[PATCH] gpg-interface: still print ssh signatures when allowed signers file is not set

[Grayson Tinker <graysontinker@gmail.com>]

"show-signature" errors when the allowed signers file is not configured,
which means that the user can't see the key that the ref was signed with
without creating and configuring the file. Change the logic so that the file
is only used when configured, and so the signature status is always displayed.

Example of previous output:
```
error: gpg.ssh.allowedSignersFile needs to be configured and exist for ssh signature verification
commit b437db5ddc38ebda223bbae2087eee90a7b1c6e2 (HEAD -> master)
No signature
Author: Grayson Tinker <graysontinker@gmail.com>
```

Example of new output:
```
commit b437db5ddc38ebda223bbae2087eee90a7b1c6e2 (HEAD -> master)
hint: Configure gpg.ssh.allowedSignersFile for automatic principal matching
Good "git" signature with ED25519-SK key SHA256:yTU4KFs/g6MY7biDSlVStB63Gi1rCKg7dOFDXbe0yuw
Author: Grayson Tinker <graysontinker@gmail.com>
```

Signed-off-by: Grayson Tinker <graysontinker@gmail.com>
---
 gpg-interface.c | 42 +++++++++++++++++++++++-------------------
 1 file changed, 23 insertions(+), 19 deletions(-)

diff --git a/gpg-interface.c b/gpg-interface.c
index dafd5371fa..ef3e1a0aa0 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -1,6 +1,7 @@
 #define USE_THE_REPOSITORY_VARIABLE
 
 #include "git-compat-util.h"
+#include "advice.h"
 #include "commit.h"
 #include "config.h"
 #include "date.h"
@@ -480,11 +481,6 @@ static int verify_ssh_signed_buffer(struct signature_check *sigc,
 		.local = 1,
 	};
 
-	if (!ssh_allowed_signers) {
-		error(_("gpg.ssh.allowedSignersFile needs to be configured and exist for ssh signature verification"));
-		return -1;
-	}
-
 	buffer_file = mks_tempfile_t(".git_vtag_tmpXXXXXX");
 	if (!buffer_file)
 		return error_errno(_("could not create temporary file"));
@@ -500,22 +496,26 @@ static int verify_ssh_signed_buffer(struct signature_check *sigc,
 		strbuf_addf(&verify_time, "-Overify-time=%s",
 			show_date(sigc->payload_timestamp, 0, verify_date_mode));
 
-	/* Find the principal from the signers */
-	strvec_pushl(&ssh_keygen.args, fmt->program,
-		     "-Y", "find-principals",
-		     "-f", ssh_allowed_signers,
-		     "-s", buffer_file->filename.buf,
-		     verify_time.buf,
-		     NULL);
-	ret = pipe_command(&ssh_keygen, NULL, 0, &ssh_principals_out, 0,
-			   &ssh_principals_err, 0);
-	if (ret && strstr(ssh_principals_err.buf, "usage:")) {
-		error(_("ssh-keygen -Y find-principals/verify is needed for ssh signature verification (available in openssh version 8.2p1+)"));
-		goto out;
+	if (ssh_allowed_signers) {
+		/* Find the principal from the signers */
+		strvec_pushl(&ssh_keygen.args, fmt->program,
+				"-Y", "find-principals",
+				"-f", ssh_allowed_signers,
+				"-s", buffer_file->filename.buf,
+				verify_time.buf,
+				NULL);
+		ret = pipe_command(&ssh_keygen, NULL, 0, &ssh_principals_out, 0,
+				&ssh_principals_err, 0);
+		if (ret && strstr(ssh_principals_err.buf, "usage:")) {
+			error(_("ssh-keygen -Y find-principals/verify is needed for ssh signature verification (available in openssh version 8.2p1+)"));
+			goto out;
+		}
 	}
-	if (ret || !ssh_principals_out.len) {
+
+	if (!ssh_allowed_signers || ret || !ssh_principals_out.len) {
 		/*
-		 * We did not find a matching principal in the allowedSigners
+		 * We did not find a matching principal in the allowedSigners,
+		 * or no allowedSigners file was configured
 		 * Check without validation
 		 */
 		child_process_init(&ssh_keygen);
@@ -528,6 +528,10 @@ static int verify_ssh_signed_buffer(struct signature_check *sigc,
 		pipe_command(&ssh_keygen, sigc->payload, sigc->payload_len,
 				   &ssh_keygen_out, 0, &ssh_keygen_err, 0);
 
+		if (!ssh_allowed_signers) {
+			advise(_("Configure gpg.ssh.allowedSignersFile for automatic principal matching\n"));
+		}
+
 		/*
 		 * Fail on unknown keys
 		 * we still call check-novalidate to display the signature info
-- 
2.54.0