PT-1747 Improved FK rebuild constraints

If we cannot rebuild constraints due to metadata lock, now the tool
won't delete the new table nor the triggers so the old table and the new
table will have the same data and since the triggers are still in place,
new_table will keep getting the updates.
Also, the tool will show the commands that the user need to run manually
to complete the process

bin/pt-online-schema-change

@@ -8371,6 +8371,7 @@ my $dont_interrupt_now = 0;
 my @drop_trigger_sqls;
 my @triggers_not_dropped;
 my $pxc_version = '0';
+my $can_drop_triggers = 1;
 
 my $triggers_info = [];
 
@@ -9545,6 +9546,7 @@ sub main {
    # called which will drop whichever triggers were created.
    my $drop_triggers = $o->get('drop-triggers');
    push @cleanup_tasks, sub {
+
       PTDEBUG && _d('Clean up triggers');
       # --plugin hook
       if ( $plugin && $plugin->can('before_drop_triggers') ) {
@@ -9555,12 +9557,12 @@ sub main {
          );
       }
 
-      if ( !$oktorun ) {
+      if ( !$oktorun || !_can_drop_triggers()) {
          print "Not dropping triggers because the tool was interrupted.  "
              . "To drop the triggers, execute:\n"
              . join("\n", @drop_trigger_sqls) . "\n";
       }
-      elsif ( !$drop_triggers  ) {
+      elsif ( !$drop_triggers || !_can_drop_triggers()) {
          print "Not dropping triggers because --no-drop-triggers was "
          . "specified.  To drop the triggers, execute:\n"
          . join("\n", @drop_trigger_sqls) . "\n";
@@ -10052,6 +10054,8 @@ sub main {
       };
       if ( $EVAL_ERROR ) {
          # TODO: improve error message and handling.
+         $can_drop_triggers=undef;
+         $oktorun=undef;
          _die("Error updating foreign key constraints: $EVAL_ERROR", ERROR_UPDATING_FKS);
       }
 
@@ -10288,6 +10292,10 @@ sub main {
 # Subroutines.
 # ############################################################################
 
+sub _can_drop_triggers {
+   return $can_drop_triggers;
+}
+
 sub validate_tries {
    my ($o) = @_;
    my @ops = qw(
@@ -11069,6 +11077,8 @@ sub rebuild_constraints {
       print ts("Rebuilding foreign key constraints...\n");
    }
 
+   my $foreign_key_checks;
+
    CHILD_TABLE:
    foreach my $child_tbl ( @$child_tables ) {
       my $table_def = $tp->get_create_table(
@@ -11126,26 +11136,47 @@ sub rebuild_constraints {
 
          $constraint =~ s/CONSTRAINT `$fk`/CONSTRAINT `$new_fk`/;
 
-         my $sql = "" . "DROP FOREIGN KEY `$fk` ";
-         #. "ADD $constraint";
+         my $sql =  "DROP FOREIGN KEY `$fk`, ADD $constraint";
          push @rebuilt_constraints, $sql;
       }
 
+      my $server_version = VersionParser->new($cxn->dbh());
+      if ($server_version >= '5.6' && $o->get('disable-fk-checks')) {
+          my $row = $cxn->dbh()->selectrow_arrayref('SELECT @@foreign_key_checks');
+          $foreign_key_checks = @$row[0];
+          ts("Disabling FK checks");
+          $cxn->dbh()->do("SET FOREIGN_KEY_CHECKS=0");
+      }
       my $sql = "ALTER TABLE $child_tbl->{name} "
               . join(', ', @rebuilt_constraints);
       print $sql, "\n" if $o->get('print');
-      if ( $o->get('execute') ) {
-         osc_retry(
-            Cxn     => $cxn,
-            Retry   => $retry,
-            tries   => $tries->{update_foreign_keys},
-            stats   => $stats,
-            code    => sub {
-               PTDEBUG && _d($sql);
-               $cxn->dbh()->do($sql);
-               $stats->{rebuilt_constraint}++;
-            },
-         );
+      if ($o->get('execute')) {
+          eval {
+              osc_retry(
+                  Cxn     => $cxn,
+                  Retry   => $retry,
+                  tries   => $tries->{update_foreign_keys},
+                  stats   => $stats,
+                  code    => sub {
+                  PTDEBUG && _d($sql);
+                  $cxn->dbh()->do($sql);
+                  $stats->{rebuilt_constraint}++;
+              },
+          );
+      };
+      if ($foreign_key_checks) {
+          ts("Re-enabling FK checks");
+          $cxn->dbh()->do("SET FOREIGN_KEY_CHECKS=$foreign_key_checks");
+      }
+      if ($EVAL_ERROR) {
+         $can_drop_triggers=undef;
+         $oktorun=undef;
+         _d("Foreing keys rebuild failed. To rebuild constraints please manually run:");
+         foreach my $cmd (@rebuilt_constraints) {
+             print "$cmd\n";
+         }
+          _die("Foreing keys were not rebuilt");
+      }
       }
    }
 
@@ -11600,6 +11631,11 @@ sub osc_retry {
          ) {
             # These errors/warnings can be retried, so don't print
             # a warning yet; do that in final_fail.
+            # If we found a lock wait timeout and $tries == 0, we are in the rebuilt_constraints part
+            # so we should keep retrying
+            if ($error =~ m/Lock wait timeout exceeded/ && !$tries->{tries}) {
+                return 1;
+            }
             $stats->{ error_event($error) }++;
             return 1;  # try again
          }
@@ -12439,6 +12475,12 @@ short form: -F; type: string
 Only read mysql options from the given file.  You must give an absolute
 pathname.
 
+=item --disable-fk-checks
+
+Disable foreign keys check during rebuild constraints. This option improves the process speed
+but it is risky since FKs checks will be disabled for a short period of time, allowing invalid
+values in tables. Please use it carefully.
+
 =item --[no]drop-new-table
 
 default: yes