object-file: adapt `stream_object_signature()` to take a stream

The function `stream_object_signature()` is responsible for verifying
whether the given object ID matches the actual hash of the object's
contents. In contrast to `check_object_signature()` it does so in a
streaming fashion so that we don't have to load the full object into
memory.

In a subsequent commit we'll want to adapt one of its callsites to pass
a preconstructed stream. Prepare for this by accepting a stream as input
that the caller needs to assemble.

While at it, improve the error reporting in `parse_object_with_flags()`
to tell apart the two failure modes.

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

1 files changed, 16 insertions, 3 deletions

diff --git a/object.c b/object.c
index 4669b8d65e..9d2c676b16 100644
--- a/object.c
+++ b/object.c
@@ -6,6 +6,7 @@
 #include "object.h"
 #include "replace-object.h"
 #include "object-file.h"
+#include "odb/streaming.h"
 #include "blob.h"
 #include "statinfo.h"
 #include "tree.h"
@@ -330,9 +331,21 @@ struct object *parse_object_with_flags(struct repository *r,
 
 	if ((!obj || obj->type == OBJ_NONE || obj->type == OBJ_BLOB) &&
 	    odb_read_object_info(r->objects, oid, NULL) == OBJ_BLOB) {
-		if (!skip_hash && stream_object_signature(r, repl) < 0) {
-			error(_("hash mismatch %s"), oid_to_hex(oid));
-			return NULL;
+		if (!skip_hash) {
+			struct odb_read_stream *stream = odb_read_stream_open(r->objects, oid, NULL);
+
+			if (!stream) {
+				error(_("unable to open object stream for %s"), oid_to_hex(oid));
+				return NULL;
+			}
+
+			if (stream_object_signature(r, stream, repl) < 0) {
+				error(_("hash mismatch %s"), oid_to_hex(oid));
+				odb_read_stream_close(stream);
+				return NULL;
+			}
+
+			odb_read_stream_close(stream);
 		}
 		parse_blob_buffer(lookup_blob(r, oid));
 		return lookup_object(r, oid);