commit-graph: Create git_commit_graph as an abstraction for the file

This change does a medium-size refactor of the git_commit_graph_file and
the interaction with the ODB. Now instead of the ODB owning a direct
reference to the git_commit_graph_file, there will be an intermediate
git_commit_graph. The main advantage of that is that now end users can
explicitly set a git_commit_graph that is eagerly checked for errors,
while still being able to lazily use the commit-graph in a regular ODB,
if the file is present.

Author: lhchavez

fuzzers/commit_graph_fuzzer.c

@@ -31,7 +31,7 @@ int LLVMFuzzerInitialize(int *argc, char ***argv)
 
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
-	git_commit_graph_file cgraph = {{0}};
+	git_commit_graph_file file = {{0}};
 	git_commit_graph_entry e;
 	git_buf commit_graph_buf = GIT_BUF_INIT;
 	git_oid oid = {{0}};
@@ -62,19 +62,19 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 		git_buf_attach_notowned(&commit_graph_buf, (char *)data, size);
 	}
 
-	if (git_commit_graph_parse(
-			    &cgraph,
+	if (git_commit_graph_file_parse(
+			    &file,
 			    (const unsigned char *)git_buf_cstr(&commit_graph_buf),
 			    git_buf_len(&commit_graph_buf))
 	    < 0)
 		goto cleanup;
 
 	/* Search for any oid, just to exercise that codepath. */
-	if (git_commit_graph_entry_find(&e, &cgraph, &oid, GIT_OID_HEXSZ) < 0)
+	if (git_commit_graph_entry_find(&e, &file, &oid, GIT_OID_HEXSZ) < 0)
 		goto cleanup;
 
 cleanup:
-	git_commit_graph_close(&cgraph);
+	git_commit_graph_file_close(&file);
 	git_buf_dispose(&commit_graph_buf);
 	return 0;
 }

include/git2/odb.h

@@ -544,6 +544,21 @@ GIT_EXTERN(size_t) git_odb_num_backends(git_odb *odb);
  */
 GIT_EXTERN(int) git_odb_get_backend(git_odb_backend **out, git_odb *odb, size_t pos);
 
+/**
+ * Set the git commit-graph for the ODB.
+ *
+ * After a successfull call, the ownership of the cgraph parameter will be
+ * transferred to libgit2, and the caller should not free it.
+ *
+ * The commit-graph can also be unset by explicitly passing NULL as the cgraph
+ * parameter.
+ *
+ * @param odb object database
+ * @param cgraph the git commit-graph
+ * @return 0 on success; error code otherwise
+ */
+GIT_EXTERN(int) git_odb_set_commit_graph(git_odb *odb, git_commit_graph *cgraph);
+
 /** @} */
 GIT_END_DECL
 #endif

include/git2/sys/commit_graph.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * 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_sys_git_commit_graph_h__
+#define INCLUDE_sys_git_commit_graph_h__
+
+#include "git2/common.h"
+#include "git2/types.h"
+
+/**
+ * @file git2/sys/commit_graph.h
+ * @brief Git commit-graph
+ * @defgroup git_commit_graph Git commit-graph APIs
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Opens a `git_commit_graph` from a path to an objects directory.
+ *
+ * This finds, opens, and validates the `commit-graph` file.
+ *
+ * @param cgraph_out the `git_commit_graph` struct to initialize.
+ * @param objects_dir the path to a git objects directory.
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_commit_graph_open(git_commit_graph **cgraph_out, const char *objects_dir);
+
+/**
+ * Frees commit-graph data. This should only be called when memory allocated
+ * using `git_commit_graph_open` is not returned to libgit2 because it was not
+ * associated with the ODB through a successful call to
+ * `git_odb_set_commit_graph`.
+ *
+ * @param cgraph the commit-graph object to free. If NULL, no action is taken.
+ */
+GIT_EXTERN(void) git_commit_graph_free(git_commit_graph *cgraph);
+
+GIT_END_DECL
+
+#endif

include/git2/types.h

@@ -102,6 +102,9 @@ typedef struct git_refdb git_refdb;
 /** A custom backend for refs */
 typedef struct git_refdb_backend git_refdb_backend;
 
+/** A git commit-graph */
+typedef struct git_commit_graph git_commit_graph;
+
 /**
  * Representation of an existing git repository,
  * including all its object contents