Split GPG interface into its own helper library

This mostly moves existing code from builtin/tag.c (for signing)
and builtin/verify-tag.c (for verifying) to a new gpg-interface.c
file to provide a more generic library interface.

 - sign_buffer() takes a payload strbuf, a signature strbuf, and a signing
   key, runs "gpg" to produce a detached signature for the payload, and
   appends it to the signature strbuf. The contents of a signed tag that
   concatenates the payload and the detached signature can be produced by
   giving the same strbuf as payload and signature strbuf.

 - verify_signed_buffer() takes a payload and a detached signature as
   <ptr, len> pairs, and runs "gpg --verify" to see if the payload matches
   the signature. It can optionally capture the output from GPG to allow
   the callers to pretty-print it in a way more suitable for their
   contexts.

"verify-tag" (aka "tag -v") used to save the whole tag contents as if it
is a detached signature, and fed gpg the payload part of the tag. It
relied on gpg to fail when the given tag is not signed but just is
annotated.  The updated run_gpg_verify() function detects the lack of
detached signature in the input, and errors out without bothering "gpg".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 138 insertions, 0 deletions

diff --git a/gpg-interface.c b/gpg-interface.c
new file mode 100644
index 0000000..ff232c8
--- /dev/null
+++ b/gpg-interface.c
@@ -0,0 +1,138 @@
+#include "cache.h"
+#include "run-command.h"
+#include "strbuf.h"
+#include "gpg-interface.h"
+#include "sigchain.h"
+
+static char *configured_signing_key;
+
+void set_signing_key(const char *key)
+{
+	free(configured_signing_key);
+	configured_signing_key = xstrdup(key);
+}
+
+int git_gpg_config(const char *var, const char *value, void *cb)
+{
+	if (!strcmp(var, "user.signingkey")) {
+		if (!value)
+			return config_error_nonbool(var);
+		set_signing_key(value);
+	}
+	return 0;
+}
+
+const char *get_signing_key(void)
+{
+	if (configured_signing_key)
+		return configured_signing_key;
+	return git_committer_info(IDENT_ERROR_ON_NO_NAME|IDENT_NO_DATE);
+}
+
+/*
+ * Create a detached signature for the contents of "buffer" and append
+ * it after "signature"; "buffer" and "signature" can be the same
+ * strbuf instance, which would cause the detached signature appended
+ * at the end.
+ */
+int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *signing_key)
+{
+	struct child_process gpg;
+	const char *args[4];
+	ssize_t len;
+	size_t i, j, bottom;
+
+	memset(&gpg, 0, sizeof(gpg));
+	gpg.argv = args;
+	gpg.in = -1;
+	gpg.out = -1;
+	args[0] = "gpg";
+	args[1] = "-bsau";
+	args[2] = signing_key;
+	args[3] = NULL;
+
+	if (start_command(&gpg))
+		return error(_("could not run gpg."));
+
+	/*
+	 * When the username signingkey is bad, program could be terminated
+	 * because gpg exits without reading and then write gets SIGPIPE.
+	 */
+	sigchain_push(SIGPIPE, SIG_IGN);
+
+	if (write_in_full(gpg.in, buffer->buf, buffer->len) != buffer->len) {
+		close(gpg.in);
+		close(gpg.out);
+		finish_command(&gpg);
+		return error(_("gpg did not accept the data"));
+	}
+	close(gpg.in);
+
+	bottom = signature->len;
+	len = strbuf_read(signature, gpg.out, 1024);
+	close(gpg.out);
+
+	sigchain_pop(SIGPIPE);
+
+	if (finish_command(&gpg) || !len || len < 0)
+		return error(_("gpg failed to sign the data"));
+
+	/* Strip CR from the line endings, in case we are on Windows. */
+	for (i = j = bottom; i < signature->len; i++)
+		if (signature->buf[i] != '\r') {
+			if (i != j)
+				signature->buf[j] = signature->buf[i];
+			j++;
+		}
+	strbuf_setlen(signature, j);
+
+	return 0;
+}
+
+/*
+ * Run "gpg" to see if the payload matches the detached signature.
+ * gpg_output_to tells where the output from "gpg" should go:
+ *   < 0: /dev/null
+ *   = 0: standard error of the calling process
+ *   > 0: the specified file descriptor
+ */
+int verify_signed_buffer(const char *payload, size_t payload_size,
+			 const char *signature, size_t signature_size,
+			 struct strbuf *gpg_output)
+{
+	struct child_process gpg;
+	const char *args_gpg[] = {"gpg", "--verify", "FILE", "-", NULL};
+	char path[PATH_MAX];
+	int fd, ret;
+
+	fd = git_mkstemp(path, PATH_MAX, ".git_vtag_tmpXXXXXX");
+	if (fd < 0)
+		return error("could not create temporary file '%s': %s",
+			     path, strerror(errno));
+	if (write_in_full(fd, signature, signature_size) < 0)
+		return error("failed writing detached signature to '%s': %s",
+			     path, strerror(errno));
+	close(fd);
+
+	memset(&gpg, 0, sizeof(gpg));
+	gpg.argv = args_gpg;
+	gpg.in = -1;
+	if (gpg_output)
+		gpg.err = -1;
+	args_gpg[2] = path;
+	if (start_command(&gpg)) {
+		unlink(path);
+		return error("could not run gpg.");
+	}
+
+	write_in_full(gpg.in, payload, payload_size);
+	close(gpg.in);
+
+	if (gpg_output)
+		strbuf_read(gpg_output, gpg.err, 0);
+	ret = finish_command(&gpg);
+
+	unlink_or_warn(path);
+
+	return ret;
+}