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, 39 insertions, 0 deletions

diff --git a/src/object.h b/src/object.h
new file mode 100644
index 000000000..bc12aad04
--- /dev/null
+++ b/src/object.h
@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2009-2012 the libgit2 contributors
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_object_h__
+#define INCLUDE_object_h__
+
+/** Base git object for inheritance */
+struct git_object {
+	git_cached_obj cached;
+	git_repository *repo;
+	git_otype type;
+};
+
+/* fully free the object; internal method, DO NOT EXPORT */
+void git_object__free(void *object);
+
+GIT_INLINE(int) git_object__dup(git_object **dest, git_object *source)
+{
+	git_cached_obj_incref(source);
+	*dest = source;
+	return 0;
+}
+
+int git_object__from_odb_object(
+	git_object **object_out,
+	git_repository *repo,
+	git_odb_object *odb_obj,
+	git_otype type);
+
+int git_object__resolve_to_type(git_object **obj, git_otype type);
+
+int git_oid__parse(git_oid *oid, const char **buffer_out, const char *buffer_end, const char *header);
+
+void git_oid__writebuf(git_buf *buf, const char *header, const git_oid *oid);
+
+#endif