Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1739

Reported by Marcus Wellhardh wellhard@roxen.com

The following testcase gives some backtraces telling that 'foo' is undefined even if a resolver is used when compiling the string. If the mutex in the code is uncommented no backtrace is returned.

  Thread.Mutex m = Thread.Mutex();

  int threads = 0;

  class Compile
  {
    void go(string txt)
    {
      threads++;
      if (mixed err = catch {
  	for(int i; i < 1000; i++)
  	{
  	  program res;

  	  // Thread.MutexKey l = m->lock();
  	  res = compile_string (
  	    txt, 0,
  	    class (object master) {
  	      mixed resolv (string id, void|string file)
  	      {
  		return "bar";
  	      }
  	    } (master()));
  	  // l = 0;
  	}
      })
  	werror (describe_backtrace (err));
      if (!--threads) exit (0);
    }
  }

  int main()
  {
    for(int t = 0; t < 10; t++)
    {
      Compile compile = Compile();
      thread_create(compile->go, "string f1() { return foo; }");
    }
    return -17;
  }

Output:
 | -:1:'foo' undefined.
 | Compilation failed.
 | /home/wellhard/projects/pike-7.2/build/sunos-5.8-i86pc/master.pike:172:
 |     master()->compile_string("string f1() { return foo;
}",0,gc.pike.Compile.__class_65595_0())
 | gc.pike:23: go("string f1() { return foo; }")