Cygwin: Remove waitloop argument from try_to_debug()

Currently, when using CYGWIN's error_start facility, the faulting
process isn't stopped while the error_start process is started when the
fault is caused by an exception. (it even seems possible in theory that
the faulting process could have exited before the error_start process
attaches).

This leads to e.g. the core dump written by CYGWIN='error_start=dumper'
in response to an exception being non-deterministic.

Remove the waitloop argument from try_to_debug(), only used in the
exception case, so the faulting process busy-waits until the error_start
process attaches.

Code archaeology to determine why the code is this way didn't really
turn up any answers, but this seems a low-risk change, as this only
changes the behaviour when:

 - a debugger isn't already attached
 - an error_start is specified in CYGWIN env var
 - an exception has occurred which will be translated to a signal

If error_start invokes something which doesn't attach using
DebugActiveProcess(), we will spin indefinitely, but that will also
currently occur for any of the existing other uses of try_to_debug(),
which default to waitloop=TRUE.

1 files changed, 8 insertions, 6 deletions

diff --git a/winsup/cygwin/exceptions.cc b/winsup/cygwin/exceptions.cc
index bb7704f..c98b92d 100644
--- a/winsup/cygwin/exceptions.cc
+++ b/winsup/cygwin/exceptions.cc
@@ -461,10 +461,8 @@ cygwin_stackdump ()
   exc.dumpstack ();
 }
 
-#define TIME_TO_WAIT_FOR_DEBUGGER 10000
-
 extern "C" int
-try_to_debug (bool waitloop)
+try_to_debug ()
 {
   if (!debugger_command)
     return 0;
@@ -532,13 +530,17 @@ try_to_debug (bool waitloop)
 			&si,
 			&pi);
 
+  /* We want to stop here and wait until the error_start process attaches.  But
+     we can't wait here for the error_start process to exit, as if it's a
+     debugger, it might want to continue this thread.  So we busy wait until a
+     debugger attaches, which stops this process, after which it can decide if
+     we continue or not. */
+
   *dbg_end = L'\0';
   if (!dbg)
     system_printf ("Failed to start debugger, %E");
   else
     {
-      if (!waitloop)
-	return dbg;
       SetThreadPriority (GetCurrentThread (), THREAD_PRIORITY_IDLE);
       while (!being_debugged ())
 	Sleep (1);
@@ -812,7 +814,7 @@ exception::handle (EXCEPTION_RECORD *e, exception_list *frame, CONTEXT *in,
   if (exit_state >= ES_SIGNAL_EXIT
       && (NTSTATUS) e->ExceptionCode != STATUS_CONTROL_C_EXIT)
     api_fatal ("Exception during process exit");
-  else if (!try_to_debug (0))
+  else if (!try_to_debug ())
     rtl_unwind (frame, e);
   else
     {