Merge branch 'ps/odb-sources' into ps/object-counting

* ps/odb-sources:
  odb/source: make `begin_transaction()` function pluggable
  odb/source: make `write_alternate()` function pluggable
  odb/source: make `read_alternates()` function pluggable
  odb/source: make `write_object_stream()` function pluggable
  odb/source: make `write_object()` function pluggable
  odb/source: make `freshen_object()` function pluggable
  odb/source: make `for_each_object()` function pluggable
  odb/source: make `read_object_stream()` function pluggable
  odb/source: make `read_object_info()` function pluggable
  odb/source: make `close()` function pluggable
  odb/source: make `reprepare()` function pluggable
  odb/source: make `free()` function pluggable
  odb/source: introduce source type for robustness
  odb: move reparenting logic into respective subsystems
  odb: embed base source in the "files" backend
  odb: introduce "files" source
  odb: split `struct odb_source` into separate header

23 files changed, 953 insertions, 354 deletions

diff --git a/Makefile b/Makefile
index f3264d0a37..e1355721d5 100644
--- a/Makefile
+++ b/Makefile
@@ -1214,6 +1214,8 @@ LIB_OBJS += object-file.o
 LIB_OBJS += object-name.o
 LIB_OBJS += object.o
 LIB_OBJS += odb.o
+LIB_OBJS += odb/source.o
+LIB_OBJS += odb/source-files.o
 LIB_OBJS += odb/streaming.o
 LIB_OBJS += oid-array.o
 LIB_OBJS += oidmap.o
diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index da059d0e26..b6f12f41d6 100644
--- a/builtin/cat-file.c
+++ b/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->packfiles, &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;
diff --git a/builtin/fast-import.c b/builtin/fast-import.c
index b8a7757cfd..a41f95191e 100644
--- a/builtin/fast-import.c
+++ b/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->packfiles,
-						 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->packfiles), &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->packfiles), &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;
diff --git a/builtin/grep.c b/builtin/grep.c
index b6cecfd9b6..e33285e5e6 100644
--- a/builtin/grep.c
+++ b/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->packfiles);
+			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);
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index b67fb0256c..d1e47279a8 100644
--- a/builtin/index-pack.c
+++ b/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->packfiles,
-					 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));
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index c1ee4d5ed7..29d930e4b1 100644
--- a/builtin/pack-objects.c
+++ b/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->packfiles, 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->packfiles->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->packfiles->packs, p);
+				packfile_list_prepend(&files->packed->packs, p);
 			if (want != -1)
 				return want;
 		}
@@ -4347,10 +4350,12 @@ 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->packfiles, &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 |
diff --git a/commit-graph.c b/commit-graph.c
index d250a729b1..f8e24145a5 100644
--- a/commit-graph.c
+++ b/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->packfiles, &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);
diff --git a/http.c b/http.c
index 7815f144de..8ea1b9d1f6 100644
--- a/http.c
+++ b/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->packfiles, p);
+	packfile_store_add_pack(files->packed, p);
 }
 
 struct http_pack_request *new_http_pack_request(
diff --git a/loose.c b/loose.c
index 56cf64b648..07333be696 100644
--- a/loose.c
+++ b/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->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->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->loose->map)
-		loose_object_map_init(&source->loose->map);
-	if (!source->loose->cache) {
-		ALLOC_ARRAY(source->loose->cache, 1);
-		oidtree_init(source->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->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->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) ?
diff --git a/meson.build b/meson.build
index 4b536e0124..b62d49b9d8 100644
--- a/meson.build
+++ b/meson.build
@@ -399,6 +399,8 @@ libgit_sources = [
   'object-name.c',
   'object.c',
   'odb.c',
+  'odb/source.c',
+  'odb/source-files.c',
   'odb/streaming.c',
   'oid-array.c',
   'oidmap.c',
diff --git a/midx.c b/midx.c
index a75ea99a0d..ab8e2611d1 100644
--- a/midx.c
+++ b/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->packfiles);
-	return source->packfiles->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->packfiles,
-				     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->packfiles->midx)
+	if (files->packed->midx)
 		return 1;
 
-	source->packfiles->midx = load_multi_pack_index(source);
+	files->packed->midx = load_multi_pack_index(source);
 
-	return !!source->packfiles->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->packfiles->midx)
-				close_midx(source->packfiles->midx);
-			source->packfiles->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;
 		}
 	}
 
diff --git a/object-file.c b/object-file.c
index 3094140055..a3ff7f586c 100644
--- a/object-file.c
+++ b/object-file.c
@@ -215,8 +215,9 @@ 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->loose, oid, &p);
+	int fd = open_loose_object(files->loose, oid, &p);
 
 	if (fd < 0)
 		return NULL;
@@ -397,6 +398,7 @@ 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;
@@ -419,7 +421,7 @@ 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->loose, oid) ? 0 : -1;
+			ret = quick_has_loose(files->loose, oid) ? 0 : -1;
 			goto out;
 		}
 
@@ -540,6 +542,16 @@ int odb_source_loose_read_object_info(struct odb_source *source,
 				      enum object_info_flags flags)
 {
 	static struct strbuf buf = STRBUF_INIT;
+
+	/*
+	 * The second read shouldn't cause new loose objects to show up, unless
+	 * there was a race condition with a secondary process. We don't care
+	 * about this case though, so we simply skip reading loose objects a
+	 * second time.
+	 */
+	if (flags & OBJECT_INFO_SECOND_READ)
+		return -1;
+
 	odb_loose_path(source, &buf, oid);
 	return read_object_info_from_path(source, buf.buf, oid, oi, flags);
 }
@@ -1867,33 +1879,34 @@ 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->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->loose->subdir_seen))
+	    (size_t) subdir_nr >= bitsizeof(files->loose->subdir_seen))
 		BUG("subdir_nr out of range");
 
-	bitmap = &source->loose->subdir_seen[word_index];
+	bitmap = &files->loose->subdir_seen[word_index];
 	if (*bitmap & mask)
-		return source->loose->cache;
-	if (!source->loose->cache) {
-		ALLOC_ARRAY(source->loose->cache, 1);
-		oidtree_init(source->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->loose->cache);
+				    files->loose->cache);
 	*bitmap |= mask;
 	strbuf_release(&buf);
-	return source->loose->cache;
+	return files->loose->cache;
 }
 
 static void odb_source_loose_clear_cache(struct odb_source_loose *loose)
@@ -1906,7 +1919,8 @@ 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->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,
diff --git a/odb.c b/odb.c
index 776de5356c..84a31084d3 100644
--- a/odb.c
+++ b/odb.c
@@ -1,6 +1,5 @@
 #include "git-compat-util.h"
 #include "abspath.h"
-#include "chdir-notify.h"
 #include "commit-graph.h"
 #include "config.h"
 #include "dir.h"
@@ -132,10 +131,10 @@ out:
 	return usable;
 }
 
-static void parse_alternates(const char *string,
-			     int sep,
-			     const char *relative_base,
-			     struct strvec *out)
+void parse_alternates(const char *string,
+		      int sep,
+		      const char *relative_base,
+		      struct strvec *out)
 {
 	struct strbuf pathbuf = STRBUF_INIT;
 	struct strbuf buf = STRBUF_INIT;
@@ -199,41 +198,6 @@ static void parse_alternates(const char *string,
 	strbuf_release(&buf);
 }
 
-static void odb_source_read_alternates(struct odb_source *source,
-				       struct strvec *out)
-{
-	struct strbuf buf = STRBUF_INIT;
-	char *path;
-
-	path = xstrfmt("%s/info/alternates", source->path);
-	if (strbuf_read_file(&buf, path, 1024) < 0) {
-		warn_on_fopen_errors(path);
-		free(path);
-		return;
-	}
-	parse_alternates(buf.buf, '\n', source->path, out);
-
-	strbuf_release(&buf);
-	free(path);
-}
-
-
-static struct odb_source *odb_source_new(struct object_database *odb,
-					 const char *path,
-					 bool local)
-{
-	struct odb_source *source;
-
-	CALLOC_ARRAY(source, 1);
-	source->odb = odb;
-	source->local = local;
-	source->path = xstrdup(path);
-	source->loose = odb_source_loose_new(source);
-	source->packfiles = packfile_store_new(source);
-
-	return source;
-}
-
 static struct odb_source *odb_add_alternate_recursively(struct object_database *odb,
 							const char *source,
 							int depth)
@@ -272,58 +236,6 @@ static struct odb_source *odb_add_alternate_recursively(struct object_database *
 	return alternate;
 }
 
-static int odb_source_write_alternate(struct odb_source *source,
-				      const char *alternate)
-{
-	struct lock_file lock = LOCK_INIT;
-	char *path = xstrfmt("%s/%s", source->path, "info/alternates");
-	FILE *in, *out;
-	int found = 0;
-	int ret;
-
-	hold_lock_file_for_update(&lock, path, LOCK_DIE_ON_ERROR);
-	out = fdopen_lock_file(&lock, "w");
-	if (!out) {
-		ret = error_errno(_("unable to fdopen alternates lockfile"));
-		goto out;
-	}
-
-	in = fopen(path, "r");
-	if (in) {
-		struct strbuf line = STRBUF_INIT;
-
-		while (strbuf_getline(&line, in) != EOF) {
-			if (!strcmp(alternate, line.buf)) {
-				found = 1;
-				break;
-			}
-			fprintf_or_die(out, "%s\n", line.buf);
-		}
-
-		strbuf_release(&line);
-		fclose(in);
-	} else if (errno != ENOENT) {
-		ret = error_errno(_("unable to read alternates file"));
-		goto out;
-	}
-
-	if (found) {
-		rollback_lock_file(&lock);
-	} else {
-		fprintf_or_die(out, "%s\n", alternate);
-		if (commit_lock_file(&lock)) {
-			ret = error_errno(_("unable to move new alternates file into place"));
-			goto out;
-		}
-	}
-
-	ret = 0;
-
-out:
-	free(path);
-	return ret;
-}
-
 void odb_add_to_alternates_file(struct object_database *odb,
 				const char *dir)
 {
@@ -373,14 +285,6 @@ struct odb_source *odb_set_temporary_primary_source(struct object_database *odb,
 	return source->next;
 }
 
-static void odb_source_free(struct odb_source *source)
-{
-	free(source->path);
-	odb_source_loose_free(source->loose);
-	packfile_store_free(source->packfiles);
-	free(source);
-}
-
 void odb_restore_primary_source(struct object_database *odb,
 				struct odb_source *restore_source,
 				const char *old_path)
@@ -714,18 +618,19 @@ static int do_oid_object_info_extended(struct object_database *odb,
 	while (1) {
 		struct odb_source *source;
 
-		/* Most likely it's a loose object. */
-		for (source = odb->sources; source; source = source->next) {
-			if (!packfile_store_read_object_info(source->packfiles, real, oi, flags) ||
-			    !odb_source_loose_read_object_info(source, real, oi, flags))
+		for (source = odb->sources; source; source = source->next)
+			if (!odb_source_read_object_info(source, real, oi, flags))
 				return 0;
-		}
 
-		/* Not a loose object; someone else may have just packed it. */
+		/*
+		 * When the object hasn't been found we try a second read and
+		 * tell the sources so. This may cause them to invalidate
+		 * caches or reload on-disk state.
+		 */
 		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->packfiles, real, oi, flags))
+				if (!odb_source_read_object_info(source, real, oi,
+								 flags | OBJECT_INFO_SECOND_READ))
 					return 0;
 		}
 
@@ -984,16 +889,10 @@ int odb_freshen_object(struct object_database *odb,
 		       const struct object_id *oid)
 {
 	struct odb_source *source;
-
 	odb_prepare_alternates(odb);
-	for (source = odb->sources; source; source = source->next) {
-		if (packfile_store_freshen_object(source->packfiles, oid))
+	for (source = odb->sources; source; source = source->next)
+		if (odb_source_freshen_object(source, oid))
 			return 1;
-
-		if (odb_source_loose_freshen_object(source, oid))
-			return 1;
-	}
-
 	return 0;
 }
 
@@ -1010,15 +909,7 @@ int odb_for_each_object(struct object_database *odb,
 		if (flags & ODB_FOR_EACH_OBJECT_LOCAL_ONLY && !source->local)
 			continue;
 
-		if (!(flags & ODB_FOR_EACH_OBJECT_PROMISOR_ONLY)) {
-			ret = odb_source_loose_for_each_object(source, request,
-							       cb, cb_data, flags);
-			if (ret)
-				return ret;
-		}
-
-		ret = packfile_store_for_each_object(source->packfiles, request,
-						     cb, cb_data, flags);
+		ret = odb_source_for_each_object(source, request, cb, cb_data, flags);
 		if (ret)
 			return ret;
 	}
@@ -1044,47 +935,15 @@ int odb_write_object_ext(struct object_database *odb,
 			 struct object_id *compat_oid,
 			 unsigned flags)
 {
-	return odb_source_loose_write_object(odb->sources, buf, len, type,
-					     oid, compat_oid, flags);
+	return odb_source_write_object(odb->sources, buf, len, type,
+				       oid, compat_oid, flags);
 }
 
 int odb_write_object_stream(struct object_database *odb,
 			    struct odb_write_stream *stream, size_t len,
 			    struct object_id *oid)
 {
-	return odb_source_loose_write_stream(odb->sources, stream, len, oid);
-}
-
-static void odb_update_commondir(const char *name UNUSED,
-				 const char *old_cwd,
-				 const char *new_cwd,
-				 void *cb_data)
-{
-	struct object_database *odb = cb_data;
-	struct tmp_objdir *tmp_objdir;
-	struct odb_source *source;
-
-	tmp_objdir = tmp_objdir_unapply_primary_odb();
-
-	/*
-	 * In theory, we only have to do this for the primary object source, as
-	 * alternates' paths are always resolved to an absolute path.
-	 */
-	for (source = odb->sources; source; source = source->next) {
-		char *path;
-
-		if (is_absolute_path(source->path))
-			continue;
-
-		path = reparent_relative_path(old_cwd, new_cwd,
-					      source->path);
-
-		free(source->path);
-		source->path = path;
-	}
-
-	if (tmp_objdir)
-		tmp_objdir_reapply_primary_odb(tmp_objdir, old_cwd, new_cwd);
+	return odb_source_write_object_stream(odb->sources, stream, len, oid);
 }
 
 struct object_database *odb_new(struct repository *repo,
@@ -1107,8 +966,6 @@ struct object_database *odb_new(struct repository *repo,
 
 	free(to_free);
 
-	chdir_notify_register(NULL, odb_update_commondir, o);
-
 	return o;
 }
 
@@ -1116,7 +973,7 @@ void odb_close(struct object_database *o)
 {
 	struct odb_source *source;
 	for (source = o->sources; source; source = source->next)
-		packfile_store_close(source->packfiles);
+		odb_source_close(source);
 	close_commit_graph(o);
 }
 
@@ -1152,8 +1009,6 @@ void odb_free(struct object_database *o)
 
 	string_list_clear(&o->submodule_source_paths, 0);
 
-	chdir_notify_unregister(NULL, odb_update_commondir, o);
-
 	free(o);
 }
 
@@ -1172,10 +1027,8 @@ void odb_reprepare(struct object_database *o)
 	o->loaded_alternates = 0;
 	odb_prepare_alternates(o);
 
-	for (source = o->sources; source; source = source->next) {
-		odb_source_loose_reprepare(source);
-		packfile_store_reprepare(source->packfiles);
-	}
+	for (source = o->sources; source; source = source->next)
+		odb_source_reprepare(source);
 
 	o->approximate_object_count_valid = 0;
 
diff --git a/odb.h b/odb.h
index 68b8ec2289..86e0365c24 100644
--- a/odb.h
+++ b/odb.h
@@ -3,6 +3,7 @@
 
 #include "hashmap.h"
 #include "object.h"
+#include "odb/source.h"
 #include "oidset.h"
 #include "oidmap.h"
 #include "string-list.h"
@@ -30,50 +31,6 @@ extern int fetch_if_missing;
  */
 char *compute_alternate_path(const char *path, struct strbuf *err);
 
-/*
- * 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
- * on-disk format. An object database can have multiple sources:
- *
- *   - The primary source, which is typically located in "$GIT_DIR/objects".
- *     This is where new objects are usually written to.
- *
- *   - Alternate sources, which are configured via "objects/info/alternates" or
- *     via the GIT_ALTERNATE_OBJECT_DIRECTORIES environment variable. These
- *     alternate sources are only used to read objects.
- */
-struct odb_source {
-	struct odb_source *next;
-
-	/* Object database that owns this object source. */
-	struct object_database *odb;
-
-	/* Private state for loose objects. */
-	struct odb_source_loose *loose;
-
-	/* Should only be accessed directly by packfile.c and midx.c. */
-	struct packfile_store *packfiles;
-
-	/*
-	 * Figure out whether this is the local source of the owning
-	 * repository, which would typically be its ".git/objects" directory.
-	 * This local object directory is usually where objects would be
-	 * written to.
-	 */
-	bool local;
-
-	/*
-	 * This object store is ephemeral, so there is no need to fsync.
-	 */
-	int will_destroy;
-
-	/*
-	 * Path to the source. If this is a relative path, it is relative to
-	 * the current working directory.
-	 */
-	char *path;
-};
-
 struct packed_git;
 struct packfile_store;
 struct cached_object_entry;
@@ -382,30 +339,6 @@ struct object_info {
  */
 #define OBJECT_INFO_INIT { 0 }
 
-/* Flags that can be passed to `odb_read_object_info_extended()`. */
-enum object_info_flags {
-	/* Invoke lookup_replace_object() on the given hash. */
-	OBJECT_INFO_LOOKUP_REPLACE = (1 << 0),
-
-	/* Do not reprepare object sources when the first lookup has failed. */
-	OBJECT_INFO_QUICK = (1 << 1),
-
-	/*
-	 * Do not attempt to fetch the object if missing (even if fetch_is_missing is
-	 * nonzero).
-	 */
-	OBJECT_INFO_SKIP_FETCH_OBJECT = (1 << 2),
-
-	/* Die if object corruption (not just an object being missing) was detected. */
-	OBJECT_INFO_DIE_IF_CORRUPT = (1 << 3),
-
-	/*
-	 * This is meant for bulk prefetching of missing blobs in a partial
-	 * clone. Implies OBJECT_INFO_SKIP_FETCH_OBJECT and OBJECT_INFO_QUICK.
-	 */
-	OBJECT_INFO_FOR_PREFETCH = (OBJECT_INFO_SKIP_FETCH_OBJECT | OBJECT_INFO_QUICK),
-};
-
 /*
  * Read object info from the object database and populate the `object_info`
  * structure. Returns 0 on success, a negative error code otherwise.
@@ -500,18 +433,6 @@ enum odb_for_each_object_flags {
 };
 
 /*
- * A callback function that can be used to iterate through objects. If given,
- * the optional `oi` parameter will be populated the same as if you would call
- * `odb_read_object_info()`.
- *
- * Returning a non-zero error code will cause iteration to abort. The error
- * code will be propagated.
- */
-typedef int (*odb_for_each_object_cb)(const struct object_id *oid,
-				      struct object_info *oi,
-				      void *cb_data);
-
-/*
  * Iterate through all objects contained in the object database. Note that
  * objects may be iterated over multiple times in case they are either stored
  * in different backends or in case they are stored in multiple sources.
@@ -579,4 +500,9 @@ int odb_write_object_stream(struct object_database *odb,
 			    struct odb_write_stream *stream, size_t len,
 			    struct object_id *oid);
 
+void parse_alternates(const char *string,
+		      int sep,
+		      const char *relative_base,
+		      struct strvec *out);
+
 #endif /* ODB_H */
diff --git a/odb/source-files.c b/odb/source-files.c
new file mode 100644
index 0000000000..14cb9adeca
--- /dev/null
+++ b/odb/source-files.c
@@ -0,0 +1,239 @@
+#include "git-compat-util.h"
+#include "abspath.h"
+#include "chdir-notify.h"
+#include "gettext.h"
+#include "lockfile.h"
+#include "object-file.h"
+#include "odb.h"
+#include "odb/source.h"
+#include "odb/source-files.h"
+#include "packfile.h"
+#include "strbuf.h"
+#include "write-or-die.h"
+
+static void odb_source_files_reparent(const char *name UNUSED,
+				      const char *old_cwd,
+				      const char *new_cwd,
+				      void *cb_data)
+{
+	struct odb_source_files *files = cb_data;
+	char *path = reparent_relative_path(old_cwd, new_cwd,
+					    files->base.path);
+	free(files->base.path);
+	files->base.path = path;
+}
+
+static void odb_source_files_free(struct odb_source *source)
+{
+	struct odb_source_files *files = odb_source_files_downcast(source);
+	chdir_notify_unregister(NULL, odb_source_files_reparent, files);
+	odb_source_loose_free(files->loose);
+	packfile_store_free(files->packed);
+	odb_source_release(&files->base);
+	free(files);
+}
+
+static void odb_source_files_close(struct odb_source *source)
+{
+	struct odb_source_files *files = odb_source_files_downcast(source);
+	packfile_store_close(files->packed);
+}
+
+static void odb_source_files_reprepare(struct odb_source *source)
+{
+	struct odb_source_files *files = odb_source_files_downcast(source);
+	odb_source_loose_reprepare(&files->base);
+	packfile_store_reprepare(files->packed);
+}
+
+static int odb_source_files_read_object_info(struct odb_source *source,
+					     const struct object_id *oid,
+					     struct object_info *oi,
+					     enum object_info_flags flags)
+{
+	struct odb_source_files *files = odb_source_files_downcast(source);
+
+	if (!packfile_store_read_object_info(files->packed, oid, oi, flags) ||
+	    !odb_source_loose_read_object_info(source, oid, oi, flags))
+		return 0;
+
+	return -1;
+}
+
+static int odb_source_files_read_object_stream(struct odb_read_stream **out,
+					       struct odb_source *source,
+					       const struct object_id *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;
+	return -1;
+}
+
+static int odb_source_files_for_each_object(struct odb_source *source,
+					    const struct object_info *request,
+					    odb_for_each_object_cb cb,
+					    void *cb_data,
+					    unsigned flags)
+{
+	struct odb_source_files *files = odb_source_files_downcast(source);
+	int ret;
+
+	if (!(flags & ODB_FOR_EACH_OBJECT_PROMISOR_ONLY)) {
+		ret = odb_source_loose_for_each_object(source, request, cb, cb_data, flags);
+		if (ret)
+			return ret;
+	}
+
+	ret = packfile_store_for_each_object(files->packed, request, cb, cb_data, flags);
+	if (ret)
+		return ret;
+
+	return 0;
+}
+
+static int odb_source_files_freshen_object(struct odb_source *source,
+					   const struct object_id *oid)
+{
+	struct odb_source_files *files = odb_source_files_downcast(source);
+	if (packfile_store_freshen_object(files->packed, oid) ||
+	    odb_source_loose_freshen_object(source, oid))
+		return 1;
+	return 0;
+}
+
+static int odb_source_files_write_object(struct odb_source *source,
+					 const void *buf, unsigned long len,
+					 enum object_type type,
+					 struct object_id *oid,
+					 struct object_id *compat_oid,
+					 unsigned flags)
+{
+	return odb_source_loose_write_object(source, buf, len, type,
+					     oid, compat_oid, flags);
+}
+
+static int odb_source_files_write_object_stream(struct odb_source *source,
+						struct odb_write_stream *stream,
+						size_t len,
+						struct object_id *oid)
+{
+	return odb_source_loose_write_stream(source, stream, len, oid);
+}
+
+static int odb_source_files_begin_transaction(struct odb_source *source,
+					      struct odb_transaction **out)
+{
+	struct odb_transaction *tx = odb_transaction_files_begin(source);
+	if (!tx)
+		return -1;
+	*out = tx;
+	return 0;
+}
+
+static int odb_source_files_read_alternates(struct odb_source *source,
+					    struct strvec *out)
+{
+	struct strbuf buf = STRBUF_INIT;
+	char *path;
+
+	path = xstrfmt("%s/info/alternates", source->path);
+	if (strbuf_read_file(&buf, path, 1024) < 0) {
+		warn_on_fopen_errors(path);
+		free(path);
+		return 0;
+	}
+	parse_alternates(buf.buf, '\n', source->path, out);
+
+	strbuf_release(&buf);
+	free(path);
+	return 0;
+}
+
+static int odb_source_files_write_alternate(struct odb_source *source,
+					    const char *alternate)
+{
+	struct lock_file lock = LOCK_INIT;
+	char *path = xstrfmt("%s/%s", source->path, "info/alternates");
+	FILE *in, *out;
+	int found = 0;
+	int ret;
+
+	hold_lock_file_for_update(&lock, path, LOCK_DIE_ON_ERROR);
+	out = fdopen_lock_file(&lock, "w");
+	if (!out) {
+		ret = error_errno(_("unable to fdopen alternates lockfile"));
+		goto out;
+	}
+
+	in = fopen(path, "r");
+	if (in) {
+		struct strbuf line = STRBUF_INIT;
+
+		while (strbuf_getline(&line, in) != EOF) {
+			if (!strcmp(alternate, line.buf)) {
+				found = 1;
+				break;
+			}
+			fprintf_or_die(out, "%s\n", line.buf);
+		}
+
+		strbuf_release(&line);
+		fclose(in);
+	} else if (errno != ENOENT) {
+		ret = error_errno(_("unable to read alternates file"));
+		goto out;
+	}
+
+	if (found) {
+		rollback_lock_file(&lock);
+	} else {
+		fprintf_or_die(out, "%s\n", alternate);
+		if (commit_lock_file(&lock)) {
+			ret = error_errno(_("unable to move new alternates file into place"));
+			goto out;
+		}
+	}
+
+	ret = 0;
+
+out:
+	free(path);
+	return ret;
+}
+
+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);
+	odb_source_init(&files->base, odb, ODB_SOURCE_FILES, path, local);
+	files->loose = odb_source_loose_new(&files->base);
+	files->packed = packfile_store_new(&files->base);
+
+	files->base.free = odb_source_files_free;
+	files->base.close = odb_source_files_close;
+	files->base.reprepare = odb_source_files_reprepare;
+	files->base.read_object_info = odb_source_files_read_object_info;
+	files->base.read_object_stream = odb_source_files_read_object_stream;
+	files->base.for_each_object = odb_source_files_for_each_object;
+	files->base.freshen_object = odb_source_files_freshen_object;
+	files->base.write_object = odb_source_files_write_object;
+	files->base.write_object_stream = odb_source_files_write_object_stream;
+	files->base.begin_transaction = odb_source_files_begin_transaction;
+	files->base.read_alternates = odb_source_files_read_alternates;
+	files->base.write_alternate = odb_source_files_write_alternate;
+
+	/*
+	 * Ideally, we would only ever store absolute paths in the source. This
+	 * is not (yet) possible though because we access and assume relative
+	 * paths in the primary ODB source in some user-facing functionality.
+	 */
+	if (!is_absolute_path(path))
+		chdir_notify_register(NULL, odb_source_files_reparent, files);
+
+	return files;
+}
diff --git a/odb/source-files.h b/odb/source-files.h
new file mode 100644
index 0000000000..23a3b4e04b
--- /dev/null
+++ b/odb/source-files.h
@@ -0,0 +1,35 @@
+#ifndef ODB_SOURCE_FILES_H
+#define ODB_SOURCE_FILES_H
+
+#include "odb/source.h"
+
+struct odb_source_loose;
+struct packfile_store;
+
+/*
+ * The files object database source uses a combination of loose objects and
+ * packfiles. It is the default backend used by Git to store objects.
+ */
+struct odb_source_files {
+	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 object_database *odb,
+					      const char *path,
+					      bool local);
+
+/*
+ * 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)
+{
+	if (source->type != ODB_SOURCE_FILES)
+		BUG("trying to downcast source of type '%d' to files", source->type);
+	return container_of(source, struct odb_source_files, base);
+}
+
+#endif
diff --git a/odb/source.c b/odb/source.c
new file mode 100644
index 0000000000..7993dcbd65
--- /dev/null
+++ b/odb/source.c
@@ -0,0 +1,38 @@
+#include "git-compat-util.h"
+#include "object-file.h"
+#include "odb/source-files.h"
+#include "odb/source.h"
+#include "packfile.h"
+
+struct odb_source *odb_source_new(struct object_database *odb,
+				  const char *path,
+				  bool local)
+{
+	return &odb_source_files_new(odb, path, local)->base;
+}
+
+void odb_source_init(struct odb_source *source,
+		     struct object_database *odb,
+		     enum odb_source_type type,
+		     const char *path,
+		     bool local)
+{
+	source->odb = odb;
+	source->type = type;
+	source->local = local;
+	source->path = xstrdup(path);
+}
+
+void odb_source_free(struct odb_source *source)
+{
+	if (!source)
+		return;
+	source->free(source);
+}
+
+void odb_source_release(struct odb_source *source)
+{
+	if (!source)
+		return;
+	free(source->path);
+}
diff --git a/odb/source.h b/odb/source.h
new file mode 100644
index 0000000000..caac558149
--- /dev/null
+++ b/odb/source.h
@@ -0,0 +1,468 @@
+#ifndef ODB_SOURCE_H
+#define ODB_SOURCE_H
+
+#include "object.h"
+
+enum odb_source_type {
+	/*
+	 * The "unknown" type, which should never be in use. This type mostly
+	 * exists to catch cases where the type field remains zeroed out.
+	 */
+	ODB_SOURCE_UNKNOWN,
+
+	/* The "files" backend that uses loose objects and packfiles. */
+	ODB_SOURCE_FILES,
+};
+
+/* Flags that can be passed to `odb_read_object_info_extended()`. */
+enum object_info_flags {
+	/* Invoke lookup_replace_object() on the given hash. */
+	OBJECT_INFO_LOOKUP_REPLACE = (1 << 0),
+
+	/* Do not reprepare object sources when the first lookup has failed. */
+	OBJECT_INFO_QUICK = (1 << 1),
+
+	/*
+	 * Do not attempt to fetch the object if missing (even if fetch_is_missing is
+	 * nonzero).
+	 */
+	OBJECT_INFO_SKIP_FETCH_OBJECT = (1 << 2),
+
+	/* Die if object corruption (not just an object being missing) was detected. */
+	OBJECT_INFO_DIE_IF_CORRUPT = (1 << 3),
+
+	/*
+	 * We have already tried reading the object, but it couldn't be found
+	 * via any of the attached sources, and are now doing a second read.
+	 * This second read asks the individual sources to also evaluate
+	 * whether any on-disk state may have changed that may have caused the
+	 * object to appear.
+	 *
+	 * This flag is for internal use, only. The second read only occurs
+	 * when `OBJECT_INFO_QUICK` was not passed.
+	 */
+	OBJECT_INFO_SECOND_READ = (1 << 4),
+
+	/*
+	 * This is meant for bulk prefetching of missing blobs in a partial
+	 * clone. Implies OBJECT_INFO_SKIP_FETCH_OBJECT and OBJECT_INFO_QUICK.
+	 */
+	OBJECT_INFO_FOR_PREFETCH = (OBJECT_INFO_SKIP_FETCH_OBJECT | OBJECT_INFO_QUICK),
+};
+
+struct object_id;
+struct object_info;
+struct odb_read_stream;
+struct odb_transaction;
+struct odb_write_stream;
+struct strvec;
+
+/*
+ * A callback function that can be used to iterate through objects. If given,
+ * the optional `oi` parameter will be populated the same as if you would call
+ * `odb_read_object_info()`.
+ *
+ * Returning a non-zero error code will cause iteration to abort. The error
+ * code will be propagated.
+ */
+typedef int (*odb_for_each_object_cb)(const struct object_id *oid,
+				      struct object_info *oi,
+				      void *cb_data);
+
+/*
+ * 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
+ * on-disk format. An object database can have multiple sources:
+ *
+ *   - The primary source, which is typically located in "$GIT_DIR/objects".
+ *     This is where new objects are usually written to.
+ *
+ *   - Alternate sources, which are configured via "objects/info/alternates" or
+ *     via the GIT_ALTERNATE_OBJECT_DIRECTORIES environment variable. These
+ *     alternate sources are only used to read objects.
+ */
+struct odb_source {
+	struct odb_source *next;
+
+	/* Object database that owns this object source. */
+	struct object_database *odb;
+
+	/* The type used by this source. */
+	enum odb_source_type type;
+
+	/*
+	 * Figure out whether this is the local source of the owning
+	 * repository, which would typically be its ".git/objects" directory.
+	 * This local object directory is usually where objects would be
+	 * written to.
+	 */
+	bool local;
+
+	/*
+	 * This object store is ephemeral, so there is no need to fsync.
+	 */
+	int will_destroy;
+
+	/*
+	 * Path to the source. If this is a relative path, it is relative to
+	 * the current working directory.
+	 */
+	char *path;
+
+	/*
+	 * This callback is expected to free the underlying object database source and
+	 * all associated resources. The function will never be called with a NULL pointer.
+	 */
+	void (*free)(struct odb_source *source);
+
+	/*
+	 * This callback is expected to close any open resources, like for
+	 * example file descriptors or connections. The source is expected to
+	 * still be usable after it has been closed. Closed resources may need
+	 * to be reopened in that case.
+	 */
+	void (*close)(struct odb_source *source);
+
+	/*
+	 * This callback is expected to clear underlying caches of the object
+	 * database source. The function is called when the repository has for
+	 * example just been repacked so that new objects will become visible.
+	 */
+	void (*reprepare)(struct odb_source *source);
+
+	/*
+	 * This callback is expected to read object information from the object
+	 * database source. The object info will be partially populated with
+	 * pointers for each bit of information that was requested by the
+	 * caller.
+	 *
+	 * The flags field is a combination of `OBJECT_INFO` flags. Only the
+	 * following fields need to be handled by the backend:
+	 *
+	 *   - `OBJECT_INFO_QUICK` indicates it is fine to use caches without
+	 *     re-verifying the data.
+	 *
+	 *   - `OBJECT_INFO_SECOND_READ` indicates that the initial object
+	 *     lookup has failed and that the object sources should check
+	 *     whether any of its on-disk state has changed that may have
+	 *     caused the object to appear. Sources are free to ignore the
+	 *     second read in case they know that the first read would have
+	 *     already surfaced the object without reloading any on-disk state.
+	 *
+	 * The callback is expected to return a negative error code in case
+	 * reading the object has failed, 0 otherwise.
+	 */
+	int (*read_object_info)(struct odb_source *source,
+				const struct object_id *oid,
+				struct object_info *oi,
+				enum object_info_flags flags);
+
+	/*
+	 * This callback is expected to create a new read stream that can be
+	 * used to stream the object identified by the given ID.
+	 *
+	 * The callback is expected to return a negative error code in case
+	 * creating the object stream has failed, 0 otherwise.
+	 */
+	int (*read_object_stream)(struct odb_read_stream **out,
+				  struct odb_source *source,
+				  const struct object_id *oid);
+
+	/*
+	 * This callback is expected to iterate over all objects stored in this
+	 * source and invoke the callback function for each of them. It is
+	 * valid to yield the same object multiple time. A non-zero exit code
+	 * from the object callback shall abort iteration.
+	 *
+	 * The optional `request` structure should serve as a template for
+	 * looking up object info for every individual iterated object. It
+	 * should not be modified directly and should instead be copied into a
+	 * separate `struct object_info` that gets passed to the callback. If
+	 * the caller passes a `NULL` pointer then the object itself shall not
+	 * be read.
+	 *
+	 * The callback is expected to return a negative error code in case the
+	 * iteration has failed to read all objects, 0 otherwise. When the
+	 * callback function returns a non-zero error code then that error code
+	 * should be returned.
+	 */
+	int (*for_each_object)(struct odb_source *source,
+			       const struct object_info *request,
+			       odb_for_each_object_cb cb,
+			       void *cb_data,
+			       unsigned flags);
+
+	/*
+	 * This callback is expected to freshen the given object so that its
+	 * last access time is set to the current time. This is used to ensure
+	 * that objects that are recent will not get garbage collected even if
+	 * they were unreachable.
+	 *
+	 * Returns 0 in case the object does not exist, 1 in case the object
+	 * has been freshened.
+	 */
+	int (*freshen_object)(struct odb_source *source,
+			      const struct object_id *oid);
+
+	/*
+	 * This callback is expected to persist the given object into the
+	 * object source. In case the object already exists it shall be
+	 * freshened.
+	 *
+	 * The flags field is a combination of `WRITE_OBJECT` flags.
+	 *
+	 * The resulting object ID (and optionally the compatibility object ID)
+	 * shall be written into the out pointers. The callback is expected to
+	 * return 0 on success, a negative error code otherwise.
+	 */
+	int (*write_object)(struct odb_source *source,
+			    const void *buf, unsigned long len,
+			    enum object_type type,
+			    struct object_id *oid,
+			    struct object_id *compat_oid,
+			    unsigned flags);
+
+	/*
+	 * This callback is expected to persist the given object stream into
+	 * the object source.
+	 *
+	 * The resulting object ID shall be written into the out pointer. The
+	 * callback is expected to return 0 on success, a negative error code
+	 * otherwise.
+	 */
+	int (*write_object_stream)(struct odb_source *source,
+				   struct odb_write_stream *stream, size_t len,
+				   struct object_id *oid);
+
+	/*
+	 * This callback is expected to create a new transaction that can be
+	 * used to write objects to. The objects shall only be persisted into
+	 * the object database when the transcation's commit function is
+	 * called. Otherwise, the objects shall be discarded.
+	 *
+	 * Returns 0 on success, in which case the `*out` pointer will have
+	 * been populated with the object database transaction. Returns a
+	 * negative error code otherwise.
+	 */
+	int (*begin_transaction)(struct odb_source *source,
+				 struct odb_transaction **out);
+
+	/*
+	 * This callback is expected to read the list of alternate object
+	 * database sources connected to it and write them into the `strvec`.
+	 *
+	 * The result is expected to be paths to the alternates. All paths must
+	 * be resolved to absolute paths.
+	 *
+	 * The callback is expected to return 0 on success, a negative error
+	 * code otherwise.
+	 */
+	int (*read_alternates)(struct odb_source *source,
+			       struct strvec *out);
+
+	/*
+	 * This callback is expected to persist the singular alternate passed
+	 * to it into its list of alternates. Any pre-existing alternates are
+	 * expected to remain active. Subsequent calls to `read_alternates` are
+	 * thus expected to yield the pre-existing list of alternates plus the
+	 * newly added alternate appended to its end.
+	 *
+	 * The callback is expected to return 0 on success, a negative error
+	 * code otherwise.
+	 */
+	int (*write_alternate)(struct odb_source *source,
+			       const char *alternate);
+};
+
+/*
+ * Allocate and initialize a new 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.
+ */
+struct odb_source *odb_source_new(struct object_database *odb,
+				  const char *path,
+				  bool local);
+
+/*
+ * 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,
+		     enum odb_source_type type,
+		     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);
+
+/*
+ * Close the object database source without releasing he underlying data. The
+ * source can still be used going forward, but it first needs to be reopened.
+ * This can be useful to reduce resource usage.
+ */
+static inline void odb_source_close(struct odb_source *source)
+{
+	source->close(source);
+}
+
+/*
+ * Reprepare the object database source and clear any caches. Depending on the
+ * backend used this may have the effect that concurrently-written objects
+ * become visible.
+ */
+static inline void odb_source_reprepare(struct odb_source *source)
+{
+	source->reprepare(source);
+}
+
+/*
+ * Read an object from the object database source identified by its object ID.
+ * Returns 0 on success, a negative error code otherwise.
+ */
+static inline int odb_source_read_object_info(struct odb_source *source,
+					      const struct object_id *oid,
+					      struct object_info *oi,
+					      enum object_info_flags flags)
+{
+	return source->read_object_info(source, oid, oi, flags);
+}
+
+/*
+ * Create a new read stream for the given object ID. Returns 0 on success, a
+ * negative error code otherwise.
+ */
+static inline int odb_source_read_object_stream(struct odb_read_stream **out,
+						struct odb_source *source,
+						const struct object_id *oid)
+{
+	return source->read_object_stream(out, source, oid);
+}
+
+/*
+ * Iterate through all objects contained in the given source and invoke the
+ * callback function for each of them. Returning a non-zero code from the
+ * callback function aborts iteration. There is no guarantee that objects
+ * are only iterated over once.
+ *
+ * The optional `request` structure serves as a template for retrieving the
+ * object info for each indvidual iterated object and will be populated as if
+ * `odb_source_read_object_info()` was called on the object. It will not be
+ * modified, the callback will instead be invoked with a separate `struct
+ * object_info` for every object. Object info will not be read when passing a
+ * `NULL` pointer.
+ *
+ * The flags is a bitfield of `ODB_FOR_EACH_OBJECT_*` flags. Not all flags may
+ * apply to a specific backend, so whether or not they are honored is defined
+ * by the implementation.
+ *
+ * Returns 0 when all objects have been iterated over, a negative error code in
+ * case iteration has failed, or a non-zero value returned from the callback.
+ */
+static inline int odb_source_for_each_object(struct odb_source *source,
+					     const struct object_info *request,
+					     odb_for_each_object_cb cb,
+					     void *cb_data,
+					     unsigned flags)
+{
+	return source->for_each_object(source, request, cb, cb_data, flags);
+}
+
+/*
+ * Freshen an object in the object database by updating its timestamp.
+ * Returns 1 in case the object has been freshened, 0 in case the object does
+ * not exist.
+ */
+static inline int odb_source_freshen_object(struct odb_source *source,
+					    const struct object_id *oid)
+{
+	return source->freshen_object(source, oid);
+}
+
+/*
+ * Write an object into the object database source. Returns 0 on success, a
+ * negative error code otherwise. Populates the given out pointers for the
+ * object ID and the compatibility object ID, if non-NULL.
+ */
+static inline int odb_source_write_object(struct odb_source *source,
+					  const void *buf, unsigned long len,
+					  enum object_type type,
+					  struct object_id *oid,
+					  struct object_id *compat_oid,
+					  unsigned flags)
+{
+	return source->write_object(source, buf, len, type, oid,
+				    compat_oid, flags);
+}
+
+/*
+ * Write an object into the object database source via a stream. The overall
+ * length of the object must be known in advance.
+ *
+ * Return 0 on success, a negative error code otherwise. Populates the given
+ * out pointer for the object ID.
+ */
+static inline int odb_source_write_object_stream(struct odb_source *source,
+						 struct odb_write_stream *stream,
+						 size_t len,
+						 struct object_id *oid)
+{
+	return source->write_object_stream(source, stream, len, oid);
+}
+
+/*
+ * Read the list of alternative object database sources from the given backend
+ * and populate the `strvec` with them. The listing is not recursive -- that
+ * is, if any of the yielded alternate sources has alternates itself, those
+ * will not be yielded as part of this function call.
+ *
+ * Return 0 on success, a negative error code otherwise.
+ */
+static inline int odb_source_read_alternates(struct odb_source *source,
+					     struct strvec *out)
+{
+	return source->read_alternates(source, out);
+}
+
+/*
+ * Write and persist a new alternate object database source for the given
+ * source. Any preexisting alternates are expected to stay valid, and the new
+ * alternate shall be appended to the end of the list.
+ *
+ * Returns 0 on success, a negative error code otherwise.
+ */
+static inline int odb_source_write_alternate(struct odb_source *source,
+					      const char *alternate)
+{
+	return source->write_alternate(source, alternate);
+}
+
+/*
+ * Create a new transaction that can be used to write objects into a temporary
+ * staging area. The objects will only be persisted when the transaction is
+ * committed.
+ *
+ * Returns 0 on success, a negative error code otherwise.
+ */
+static inline int odb_source_begin_transaction(struct odb_source *source,
+					       struct odb_transaction **out)
+{
+	return source->begin_transaction(source, out);
+}
+
+#endif
diff --git a/odb/streaming.c b/odb/streaming.c
index 4a4474f891..a4355cd245 100644
--- a/odb/streaming.c
+++ b/odb/streaming.c
@@ -6,11 +6,9 @@
 #include "convert.h"
 #include "environment.h"
 #include "repository.h"
-#include "object-file.h"
 #include "odb.h"
 #include "odb/streaming.h"
 #include "replace-object.h"
-#include "packfile.h"
 
 #define FILTER_BUFFER (1024*16)
 
@@ -186,11 +184,9 @@ static int istream_source(struct odb_read_stream **out,
 	struct odb_source *source;
 
 	odb_prepare_alternates(odb);
-	for (source = odb->sources; source; source = source->next) {
-		if (!packfile_store_read_object_stream(out, source->packfiles, oid) ||
-		    !odb_source_loose_read_object_stream(out, source, oid))
+	for (source = odb->sources; source; source = source->next)
+		if (!odb_source_read_object_stream(out, source, oid))
 			return 0;
-	}
 
 	return open_istream_incore(out, odb, oid);
 }
diff --git a/packfile.c b/packfile.c
index 4a6d4a80ea..215a23e42b 100644
--- a/packfile.c
+++ b/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->packfiles->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->packfiles->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->packfiles->midx &&
-	      midx_contains_pack(data->source->packfiles->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->packfiles,
+		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->packfiles->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;
 	}
@@ -2175,11 +2181,19 @@ int packfile_store_freshen_object(struct packfile_store *store,
 int packfile_store_read_object_info(struct packfile_store *store,
 				    const struct object_id *oid,
 				    struct object_info *oi,
-				    enum object_info_flags flags UNUSED)
+				    enum object_info_flags flags)
 {
 	struct pack_entry e;
 	int ret;
 
+	/*
+	 * In case the first read didn't surface the object, we have to reload
+	 * packfiles. This may cause us to discover new packfiles that have
+	 * been added since the last time we have prepared the packfile store.
+	 */
+	if (flags & OBJECT_INFO_SECOND_READ)
+		packfile_store_reprepare(store);
+
 	if (!find_pack_entry(store, oid, &e))
 		return 1;
 
@@ -2254,7 +2268,8 @@ 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->packfiles, 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 +2284,10 @@ 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->packfiles, flags);
+		cache = packfile_store_get_kept_pack_cache(files->packed, flags);
 
 		for (; *cache; cache++) {
 			struct packed_git *p = *cache;
diff --git a/packfile.h b/packfile.h
index 85ce44f8ee..8b04a258a7 100644
--- a/packfile.h
+++ b/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->packfiles);
+		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->packfiles);
+		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;
diff --git a/tmp-objdir.c b/tmp-objdir.c
index 9f5a1788cd..e436eed07e 100644
--- a/tmp-objdir.c
+++ b/tmp-objdir.c
@@ -36,6 +36,21 @@ static void tmp_objdir_free(struct tmp_objdir *t)
 	free(t);
 }
 
+static void tmp_objdir_reparent(const char *name UNUSED,
+				const char *old_cwd,
+				const char *new_cwd,
+				void *cb_data)
+{
+	struct tmp_objdir *t = cb_data;
+	char *path;
+
+	path = reparent_relative_path(old_cwd, new_cwd,
+				      t->path.buf);
+	strbuf_reset(&t->path);
+	strbuf_addstr(&t->path, path);
+	free(path);
+}
+
 int tmp_objdir_destroy(struct tmp_objdir *t)
 {
 	int err;
@@ -51,6 +66,7 @@ int tmp_objdir_destroy(struct tmp_objdir *t)
 
 	err = remove_dir_recursively(&t->path, 0);
 
+	chdir_notify_unregister(NULL, tmp_objdir_reparent, t);
 	tmp_objdir_free(t);
 
 	return err;
@@ -137,6 +153,9 @@ struct tmp_objdir *tmp_objdir_create(struct repository *r,
 	strbuf_addf(&t->path, "%s/tmp_objdir-%s-XXXXXX",
 		    repo_get_object_directory(r), prefix);
 
+	if (!is_absolute_path(t->path.buf))
+		chdir_notify_register(NULL, tmp_objdir_reparent, t);
+
 	if (!mkdtemp(t->path.buf)) {
 		/* free, not destroy, as we never touched the filesystem */
 		tmp_objdir_free(t);
@@ -315,26 +334,3 @@ void tmp_objdir_replace_primary_odb(struct tmp_objdir *t, int will_destroy)
 							  t->path.buf, will_destroy);
 	t->will_destroy = will_destroy;
 }
-
-struct tmp_objdir *tmp_objdir_unapply_primary_odb(void)
-{
-	if (!the_tmp_objdir || !the_tmp_objdir->prev_source)
-		return NULL;
-
-	odb_restore_primary_source(the_tmp_objdir->repo->objects,
-				   the_tmp_objdir->prev_source, the_tmp_objdir->path.buf);
-	the_tmp_objdir->prev_source = NULL;
-	return the_tmp_objdir;
-}
-
-void tmp_objdir_reapply_primary_odb(struct tmp_objdir *t, const char *old_cwd,
-		const char *new_cwd)
-{
-	char *path;
-
-	path = reparent_relative_path(old_cwd, new_cwd, t->path.buf);
-	strbuf_reset(&t->path);
-	strbuf_addstr(&t->path, path);
-	free(path);
-	tmp_objdir_replace_primary_odb(t, t->will_destroy);
-}
diff --git a/tmp-objdir.h b/tmp-objdir.h
index fceda14979..ccf800faa7 100644
--- a/tmp-objdir.h
+++ b/tmp-objdir.h
@@ -68,19 +68,4 @@ void tmp_objdir_add_as_alternate(const struct tmp_objdir *);
  */
 void tmp_objdir_replace_primary_odb(struct tmp_objdir *, int will_destroy);
 
-/*
- * If the primary object database was replaced by a temporary object directory,
- * restore it to its original value while keeping the directory contents around.
- * Returns NULL if the primary object database was not replaced.
- */
-struct tmp_objdir *tmp_objdir_unapply_primary_odb(void);
-
-/*
- * Reapplies the former primary temporary object database, after potentially
- * changing its relative path.
- */
-void tmp_objdir_reapply_primary_odb(struct tmp_objdir *, const char *old_cwd,
-		const char *new_cwd);
-
-
 #endif /* TMP_OBJDIR_H */