fsck: provide a function to fsck buffer without object struct

The fsck code has been slowly moving away from requiring an object
struct in commits like 103fb6d43b (fsck: accept an oid instead of a
"struct tag" for fsck_tag(), 2019-10-18), c5b4269b57 (fsck: accept an
oid instead of a "struct commit" for fsck_commit(), 2019-10-18), etc.

However, the only external interface that fsck.c provides is
fsck_object(), which requires an object struct, then promptly discards
everything except its oid and type. Let's factor out the post-discard
part of that function as fsck_buffer(), leaving fsck_object() as a thin
wrapper around it. That will provide more flexibility for callers which
may not have a struct.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.c

@@ -1237,19 +1237,26 @@ int fsck_object(struct object *obj, void *data, unsigned long size,
 	if (!obj)
 		return report(options, NULL, OBJ_NONE, FSCK_MSG_BAD_OBJECT_SHA1, "no valid object to fsck");
 
-	if (obj->type == OBJ_BLOB)
-		return fsck_blob(&obj->oid, data, size, options);
-	if (obj->type == OBJ_TREE)
-		return fsck_tree(&obj->oid, data, size, options);
-	if (obj->type == OBJ_COMMIT)
-		return fsck_commit(&obj->oid, data, size, options);
-	if (obj->type == OBJ_TAG)
-		return fsck_tag(&obj->oid, data, size, options);
+	return fsck_buffer(&obj->oid, obj->type, data, size, options);
+}
 
-	return report(options, &obj->oid, obj->type,
+int fsck_buffer(const struct object_id *oid, enum object_type type,
+		void *data, unsigned long size,
+		struct fsck_options *options)
+{
+	if (type == OBJ_BLOB)
+		return fsck_blob(oid, data, size, options);
+	if (type == OBJ_TREE)
+		return fsck_tree(oid, data, size, options);
+	if (type == OBJ_COMMIT)
+		return fsck_commit(oid, data, size, options);
+	if (type == OBJ_TAG)
+		return fsck_tag(oid, data, size, options);
+
+	return report(options, oid, type,
 		      FSCK_MSG_UNKNOWN_TYPE,
 		      "unknown type '%d' (internal fsck error)",
-		      obj->type);
+		      type);
 }
 
 int fsck_error_function(struct fsck_options *o,

fsck.h

@@ -183,6 +183,14 @@ int fsck_walk(struct object *obj, void *data, struct fsck_options *options);
 int fsck_object(struct object *obj, void *data, unsigned long size,
 	struct fsck_options *options);
 
+/*
+ * Same as fsck_object(), but for when the caller doesn't have an object
+ * struct.
+ */
+int fsck_buffer(const struct object_id *oid, enum object_type,
+		void *data, unsigned long size,
+		struct fsck_options *options);
+
 /*
  * fsck a tag, and pass info about it back to the caller. This is
  * exposed fsck_object() internals for git-mktag(1).