Fix TxnScopeGuard misbehaving on externally set $@ without inner exce…

…ptions

The guard is supposed to warn when it goes out of scope without a commit.
However this doesn't work if $@ was already set before the guard was created
and there were no eval{}s to clear it between definition and destruction.

Fixing this by storing a weakref to the current exception (if any) and
disregarding the contents of $@ if they match the ref we started with.

Changes

@@ -15,6 +15,8 @@ Revision history for DBIx::Class
         - The internal carp module now correctly skips CAG frames when
           reporting a callsite
         - Fix test failures on perl < 5.8.7 and new Package::Stash::XS
+        - Fix TxnScopeGuard not behaving correctly when $@ is set at the
+          time of $guard instantiation
 
     * Misc
         - No longer depend on Variable::Magic now that a pure-perl

lib/DBIx/Class/Storage/TxnScopeGuard.pm

@@ -3,7 +3,7 @@ package DBIx::Class::Storage::TxnScopeGuard;
 use strict;
 use warnings;
 use Try::Tiny;
-use Scalar::Util qw/weaken blessed/;
+use Scalar::Util qw/weaken blessed refaddr/;
 use DBIx::Class;
 use DBIx::Class::Exception;
 use DBIx::Class::Carp;
@@ -14,9 +14,25 @@ my ($guards_count, $compat_handler, $foreign_handler);
 sub new {
   my ($class, $storage) = @_;
 
+  my $guard = {
+    inactivated => 0,
+    storage => $storage,
+  };
+
+  # we are starting with an already set $@ - in order for things to work we need to
+  # be able to recognize it upon destruction - store its weakref
+  # recording it before doing the txn_begin stuff
+  if (defined $@ and $@ ne '') {
+    $guard->{existing_exception_ref} = (ref $@ ne '') ? $@ : \$@;
+    weaken $guard->{existing_exception_ref};
+  }
+
   $storage->txn_begin;
-  my $guard = bless [ 0, $storage, $storage->_dbh ], ref $class || $class;
 
+  $guard->{dbh} = $storage->_dbh;
+  weaken $guard->{dbh};
+
+  bless $guard, ref $class || $class;
 
   # install a callback carefully
   if (DBIx::Class::_ENV_::INVISIBLE_DOLLAR_AT and !$guards_count) {
@@ -53,19 +69,21 @@ sub new {
 
   $guards_count++;
 
-  weaken ($guard->[2]);
   $guard;
 }
 
 sub commit {
   my $self = shift;
 
-  $self->[1]->txn_commit;
-  $self->[0] = 1;
+  $self->{storage}->throw_exception("Refusing to execute multiple commits on scope guard $self")
+    if $self->{inactivated};
+
+  $self->{storage}->txn_commit;
+  $self->{inactivated} = 1;
 }
 
 sub DESTROY {
-  my ($dismiss, $storage) = @{$_[0]};
+  my $self = shift;
 
   $guards_count--;
 
@@ -90,24 +108,34 @@ sub DESTROY {
     undef $foreign_handler;
   }
 
-  return if $dismiss;
+  return if $self->{inactivated};
 
-  # if our dbh is not ours anymore, the weakref will go undef
-  $storage->_verify_pid;
-  return unless $_[0]->[2];
+  # if our dbh is not ours anymore, the $dbh weakref will go undef
+  $self->{storage}->_verify_pid;
+  return unless $self->{dbh};
 
-  my $exception = $@;
+  my $exception = $@ if (
+    defined $@
+      and
+    $@ ne ''
+      and
+    (
+      ! defined $self->{existing_exception_ref}
+        or
+      refaddr( ref $@ eq '' ? \$@ : $@ ) != refaddr($self->{existing_exception_ref})
+    )
+  );
 
   {
     local $@;
 
     carp 'A DBIx::Class::Storage::TxnScopeGuard went out of scope without explicit commit or error. Rolling back.'
-      unless $exception;
+      unless defined $exception;
 
     my $rollback_exception;
     # do minimal connectivity check due to weird shit like
     # https://rt.cpan.org/Public/Bug/Display.html?id=62370
-    try { $storage->_seems_connected && $storage->txn_rollback }
+    try { $self->{storage}->_seems_connected && $self->{storage}->txn_rollback }
     catch { $rollback_exception = shift };
 
     if ( $rollback_exception and (

t/storage/txn_scope_guard.t

@@ -116,7 +116,9 @@ use DBICTest;
 }
 
 # make sure it warns *big* on failed rollbacks
-{
+# test with and without a poisoned $@
+for my $poison (0,1) {
+
   my $schema = DBICTest->init_schema();
 
   no strict 'refs';
@@ -160,11 +162,12 @@ use DBICTest;
     }
   };
   {
+      eval { die 'GIFT!' if $poison };
       my $guard = $schema->txn_scope_guard;
       $schema->resultset ('Artist')->create ({ name => 'bohhoo'});
   }
 
-  is (@w, 2, 'Both expected warnings found');
+  is (@w, 2, 'Both expected warnings found' . ($poison ? ' (after $@ poisoning)' : '') );
 
   # just to mask off warning since we could not disconnect above
   $schema->storage->_dbh->disconnect;