commit-graph: Introduce a parser for commit-graph files

This change is the first in a series to add support for git's
commit-graph. This should speed up commit graph traversals by avoiding
object parsing and allowing some operations to terminate earlier.

Part of: #5757

fuzzers/commit_graph_fuzzer.c

@@ -0,0 +1,75 @@
+/*
+ * libgit2 commit-graph fuzzer target.
+ *
+ * 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.
+ */
+
+#include <stdio.h>
+
+#include "git2.h"
+
+#include "buffer.h"
+#include "common.h"
+#include "futils.h"
+#include "hash.h"
+#include "commit_graph.h"
+
+int LLVMFuzzerInitialize(int *argc, char ***argv)
+{
+	GIT_UNUSED(argc);
+	GIT_UNUSED(argv);
+
+	if (git_libgit2_init() < 0) {
+		fprintf(stderr, "Failed to initialize libgit2\n");
+		abort();
+	}
+	return 0;
+}
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+	git_commit_graph_file cgraph = {{0}};
+	git_buf commit_graph_buf = GIT_BUF_INIT;
+	git_oid oid = {{0}};
+	bool append_hash = false;
+
+	if (size < 4)
+		return 0;
+
+	/*
+	 * If the first byte in the stream has the high bit set, append the
+	 * SHA1 hash so that the file is somewhat valid.
+	 */
+	append_hash = *data & 0x80;
+	/* Keep a 4-byte alignment to avoid unaligned accesses. */
+	data += 4;
+	size -= 4;
+
+	if (append_hash) {
+		if (git_buf_init(&commit_graph_buf, size + sizeof(oid)) < 0)
+			goto cleanup;
+		if (git_hash_buf(&oid, data, size) < 0) {
+			fprintf(stderr, "Failed to compute the SHA1 hash\n");
+			abort();
+		}
+		memcpy(commit_graph_buf.ptr, data, size);
+		memcpy(commit_graph_buf.ptr + size, &oid, sizeof(oid));
+	} else {
+		git_buf_attach_notowned(&commit_graph_buf, (char *)data, size);
+	}
+
+	if (git_commit_graph_parse(
+			    &cgraph,
+			    (const unsigned char *)git_buf_cstr(&commit_graph_buf),
+			    git_buf_len(&commit_graph_buf))
+	    < 0)
+		goto cleanup;
+
+cleanup:
+	git_commit_graph_close(&cgraph);
+	git_buf_dispose(&commit_graph_buf);
+	return 0;
+}