Reorg internal odb read header and object lookup

Often `git_odb_read_header` will "fail" and have to read the
entire object into memory instead of just the header.  When this
happens, the object is loaded and then disposed of immediately,
which makes it difficult to efficiently use the header information
to decide if the object should be loaded (since attempting to do
so will often result in loading the object twice).

This commit takes the existing code and reorganizes it to have
two new functions:

- `git_odb__read_header_or_object` which acts just like the old
  read header function except that it returns the object, too, if
  it was forced to load the whole thing.  It then becomes the
  callers responsibility to free the `git_odb_object`.
- `git_object__from_odb_object` which was extracted from the old
  `git_object_lookup` and creates a subclass of `git_object` from
  an existing `git_odb_object` (separating the ODB lookup from the
  `git_object` creation).  This allows you to use the first header
  reading function efficiently without instantiating the
  `git_odb_object` twice.

There is no net change to the behavior of any of the existing
functions, but this allows internal code to tap into the ODB
lookup and object creation to be more efficient.

1 files changed, 54 insertions, 44 deletions

diff --git a/src/object.c b/src/object.c
index 5130d97ac..2e45eb86a 100644
--- a/src/object.c
+++ b/src/object.c
@@ -77,6 +77,58 @@ static int create_object(git_object **object_out, git_otype type)
 	return 0;
 }
 
+int git_object__from_odb_object(
+	git_object **object_out,
+	git_repository *repo,
+	git_odb_object *odb_obj,
+	git_otype type)
+{
+	int error;
+	git_object *object = NULL;
+
+	if (type != GIT_OBJ_ANY && type != odb_obj->raw.type) {
+		giterr_set(GITERR_ODB, "The requested type does not match the type in the ODB");
+		return GIT_ENOTFOUND;
+	}
+
+	type = odb_obj->raw.type;
+
+	if ((error = create_object(&object, type)) < 0)
+		return error;
+
+	/* Initialize parent object */
+	git_oid_cpy(&object->cached.oid, &odb_obj->cached.oid);
+	object->repo = repo;
+
+	switch (type) {
+	case GIT_OBJ_COMMIT:
+		error = git_commit__parse((git_commit *)object, odb_obj);
+		break;
+
+	case GIT_OBJ_TREE:
+		error = git_tree__parse((git_tree *)object, odb_obj);
+		break;
+
+	case GIT_OBJ_TAG:
+		error = git_tag__parse((git_tag *)object, odb_obj);
+		break;
+
+	case GIT_OBJ_BLOB:
+		error = git_blob__parse((git_blob *)object, odb_obj);
+		break;
+
+	default:
+		break;
+	}
+
+	if (error < 0)
+		git_object__free(object);
+	else
+		*object_out = git_cache_try_store(&repo->objects, object);
+
+	return error;
+}
+
 int git_object_lookup_prefix(
 	git_object **object_out,
 	git_repository *repo,
@@ -148,53 +200,11 @@ int git_object_lookup_prefix(
 	if (error < 0)
 		return error;
 
-	if (type != GIT_OBJ_ANY && type != odb_obj->raw.type) {
-		git_odb_object_free(odb_obj);
-		giterr_set(GITERR_ODB, "The given type does not match the type on the ODB");
-		return GIT_ENOTFOUND;
-	}
-
-	type = odb_obj->raw.type;
-
-	if (create_object(&object, type) < 0) {
-		git_odb_object_free(odb_obj);
-		return -1;
-	}
-
-	/* Initialize parent object */
-	git_oid_cpy(&object->cached.oid, &odb_obj->cached.oid);
-	object->repo = repo;
-
-	switch (type) {
-	case GIT_OBJ_COMMIT:
-		error = git_commit__parse((git_commit *)object, odb_obj);
-		break;
-
-	case GIT_OBJ_TREE:
-		error = git_tree__parse((git_tree *)object, odb_obj);
-		break;
-
-	case GIT_OBJ_TAG:
-		error = git_tag__parse((git_tag *)object, odb_obj);
-		break;
-
-	case GIT_OBJ_BLOB:
-		error = git_blob__parse((git_blob *)object, odb_obj);
-		break;
-
-	default:
-		break;
-	}
+	error = git_object__from_odb_object(object_out, repo, odb_obj, type);
 
 	git_odb_object_free(odb_obj);
 
-	if (error < 0) {
-		git_object__free(object);
-		return -1;
-	}
-
-	*object_out = git_cache_try_store(&repo->objects, object);
-	return 0;
+	return error;
 }
 
 int git_object_lookup(git_object **object_out, git_repository *repo, const git_oid *id, git_otype type) {