object: implement function to parse raw data

Now that we have implement functions to parse all git objects from raw
data, we can implement a generic function `git_object__from_raw` to
create a structure of type `git_object`. This allows us to parse and
interpret objects from raw data without having to touch the ODB at all,
which is especially useful for object verification prior to accepting
them into the repository.

src/object.c

@@ -12,6 +12,7 @@
 #include "repository.h"
 
 #include "commit.h"
+#include "hash.h"
 #include "tree.h"
 #include "blob.h"
 #include "oid.h"
@@ -19,38 +20,86 @@
 
 bool git_object__strict_input_validation = true;
 
+extern int git_odb_hash(git_oid *out, const void *data, size_t len, git_otype type);
+
 typedef struct {
 	const char	*str;	/* type name string */
 	size_t		size;	/* size in bytes of the object structure */
 
 	int  (*parse)(void *self, git_odb_object *obj);
+	int  (*parse_raw)(void *self, const char *data, size_t size);
 	void (*free)(void *self);
 } git_object_def;
 
 static git_object_def git_objects_table[] = {
 	/* 0 = GIT_OBJ__EXT1 */
-	{ "", 0, NULL, NULL },
+	{ "", 0, NULL, NULL, NULL },
 
 	/* 1 = GIT_OBJ_COMMIT */
-	{ "commit", sizeof(git_commit), git_commit__parse, git_commit__free },
+	{ "commit", sizeof(git_commit), git_commit__parse, git_commit__parse_raw, git_commit__free },
 
 	/* 2 = GIT_OBJ_TREE */
-	{ "tree", sizeof(git_tree), git_tree__parse, git_tree__free },
+	{ "tree", sizeof(git_tree), git_tree__parse, git_tree__parse_raw, git_tree__free },
 
 	/* 3 = GIT_OBJ_BLOB */
-	{ "blob", sizeof(git_blob), git_blob__parse, git_blob__free },
+	{ "blob", sizeof(git_blob), git_blob__parse, git_blob__parse_raw, git_blob__free },
 
 	/* 4 = GIT_OBJ_TAG */
-	{ "tag", sizeof(git_tag), git_tag__parse, git_tag__free },
+	{ "tag", sizeof(git_tag), git_tag__parse, git_tag__parse_raw, git_tag__free },
 
 	/* 5 = GIT_OBJ__EXT2 */
-	{ "", 0, NULL, NULL },
+	{ "", 0, NULL, NULL, NULL },
 	/* 6 = GIT_OBJ_OFS_DELTA */
-	{ "OFS_DELTA", 0, NULL, NULL },
+	{ "OFS_DELTA", 0, NULL, NULL, NULL },
 	/* 7 = GIT_OBJ_REF_DELTA */
-	{ "REF_DELTA", 0, NULL, NULL },
+	{ "REF_DELTA", 0, NULL, NULL, NULL },
 };
 
+int git_object__from_raw(
+	git_object **object_out,
+	const char *data,
+	size_t size,
+	git_otype type)
+{
+	git_object_def *def;
+	git_object *object;
+	size_t object_size;
+	int error;
+
+	assert(object_out);
+	*object_out = NULL;
+
+	/* Validate type match */
+	if (type != GIT_OBJ_BLOB && type != GIT_OBJ_TREE && type != GIT_OBJ_COMMIT && type != GIT_OBJ_TAG) {
+		giterr_set(GITERR_INVALID, "the requested type is invalid");
+		return GIT_ENOTFOUND;
+	}
+
+	if ((object_size = git_object__size(type)) == 0) {
+		giterr_set(GITERR_INVALID, "the requested type is invalid");
+		return GIT_ENOTFOUND;
+	}
+
+	/* Allocate and initialize base object */
+	object = git__calloc(1, object_size);
+	GITERR_CHECK_ALLOC(object);
+	object->cached.flags = GIT_CACHE_STORE_PARSED;
+	object->cached.type = type;
+	git_odb_hash(&object->cached.oid, data, size, type);
+
+	/* Parse raw object data */
+	def = &git_objects_table[type];
+	assert(def->free && def->parse_raw);
+
+	if ((error = def->parse_raw(object, data, size)) < 0)
+		def->free(object);
+
+	git_cached_obj_incref(object);
+	*object_out = object;
+
+	return 0;
+}
+
 int git_object__from_odb_object(
 	git_object **object_out,
 	git_repository *repo,

src/object.h

@@ -22,6 +22,17 @@ struct git_object {
 /* fully free the object; internal method, DO NOT EXPORT */
 void git_object__free(void *object);
 
+/*
+ * Parse object from raw data. Note that the resulting object is
+ * tied to the lifetime of the data, as some objects simply point
+ * into it.
+ */
+int git_object__from_raw(
+	git_object **object_out,
+	const char *data,
+	size_t size,
+	git_otype type);
+
 int git_object__from_odb_object(
 	git_object **object_out,
 	git_repository *repo,