verify-commit: scriptable commit signature verification

Commit signatures can be verified using "git show -s --show-signature"
or the "%G?" pretty format and parsing the output, which is well suited
for user inspection, but not for scripting.

Provide a command "verify-commit" which is analogous to "verify-tag": It
returns 0 for good signatures and non-zero otherwise, has the gpg output
on stderr and (optionally) the commit object on stdout, sans the
signature, just like "verify-tag" does.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/verify-commit.c

@@ -0,0 +1,93 @@
+/*
+ * Builtin "git commit-commit"
+ *
+ * Copyright (c) 2014 Michael J Gruber <git@drmicha.warpmail.net>
+ *
+ * Based on git-verify-tag
+ */
+#include "cache.h"
+#include "builtin.h"
+#include "commit.h"
+#include "run-command.h"
+#include <signal.h>
+#include "parse-options.h"
+#include "gpg-interface.h"
+
+static const char * const verify_commit_usage[] = {
+		N_("git verify-commit [-v|--verbose] <commit>..."),
+		NULL
+};
+
+static int run_gpg_verify(const unsigned char *sha1, const char *buf, unsigned long size, int verbose)
+{
+	struct signature_check signature_check;
+
+	memset(&signature_check, 0, sizeof(signature_check));
+
+	check_commit_signature(lookup_commit(sha1), &signature_check);
+
+	if (verbose && signature_check.payload)
+		fputs(signature_check.payload, stdout);
+
+	if (signature_check.gpg_output)
+		fputs(signature_check.gpg_output, stderr);
+
+	signature_check_clear(&signature_check);
+	return signature_check.result != 'G';
+}
+
+static int verify_commit(const char *name, int verbose)
+{
+	enum object_type type;
+	unsigned char sha1[20];
+	char *buf;
+	unsigned long size;
+	int ret;
+
+	if (get_sha1(name, sha1))
+		return error("commit '%s' not found.", name);
+
+	buf = read_sha1_file(sha1, &type, &size);
+	if (!buf)
+		return error("%s: unable to read file.", name);
+	if (type != OBJ_COMMIT)
+		return error("%s: cannot verify a non-commit object of type %s.",
+				name, typename(type));
+
+	ret = run_gpg_verify(sha1, buf, size, verbose);
+
+	free(buf);
+	return ret;
+}
+
+static int git_verify_commit_config(const char *var, const char *value, void *cb)
+{
+	int status = git_gpg_config(var, value, cb);
+	if (status)
+		return status;
+	return git_default_config(var, value, cb);
+}
+
+int cmd_verify_commit(int argc, const char **argv, const char *prefix)
+{
+	int i = 1, verbose = 0, had_error = 0;
+	const struct option verify_commit_options[] = {
+		OPT__VERBOSE(&verbose, N_("print commit contents")),
+		OPT_END()
+	};
+
+	git_config(git_verify_commit_config, NULL);
+
+	argc = parse_options(argc, argv, prefix, verify_commit_options,
+			     verify_commit_usage, PARSE_OPT_KEEP_ARGV0);
+	if (argc <= i)
+		usage_with_options(verify_commit_usage, verify_commit_options);
+
+	/* sometimes the program was terminated because this signal
+	 * was received in the process of writing the gpg input: */
+	signal(SIGPIPE, SIG_IGN);
+	while (i < argc)
+		if (verify_commit(argv[i++], verbose))
+			had_error = 1;
+	return had_error;
+}