 0;

odb/source-files.c

@@ -1,5 +1,6 @@
 #include "git-compat-util.h"
 #include "object-file.h"
+#include "odb/source.h"
 #include "odb/source-files.h"
 #include "packfile.h"
 
@@ -9,15 +10,20 @@ void odb_source_files_free(struct odb_source_files *files)
 		return;
 	odb_source_loose_free(files->loose);
 	packfile_store_free(files->packed);
+	odb_source_release(&files->base);
 	free(files);
 }
 
-struct odb_source_files *odb_source_files_new(struct odb_source *source)
+struct odb_source_files *odb_source_files_new(struct object_database *odb,
+					      const char *path,
+					      bool local)
 {
 	struct odb_source_files *files;
+
 	CALLOC_ARRAY(files, 1);
-	files->source = source;
-	files->loose = odb_source_loose_new(source);
-	files->packed = packfile_store_new(source);
+	odb_source_init(&files->base, odb, path, local);
+	files->loose = odb_source_loose_new(&files->base);
+	files->packed = packfile_store_new(&files->base);
+
 	return files;
 }

odb/source-files.h

@@ -1,8 +1,9 @@
 #ifndef ODB_SOURCE_FILES_H
 #define ODB_SOURCE_FILES_H
 
+#include "odb/source.h"
+
 struct odb_source_loose;
-struct odb_source;
 struct packfile_store;
 
 /*
@@ -10,15 +11,26 @@ struct packfile_store;
  * packfiles. It is the default backend used by Git to store objects.
  */
 struct odb_source_files {
-	struct odb_source *source;
+	struct odb_source base;
 	struct odb_source_loose *loose;
 	struct packfile_store *packed;
 };
 
 /* Allocate and initialize a new object source. */
-struct odb_source_files *odb_source_files_new(struct odb_source *source);
+struct odb_source_files *odb_source_files_new(struct object_database *odb,
+					      const char *path,
+					      bool local);
 
 /* Free the object source and release all associated resources. */
 void odb_source_files_free(struct odb_source_files *files);
 
+/*
+ * Cast the given object database source to the files backend. This will cause
+ * a BUG in case the source doesn't use this backend.
+ */
+static inline struct odb_source_files *odb_source_files_downcast(struct odb_source *source)
+{
+	return container_of(source, struct odb_source_files, base);
+}
+
 #endif

odb/source.c

@@ -1,5 +1,6 @@
 #include "git-compat-util.h"
 #include "object-file.h"
+#include "odb/source-files.h"
 #include "odb/source.h"
 #include "packfile.h"
 
@@ -7,20 +8,31 @@ struct odb_source *odb_source_new(struct object_database *odb,
 				  const char *path,
 				  bool local)
 {
-	struct odb_source *source;
+	return &odb_source_files_new(odb, path, local)->base;
+}
 
-	CALLOC_ARRAY(source, 1);
+void odb_source_init(struct odb_source *source,
+		     struct object_database *odb,
+		     const char *path,
+		     bool local)
+{
 	source->odb = odb;
 	source->local = local;
 	source->path = xstrdup(path);
-	source->files = odb_source_files_new(source);
-
-	return source;
 }
 
 void odb_source_free(struct odb_source *source)
 {
-	free(source->path);
-	odb_source_files_free(source->files);
-	free(source);
+	struct odb_source_files *files;
+	if (!source)
+		return;
+	files = odb_source_files_downcast(source);
+	odb_source_files_free(files);
+}
+
+void odb_source_release(struct odb_source *source)
+{
+	if (!source)
+		return;
+	free(source->path);
 }

odb/source.h

@@ -1,8 +1,6 @@
 #ifndef ODB_SOURCE_H
 #define ODB_SOURCE_H
 
-#include "odb/source-files.h"
-
 /*
  * The source is the part of the object database that stores the actual
  * objects. It thus encapsulates the logic to read and write the specific
@@ -21,9 +19,6 @@ struct odb_source {
 	/* Object database that owns this object source. */
 	struct object_database *odb;
 
-	/* The backend used to store objects. */
-	struct odb_source_files *files;
-
 	/*
 	 * Figure out whether this is the local source of the owning
 	 * repository, which would typically be its ".git/objects" directory.
@@ -53,7 +48,31 @@ struct odb_source *odb_source_new(struct object_database *odb,
 				  const char *path,
 				  bool local);
 
-/* Free the object database source, releasing all associated resources. */
+/*
+ * Initialize the source for the given object database located at `path`.
+ * `local` indicates whether or not the source is the local and thus primary
+ * object source of the object database.
+ *
+ * This function is only supposed to be called by specific object source
+ * implementations.
+ */
+void odb_source_init(struct odb_source *source,
+		     struct object_database *odb,
+		     const char *path,
+		     bool local);
+
+/*
+ * Free the object database source, releasing all associated resources and
+ * freeing the structure itself.
+ */
 void odb_source_free(struct odb_source *source);
 
+/*
+ * Release the object database source, releasing all associated resources.
+ *
+ * This function is only supposed to be called by specific object source
+ * implementations.
+ */
+void odb_source_release(struct odb_source *source);
+
 #endif

odb/streaming.c

@@ -187,7 +187,8 @@ static int istream_source(struct odb_read_stream **out,
 
 	odb_prepare_alternates(odb);
 	for (source = odb->sources; source; source = source->next) {
-		if (!packfile_store_read_object_stream(out, source->files->packed, oid) ||
+		struct odb_source_files *files = odb_source_files_downcast(source);
+		if (!packfile_store_read_object_stream(out, files->packed, oid) ||
 		    !odb_source_loose_read_object_stream(out, source, oid))
 			return 0;
 	}

packfile.c

@@ -362,9 +362,11 @@ static int unuse_one_window(struct object_database *odb)
 	struct packed_git *lru_p = NULL;
 	struct pack_window *lru_w = NULL, *lru_l = NULL;
 
-	for (source = odb->sources; source; source = source->next)
-		for (e = source->files->packed->packs.head; e; e = e->next)
+	for (source = odb->sources; source; source = source->next) {
+		struct odb_source_files *files = odb_source_files_downcast(source);
+		for (e = files->packed->packs.head; e; e = e->next)
 			scan_windows(e->pack, &lru_p, &lru_w, &lru_l);
+	}
 
 	if (lru_p) {
 		munmap(lru_w->base, lru_w->len);
@@ -537,7 +539,8 @@ static int close_one_pack(struct repository *r)
 	int accept_windows_inuse = 1;
 
 	for (source = r->objects->sources; source; source = source->next) {
-		for (e = source->files->packed->packs.head; e; e = e->next) {
+		struct odb_source_files *files = odb_source_files_downcast(source);
+		for (e = files->packed->packs.head; e; e = e->next) {
 			if (e->pack->pack_fd == -1)
 				continue;
 			find_lru_pack(e->pack, &lru_p, &mru_w, &accept_windows_inuse);
@@ -987,13 +990,14 @@ static void prepare_pack(const char *full_name, size_t full_name_len,
 			 const char *file_name, void *_data)
 {
 	struct prepare_pack_data *data = (struct prepare_pack_data *)_data;
+	struct odb_source_files *files = odb_source_files_downcast(data->source);
 	size_t base_len = full_name_len;
 
 	if (strip_suffix_mem(full_name, &base_len, ".idx") &&
-	    !(data->source->files->packed->midx &&
-	      midx_contains_pack(data->source->files->packed->midx, file_name))) {
+	    !(files->packed->midx &&
+	      midx_contains_pack(files->packed->midx, file_name))) {
 		char *trimmed_path = xstrndup(full_name, full_name_len);
-		packfile_store_load_pack(data->source->files->packed,
+		packfile_store_load_pack(files->packed,
 					 trimmed_path, data->source->local);
 		free(trimmed_path);
 	}
@@ -1247,8 +1251,10 @@ const struct packed_git *has_packed_and_bad(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 packfile_list_entry *e;
-		for (e = source->files->packed->packs.head; e; e = e->next)
+
+		for (e = files->packed->packs.head; e; e = e->next)
 			if (oidset_contains(&e->pack->bad_objects, oid))
 				return e->pack;
 	}
@@ -2254,7 +2260,8 @@ int has_object_pack(struct repository *r, const struct object_id *oid)
 
 	odb_prepare_alternates(r->objects);
 	for (source = r->objects->sources; source; source = source->next) {
-		int ret = find_pack_entry(source->files->packed, oid, &e);
+		struct odb_source_files *files = odb_source_files_downcast(source);
+		int ret = find_pack_entry(files->packed, oid, &e);
 		if (ret)
 			return ret;
 	}
@@ -2269,9 +2276,10 @@ int has_object_kept_pack(struct repository *r, const struct object_id *oid,
 	struct pack_entry e;
 
 	for (source = r->objects->sources; source; source = source->next) {
+		struct odb_source_files *files = odb_source_files_downcast(source);
 		struct packed_git **cache;
 
-		cache = packfile_store_get_kept_pack_cache(source->files->packed, flags);
+		cache = packfile_store_get_kept_pack_cache(files->packed, flags);
 
 		for (; *cache; cache++) {
 			struct packed_git *p = *cache;

packfile.h

@@ -4,6 +4,7 @@
 #include "list.h"
 #include "object.h"
 #include "odb.h"
+#include "odb/source-files.h"
 #include "oidset.h"
 #include "repository.h"
 #include "strmap.h"
@@ -192,7 +193,8 @@ static inline struct repo_for_each_pack_data repo_for_eack_pack_data_init(struct
 	odb_prepare_alternates(repo->objects);
 
 	for (struct odb_source *source = repo->objects->sources; source; source = source->next) {
-		struct packfile_list_entry *entry = packfile_store_get_packs(source->files->packed);
+		struct odb_source_files *files = odb_source_files_downcast(source);
+		struct packfile_list_entry *entry = packfile_store_get_packs(files->packed);
 		if (!entry)
 			continue;
 		data.source = source;
@@ -212,7 +214,8 @@ static inline void repo_for_each_pack_data_next(struct repo_for_each_pack_data *
 		return;
 
 	for (source = data->source->next; source; source = source->next) {
-		struct packfile_list_entry *entry = packfile_store_get_packs(source->files->packed);
+		struct odb_source_files *files = odb_source_files_downcast(source);
+		struct packfile_list_entry *entry = packfile_store_get_packs(files->packed);
 		if (!entry)
 			continue;
 		data->source = source;