ssh signing: return an error when signature cannot be read

If the signature file cannot be read we print an error message but do
not return an error to the caller. In practice it seems unlikely that
the file would be unreadable if the call to ssh-keygen succeeds.

The unlink_or_warn() call is moved to the end of the function so that
we always try and remove the signature file. This isn't strictly
necessary at the moment but it protects us against any extra code
being added between trying to read the signature file and the cleanup
at the end of the function in the future. unlink_or_warn() only prints
a warning if it exists and cannot be removed.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.c

@@ -1043,12 +1043,11 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
 	strbuf_addbuf(&ssh_signature_filename, &buffer_file->filename);
 	strbuf_addstr(&ssh_signature_filename, ".sig");
 	if (strbuf_read_file(signature, ssh_signature_filename.buf, 0) < 0) {
-		error_errno(
+		ret = error_errno(
 			_("failed reading ssh signing data buffer from '%s'"),
 			ssh_signature_filename.buf);
+		goto out;
 	}
-	unlink_or_warn(ssh_signature_filename.buf);
-
 	/* Strip CR from the line endings, in case we are on Windows. */
 	remove_cr_after(signature, bottom);
 
@@ -1057,6 +1056,8 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
 		delete_tempfile(&key_file);
 	if (buffer_file)
 		delete_tempfile(&buffer_file);
+	if (ssh_signature_filename.len)
+		unlink_or_warn(ssh_signature_filename.buf);
 	strbuf_release(&signer_stderr);
 	strbuf_release(&ssh_signature_filename);
 	FREE_AND_NULL(ssh_signing_key_file);