Patch of https://bugs.launchpad.net/percona-toolkit/+bug/1498128

pt-online-schema-change.patch

--- /a/pt-online-schema-change	2016-03-07 09:09:45.000000000 +0000
+++ /b/pt-online-schema-change	2016-06-15 12:51:38.635743830 +0100
@@ -10031,11 +10031,7 @@
       # This is in contrast to previous behavior were we added underscores
       # indefinitely, sometimes exceeding the allowed name limit
       # https://bugs.launchpad.net/percona-toolkit/+bug/1215587 
-      if ( $sql =~ /CONSTRAINT `_/ ) {
-         $sql =~ s/^  CONSTRAINT `_/  CONSTRAINT `/gm;
-      } else {
-         $sql =~ s/^  CONSTRAINT `/  CONSTRAINT `_/gm;
-      }
+      $sql =~ s/^  CONSTRAINT `(_?)/' CONSTRAINT `'.($1 eq '' ? '_' : '')/gme;
       if ( $o->get('default-engine') ) {
          $sql =~ s/\s+ENGINE=\S+//;
       }