send-email: finer-grained SMTP error handling

Code captured errors but did not process them further.
This treated all failures the same without distinguishing SMTP status.

Add handle-smtp_error to extract SMTP status codes using a regex (as
defined in RFC 5321) and handle errors as follows:

- No error present:
	- If a result is provided, return 1 to indicate success.
	- Otherwise, return 0 to indicate failure.

- Error present with a captured three-digit status code:
	- For 4yz (transient errors), return 1 and allow retries.
	- For 5yz (permanent errors), return 0 to indicate failure.
	- For any other recognized status code, return 1, treating it as
	a transient error.

- Error present but no status code found:
	- Return 1 as a transient error.

Signed-off-by: Zheng Yuting <05ZYT30@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/git-send-email.perl b/git-send-email.perl
index 0f05f55e50d23ab91a8915e7d90ea03776f08175..1f613fa979df4513c7a07c478c9c59b787fa3e9f 100755 (executable)
--- a/git-send-email.perl
+++ b/git-send-email.perl
@@ -1454,14 +1454,40 @@ sub smtp_auth_maybe {
                        $error = $@ || 'Unknown error';
                };
 
-               # NOTE: SMTP status code handling will be added in a subsequent commit,
-               # return 1 when failed due to non-credential reasons
-               return $error ? 1 : ($result ? 1 : 0);
+               return ($error
+                       ? handle_smtp_error($error)
+                       : ($result ? 1 : 0));
        });
 
        return $auth;
 }
 
+sub handle_smtp_error {
+       my ($error) = @_;
+
+       # Parse SMTP status code from error message in:
+       # https://www.rfc-editor.org/rfc/rfc5321.html
+       if ($error =~ /\b(\d{3})\b/) {
+               my $status_code = $1;
+               if ($status_code =~ /^4/) {
+                       # 4yz: Transient Negative Completion reply
+                       warn "SMTP transient error (status code $status_code): $error";
+                       return 1;
+               } elsif ($status_code =~ /^5/) {
+                       # 5yz: Permanent Negative Completion reply
+                       warn "SMTP permanent error (status code $status_code): $error";
+                       return 0;
+               }
+               # If no recognized status code is found, treat as transient error
+               warn "SMTP unknown error: $error. Treating as transient failure.";
+               return 1;
+       }
+
+       # If no status code is found, treat as transient error
+       warn "SMTP generic error: $error";
+       return 1;
+}
+
 sub ssl_verify_params {
        eval {
                require IO::Socket::SSL;