Unbreak interactive GPG prompt upon signing

With the recent update in efee955 (gpg-interface: check gpg signature
creation status, 2016-06-17), we ask GPG to send all status updates to
stderr, and then catch the stderr in an strbuf.

But GPG might fail, and send error messages to stderr. And we simply
do not show them to the user.

Even worse: this swallows any interactive prompt for a passphrase. And
detaches stderr from the tty so that the passphrase cannot be read.

So while the first problem could be fixed (by printing the captured
stderr upon error), the second problem cannot be easily fixed, and
presents a major regression.

So let's just revert commit efee955.

This fixes #871

Cc: Michael J Gruber <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

gpg-interface.c

@@ -365,11 +365,9 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 	struct child_process gpg = CHILD_PROCESS_INIT;
 	int ret;
 	size_t i, j, bottom;
-	struct strbuf gpg_status = STRBUF_INIT;
 
 	argv_array_pushl(&gpg.args,
 			 use_format->program,
-			 "--status-fd=2",
 			 "-bsau", signing_key,
 			 NULL);
 
@@ -381,12 +379,10 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 	 */
 	sigchain_push(SIGPIPE, SIG_IGN);
 	ret = pipe_command(&gpg, buffer->buf, buffer->len,
-			   signature, 1024, &gpg_status, 0);
+			   signature, 1024, NULL, 0);
 	sigchain_pop(SIGPIPE);
 
-	ret |= !strstr(gpg_status.buf, "\n[GNUPG:] SIG_CREATED ");
-	strbuf_release(&gpg_status);
-	if (ret)
+	if (ret || signature->len == bottom)
 		return error(_("gpg failed to sign the data"));
 
 	/* Strip CR from the line endings, in case we are on Windows. */

t/t7004-tag.sh

@@ -1365,26 +1365,13 @@ test_expect_success GPG \
 	'test_config user.signingkey BobTheMouse &&
 	test_must_fail git tag -s -m tail tag-gpg-failure'
 
-# try to produce invalid signature
-test_expect_success GPG \
-	'git tag -s fails if gpg is misconfigured (bad signature format)' \
-	'test_config gpg.program echo &&
-	 test_must_fail git tag -s -m tail tag-gpg-failure'
-
 # try to sign with bad user.signingkey
 test_expect_success GPGSM \
 	'git tag -s fails if gpgsm is misconfigured (bad key)' \
 	'test_config user.signingkey BobTheMouse &&
 	 test_config gpg.format x509 &&
 	 test_must_fail git tag -s -m tail tag-gpg-failure'
 
-# try to produce invalid signature
-test_expect_success GPGSM \
-	'git tag -s fails if gpgsm is misconfigured (bad signature format)' \
-	'test_config gpg.x509.program echo &&
-	 test_config gpg.format x509 &&
-	 test_must_fail git tag -s -m tail tag-gpg-failure'
-
 # try to verify without gpg:
 
 rm -rf gpghome