[PATCH] tag: fix sign_buffer() call to create a signed tag

[Junio C Hamano <gitster@pobox.com>]

The command "git tag -s" internally calls sign_buffer() to make a
cryptographic signature using the chosen backend like GPG and SSH.
The internal helper functions used by "git tag" implementation seem
to use a "negative return values are errors, zero or positive return
values are not" convention, and there are places (e.g., verify_tag()
that calls gpg_verify_tag()) that these internal helper functions
translate return values that signal errors to conform to this
convention, but do_sign() that calls sign_buffer() forgets to do so.

Fix it, so that a failed call to sign_buffer() that can return the
exit status from pipe_command() will not be overlooked.

Reported-by: Sergey Kosukhin <skosukhin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

 * We alternatively could fix individual sign_buffer() backend that
   signals an error with a positive value (sign_buffer_ssh() in this
   case) to return a negative value, but this would hopefully be
   more future-proof.

 builtin/tag.c   | 2 +-
 gpg-interface.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/builtin/tag.c b/builtin/tag.c
index 3918eacbb5..b28ead06ea 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -176,7 +176,7 @@ static int verify_tag(const char *name, const char *ref UNUSED,
 
 static int do_sign(struct strbuf *buffer)
 {
-	return sign_buffer(buffer, buffer, get_signing_key());
+	return sign_buffer(buffer, buffer, get_signing_key()) ? -1 : 0;
 }
 
 static const char tag_template[] =
diff --git a/gpg-interface.h b/gpg-interface.h
index 143cdc1c02..7cd98161f7 100644
--- a/gpg-interface.h
+++ b/gpg-interface.h
@@ -66,7 +66,7 @@ size_t parse_signed_buffer(const char *buf, size_t size);
  * 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.
+ * at the end.  Returns 0 on success, non-zero on failure.
  */
 int sign_buffer(struct strbuf *buffer, struct strbuf *signature,
 		const char *signing_key);
-- 
2.43.0-561-g235986be82