last does not work in a do {} block

There is no point in continuing with the auth() function if there are no
mechanisms left to try (it would only issue an AUTH command with no
arguments, which is a syntax error) so simply return instead of trying to
jump out of the do {} block.

Also return if we cannot get the failed mechanism from the client because
in that case we have no way of changing the list of mechanisms to try.

Partially fixes CPAN RT#100235.

2 files changed, 3 insertions, 2 deletions

diff --git a/Changes b/Changes
index b1c737b..8928e1d 100644
--- a/Changes
+++ b/Changes
@@ -2,7 +2,7 @@ Revision history for Perl distribution libnet
 
 3.05 Development
 
-    - TODO
+    - Fixed infinite loop in Net::SMTP::auth().  [CPAN RT#100235]
 
 3.04 2014-11-29
 
diff --git a/lib/Net/SMTP.pm b/lib/Net/SMTP.pm
index 57f51cc..afd017a 100644
--- a/lib/Net/SMTP.pm
+++ b/lib/Net/SMTP.pm
@@ -193,10 +193,11 @@ sub auth {
     if ($client) {
       # $client mechanism failed, so we need to exclude this mechanism from list
       my $failed_mechanism = $client->mechanism;
+      return unless defined $failed_mechanism;
       $self->debug_text("Auth mechanism failed: $failed_mechanism")
         if $self->debug;
       $mechanisms =~ s/\b\Q$failed_mechanism\E\b//;
-      last unless $mechanisms =~ /\S/;
+      return unless $mechanisms =~ /\S/;
       $sasl->mechanism($mechanisms);
     }