gpg-interface: limit search for primary key fingerprint

The VALIDSIG status line from GnuPG with --status-fd is documented to
have 9 required and 1 optional fields [1].  The final, and optional,
field is used to specify the fingerprint of the primary key that made
the signature in case it was made by a subkey.  However, this field is
only available for OpenPGP signatures; not for CMS/X.509.

If the VALIDSIG status line does not have the optional 10th field, the
current code will continue reading onto the next status line.  And this
is the case for non-OpenPGP signatures [1].

The consequence is that a subsequent status line may be considered as
the "primary key" for signatures that does not have an actual primary
key.

Limit the search of these 9 or 10 fields to the single line to avoid
this problem.  If the 10th field is missing, report that there is no
primary key fingerprint.

[Reference]

[1] GnuPG Details, General status codes
https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg.git;a=blob;f=doc/DETAILS;h=6ce340e8c04794add995e84308bb3091450bd28f;hb=HEAD#l483

The documentation says:

    VALIDSIG <args>

    The args are:

    - <fingerprint_in_hex>
    - <sig_creation_date>
    - <sig-timestamp>
    - <expire-timestamp>
    - <sig-version>
    - <reserved>
    - <pubkey-algo>
    - <hash-algo>
    - <sig-class>
    - [ <primary-key-fpr> ]

    This status indicates that the signature is cryptographically
    valid. [...] PRIMARY-KEY-FPR is the fingerprint of the primary key
    or identical to the first argument.

    The primary-key-fpr parameter is used for OpenPGP and not available
    for CMS signatures.  [...]

Signed-off-by: Hans Jerry Illikainen <hji@dyntopia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.c

@@ -156,21 +156,33 @@ static void parse_gpg_output(struct signature_check *sigc)
 				}
 				/* Do we have fingerprint? */
 				if (sigcheck_gpg_status[i].flags & GPG_STATUS_FINGERPRINT) {
+					const char *limit;
+					char **field;
+
 					next = strchrnul(line, ' ');
 					replace_cstring(&sigc->fingerprint, line, next);
 
-					/* Skip interim fields */
+					/*
+					 * Skip interim fields.  The search is
+					 * limited to the same line since only
+					 * OpenPGP signatures has a field with
+					 * the primary fingerprint.
+					 */
+					limit = strchrnul(line, '\n');
 					for (j = 9; j > 0; j--) {
-						if (!*next)
+						if (!*next || limit <= next)
 							break;
 						line = next + 1;
 						next = strchrnul(line, ' ');
 					}
 
-					next = strchrnul(line, '\n');
-					free(sigc->primary_key_fingerprint);
-					replace_cstring(&sigc->primary_key_fingerprint,
-							line, next);
+					field = &sigc->primary_key_fingerprint;
+					if (!j) {
+						next = strchrnul(line, '\n');
+						replace_cstring(field, line, next);
+					} else {
+						replace_cstring(field, NULL, NULL);
+					}
 				}
 
 				break;

t/t4202-log.sh

@@ -1555,6 +1555,14 @@ test_expect_success GPG 'setup signed branch' '
 	git commit -S -m signed_commit
 '
 
+test_expect_success GPG 'setup signed branch with subkey' '
+	test_when_finished "git reset --hard && git checkout master" &&
+	git checkout -b signed-subkey master &&
+	echo foo >foo &&
+	git add foo &&
+	git commit -SB7227189 -m signed_commit
+'
+
 test_expect_success GPGSM 'setup signed branch x509' '
 	test_when_finished "git reset --hard && git checkout master" &&
 	git checkout -b signed-x509 master &&
@@ -1565,6 +1573,18 @@ test_expect_success GPGSM 'setup signed branch x509' '
 	git commit -S -m signed_commit
 '
 
+test_expect_success GPGSM 'log x509 fingerprint' '
+	echo "F8BF62E0693D0694816377099909C779FA23FD65 | " >expect &&
+	git log -n1 --format="%GF | %GP" signed-x509 >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success GPGSM 'log OpenPGP fingerprint' '
+	echo "D4BE22311AD3131E5EDA29A461092E85B7227189" > expect &&
+	git log -n1 --format="%GP" signed-subkey >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success GPG 'log --graph --show-signature' '
 	git log --graph --show-signature -n1 signed >actual &&
 	grep "^| gpg: Signature made" actual &&