git-tag -s must fail if gpg cannot sign the tag.

Most of this patch code and message was written by Shawn O. Pearce.
I made some tests to know what the problem was, and then I changed
the code related with the SIGPIPE signal.

If the user has misconfigured `user.signingkey` in their .git/config
or just doesn't have any secret keys on their keyring and they ask
for a signed tag with `git tag -s` we better make sure the resulting
tag was actually signed by gpg.

Prior versions of builtin git-tag allowed this failure to slip
by without error as they were not checking the return value of
the finish_command() so they did not notice when gpg exited with
an error exit status.  They also did not fail if gpg produced an
empty output or if read_in_full received an error from the read
system call while trying to read the pipe back from gpg.

Finally, we did not actually honor any return value from the do_sign
function as it returns ssize_t but was being stored into an unsigned
long.  This caused the compiler to optimize out the die condition,
allowing git-tag to continue along and create the tag object.

However, when gpg gets a wrong username, it exits before any read was done
and then the writing process receives SIGPIPE and program is terminated.
By ignoring this signal, anyway, the function write_or_die gets EPIPE from
write_in_full and exits returning 0 to the system without a message.
Here we better call to write_in_full directly so we can fail
printing a message and return safely to the caller.

With these issues fixed `git-tag -s` will now fail to create the
tag and will report a non-zero exit status to its caller, thereby
allowing automated helper scripts to detect (and recover from)
failure if gpg is not working properly.

Proposed-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Carlos Rica <jasampler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-tag.c

@@ -200,6 +200,10 @@ static ssize_t do_sign(char *buffer, size_t size, size_t max)
 			bracket[1] = '\0';
 	}
 
+	/* When the username signingkey is bad, program could be terminated
+	 * because gpg exits without reading and then write gets SIGPIPE. */
+	signal(SIGPIPE, SIG_IGN);
+
 	memset(&gpg, 0, sizeof(gpg));
 	gpg.argv = args;
 	gpg.in = -1;
@@ -212,12 +216,17 @@ static ssize_t do_sign(char *buffer, size_t size, size_t max)
 	if (start_command(&gpg))
 		return error("could not run gpg.");
 
-	write_or_die(gpg.in, buffer, size);
+	if (write_in_full(gpg.in, buffer, size) != size) {
+		close(gpg.in);
+		finish_command(&gpg);
+		return error("gpg did not accept the tag data");
+	}
 	close(gpg.in);
 	gpg.close_in = 0;
 	len = read_in_full(gpg.out, buffer + size, max - size);
 
-	finish_command(&gpg);
+	if (finish_command(&gpg) || !len || len < 0)
+		return error("gpg failed to sign the tag");
 
 	if (len == max - size)
 		return error("could not read the entire signature from gpg.");
@@ -310,9 +319,10 @@ static void create_tag(const unsigned char *object, const char *tag,
 	size += header_len;
 
 	if (sign) {
-		size = do_sign(buffer, size, max_size);
-		if (size < 0)
+		ssize_t r = do_sign(buffer, size, max_size);
+		if (r < 0)
 			die("unable to sign the tag");
+		size = r;
 	}
 
 	if (write_sha1_file(buffer, size, tag_type, result) < 0)

t/t7004-tag.sh

@@ -990,6 +990,13 @@ test_expect_success \
 	git diff expect actual
 '
 
+# try to sign with bad user.signingkey
+git config user.signingkey BobTheMouse
+test_expect_failure \
+	'git-tag -s fails if gpg is misconfigured' \
+	'git tag -s -m tail tag-gpg-failure'
+git config --unset user.signingkey
+
 # try to verify without gpg:
 
 rm -rf gpghome