gpg-interface.c: support getting key fingerprint via %GF format

Support processing VALIDSIG status that provides additional information
for valid signatures.  Use this information to propagate signing key
fingerprint and expose it via %GF pretty format.  This format can be
used to build safer key verification systems that verify the key via
complete fingerprint rather than short/long identifier provided by %GK.

Signed-off-by: Michał Górny <mgorny@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -153,6 +153,7 @@ endif::git-rev-list[]
   and "N" for no signature
 - '%GS': show the name of the signer for a signed commit
 - '%GK': show the key used to sign a signed commit
+- '%GF': show the fingerprint of the key used to sign a signed commit
 - '%gD': reflog selector, e.g., `refs/stash@{1}` or
   `refs/stash@{2 minutes ago`}; the format follows the rules described
   for the `-g` option. The portion before the `@` is the refname as

gpg-interface.c

@@ -73,6 +73,7 @@ void signature_check_clear(struct signature_check *sigc)
 	FREE_AND_NULL(sigc->gpg_status);
 	FREE_AND_NULL(sigc->signer);
 	FREE_AND_NULL(sigc->key);
+	FREE_AND_NULL(sigc->fingerprint);
 }
 
 /* An exclusive status -- only one of them can appear in output */
@@ -81,6 +82,8 @@ void signature_check_clear(struct signature_check *sigc)
 #define GPG_STATUS_KEYID	(1<<1)
 /* The status includes user identifier */
 #define GPG_STATUS_UID		(1<<2)
+/* The status includes key fingerprints */
+#define GPG_STATUS_FINGERPRINT	(1<<3)
 
 /* Short-hand for standard exclusive *SIG status with keyid & UID */
 #define GPG_STATUS_STDSIG	(GPG_STATUS_EXCLUSIVE|GPG_STATUS_KEYID|GPG_STATUS_UID)
@@ -98,6 +101,7 @@ static struct {
 	{ 'X', "EXPSIG ", GPG_STATUS_STDSIG },
 	{ 'Y', "EXPKEYSIG ", GPG_STATUS_STDSIG },
 	{ 'R', "REVKEYSIG ", GPG_STATUS_STDSIG },
+	{ 0, "VALIDSIG ", GPG_STATUS_FINGERPRINT },
 };
 
 static void parse_gpg_output(struct signature_check *sigc)
@@ -123,7 +127,8 @@ static void parse_gpg_output(struct signature_check *sigc)
 						goto found_duplicate_status;
 				}
 
-				sigc->result = sigcheck_gpg_status[i].result;
+				if (sigcheck_gpg_status[i].result)
+					sigc->result = sigcheck_gpg_status[i].result;
 				/* Do we have key information? */
 				if (sigcheck_gpg_status[i].flags & GPG_STATUS_KEYID) {
 					next = strchrnul(line, ' ');
@@ -137,6 +142,12 @@ static void parse_gpg_output(struct signature_check *sigc)
 						sigc->signer = xmemdupz(line, next - line);
 					}
 				}
+				/* Do we have fingerprint? */
+				if (sigcheck_gpg_status[i].flags & GPG_STATUS_FINGERPRINT) {
+					next = strchrnul(line, ' ');
+					free(sigc->fingerprint);
+					sigc->fingerprint = xmemdupz(line, next - line);
+				}
 
 				break;
 			}
@@ -154,6 +165,7 @@ found_duplicate_status:
 	 */
 	sigc->result = 'E';
 	/* Clear partial data to avoid confusion */
+	FREE_AND_NULL(sigc->fingerprint);
 	FREE_AND_NULL(sigc->signer);
 	FREE_AND_NULL(sigc->key);
 }

gpg-interface.h

@@ -23,6 +23,7 @@ struct signature_check {
 	char result;
 	char *signer;
 	char *key;
+	char *fingerprint;
 };
 
 void signature_check_clear(struct signature_check *sigc);

pretty.c

@@ -1256,6 +1256,10 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			if (c->signature_check.key)
 				strbuf_addstr(sb, c->signature_check.key);
 			break;
+		case 'F':
+			if (c->signature_check.fingerprint)
+				strbuf_addstr(sb, c->signature_check.fingerprint);
+			break;
 		default:
 			return 0;
 		}

t/t7510-signed-commit.sh

@@ -175,8 +175,9 @@ test_expect_success GPG 'show good signature with custom format' '
 	G
 	13B6F51ECDDE430D
 	C O Mitter <committer@example.com>
+	73D758744BE721698EC54E8713B6F51ECDDE430D
 	EOF
-	git log -1 --format="%G?%n%GK%n%GS" sixth-signed >actual &&
+	git log -1 --format="%G?%n%GK%n%GS%n%GF" sixth-signed >actual &&
 	test_cmp expect actual
 '
 
@@ -185,8 +186,9 @@ test_expect_success GPG 'show bad signature with custom format' '
 	B
 	13B6F51ECDDE430D
 	C O Mitter <committer@example.com>
+
 	EOF
-	git log -1 --format="%G?%n%GK%n%GS" $(cat forged1.commit) >actual &&
+	git log -1 --format="%G?%n%GK%n%GS%n%GF" $(cat forged1.commit) >actual &&
 	test_cmp expect actual
 '
 
@@ -195,8 +197,9 @@ test_expect_success GPG 'show untrusted signature with custom format' '
 	U
 	61092E85B7227189
 	Eris Discordia <discord@example.net>
+	D4BE22311AD3131E5EDA29A461092E85B7227189
 	EOF
-	git log -1 --format="%G?%n%GK%n%GS" eighth-signed-alt >actual &&
+	git log -1 --format="%G?%n%GK%n%GS%n%GF" eighth-signed-alt >actual &&
 	test_cmp expect actual
 '
 
@@ -205,8 +208,9 @@ test_expect_success GPG 'show unknown signature with custom format' '
 	E
 	61092E85B7227189
 
+
 	EOF
-	GNUPGHOME="$GNUPGHOME_NOT_USED" git log -1 --format="%G?%n%GK%n%GS" eighth-signed-alt >actual &&
+	GNUPGHOME="$GNUPGHOME_NOT_USED" git log -1 --format="%G?%n%GK%n%GS%n%GF" eighth-signed-alt >actual &&
 	test_cmp expect actual
 '
 
@@ -215,8 +219,9 @@ test_expect_success GPG 'show lack of signature with custom format' '
 	N
 
 
+
 	EOF
-	git log -1 --format="%G?%n%GK%n%GS" seventh-unsigned >actual &&
+	git log -1 --format="%G?%n%GK%n%GS%n%GF" seventh-unsigned >actual &&
 	test_cmp expect actual
 '
 
@@ -255,8 +260,9 @@ test_expect_success GPG 'show double signature with custom format' '
 	E
 
 
+
 	EOF
-	git log -1 --format="%G?%n%GK%n%GS" $(cat double-commit.commit) >actual &&
+	git log -1 --format="%G?%n%GK%n%GS%n%GF" $(cat double-commit.commit) >actual &&
 	test_cmp expect actual
 '