Implement mysql-test-run.pl option to limit the number of saved core

files. This helps stability of multiple parallel automated test runs,
avoiding the situation where one bad build fills up disk with 1000s of
core files, causing failures in other test runs.

mysql-test/mysql-test-run.pl

@@ -300,6 +300,8 @@ our %mysqld_variables;
 
 my $source_dist= 0;
 
+our $opt_max_save_core= 5;
+my $num_saved_cores= 0;  # Number of core files saved in vardir/log/ so far.
 
 ######################################################################
 #
@@ -588,6 +590,7 @@ sub command_line_setup () {
              'strace-client'            => \$opt_strace_client,
              'master-binary=s'          => \$exe_master_mysqld,
              'slave-binary=s'           => \$exe_slave_mysqld,
+             'max-save-core=i'          => \$opt_max_save_core,
 
              # Coverage, profiling etc
              'gcov'                     => \$opt_gcov,
@@ -3295,10 +3298,12 @@ sub save_files_before_restore($$) {
   # Look for core files
   foreach my $core_file ( glob("$data_dir/core*") )
   {
+    last if $opt_max_save_core > 0 && $num_saved_cores >= $opt_max_save_core;
     my $core_name= basename($core_file);
     mtr_report("Saving $core_name");
     mkdir($save_name) if ! -d $save_name;
     rename("$core_file", "$save_name/$core_name");
+    ++$num_saved_cores;
   }
 }
 
@@ -4891,6 +4896,9 @@ Options for debugging the product
   master-binary=PATH    Specify the master "mysqld" to use
   slave-binary=PATH     Specify the slave "mysqld" to use
   strace-client         Create strace output for mysqltest client
+  max-save-core         Limit the number of core files saved (to avoid filling
+                        up disks for heavily crashing server). Defaults to
+                        $opt_max_save_core, set to 0 for no limit.
 
 Options for coverage, profiling etc