builtin/verify-tag.c: ignore SIGPIPE in gpg-interface

The verify_signed_buffer() function may trigger a SIGPIPE when the
GPG child process terminates early (due to a bad keyid, for example)
and Git tries to write to it afterwards.  Previously, ignoring
SIGPIPE was done in builtin/verify-tag.c to avoid this issue.

However, any other caller who wants to call verify_signed_buffer()
would have to do the same.

Use sigchain_push(SIGPIPE, SIG_IGN) in verify_signed_buffer(),
pretty much like in sign_buffer(), so that any caller is not
required to perform this task.

This will avoid possible mistakes by further developers using
verify_signed_buffer().

Signed-off-by: Santiago Torres <santiago@nyu.edu>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/verify-tag.c

@@ -95,9 +95,6 @@ int cmd_verify_tag(int argc, const char **argv, const char *prefix)
 	if (verbose)
 		flags |= GPG_VERIFY_VERBOSE;
 
-	/* 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_tag(argv[i++], flags))
 			had_error = 1;

gpg-interface.c

@@ -237,6 +237,7 @@ int verify_signed_buffer(const char *payload, size_t payload_size,
 		return error(_("could not run gpg."));
 	}
 
+	sigchain_push(SIGPIPE, SIG_IGN);
 	write_in_full(gpg.in, payload, payload_size);
 	close(gpg.in);
 
@@ -250,6 +251,7 @@ int verify_signed_buffer(const char *payload, size_t payload_size,
 	close(gpg.out);
 
 	ret = finish_command(&gpg);
+	sigchain_pop(SIGPIPE);
 
 	unlink_or_warn(path);