Finally! Make './mtr --valgrind-mysqld --gdb' to work.

It has its limitations, e.g. it assumes that there's only one
gdb and only one valgrind process is running. And a hard-coded
one-second delay might be too short for slow machines.

Still, it's better than "doesn't work at all"

mysql-test/lib/My/SafeProcess.pm

@@ -84,7 +84,7 @@ sub is_child {
 }
 
 
-my @safe_process_cmd;
+our @safe_process_cmd;
 my $safe_kill;
 my $bindir;
 

mysql-test/mysql-test-run.pl

@@ -5378,7 +5378,7 @@ sub mysqld_start ($$) {
   my $args;
   mtr_init_args(\$args);
 
-  if ( $opt_valgrind_mysqld )
+  if ( $opt_valgrind_mysqld and not $opt_gdb and not $opt_manual_gdb )
   {
     valgrind_arguments($args, \$exe);
   }
@@ -5981,11 +5981,20 @@ sub gdb_arguments {
   unlink($gdb_init_file);
 
   # Put $args into a single string
-  my $str= join(" ", @$$args);
   $input = $input ? "< $input" : "";
 
-  # write init file for mysqld or client
-  mtr_tofile($gdb_init_file, "set args $str $input\n");
+  if ($type ne 'client' and $opt_valgrind_mysqld) {
+    my $v = $$exe;
+    my $vargs = [];
+    valgrind_arguments($vargs, \$v);
+    mtr_tofile($gdb_init_file, <<EOF);
+shell @My::SafeProcess::safe_process_cmd --parent-pid=`pgrep -x gdb` -- $v --vgdb-error=0 @$vargs @$$args &
+shell sleep 1
+target remote | /usr/lib64/valgrind/../../bin/vgdb
+EOF
+  } else {
+    mtr_tofile($gdb_init_file, "set args @$$args $input\n");
+  }
 
   if ( $opt_manual_gdb )
   {