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>

1 files changed, 16 insertions, 12 deletions

diff --git a/object-file.c b/object-file.c
index db66ae5ebe..7ef8291a48 100644
--- a/object-file.c
+++ b/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,