Cygwin: Only return true from try_to_debug() if we launched a JIT debugger

This fixes constantly replaying the exception if we have a segfault
while a debugger is already attached, e.g. stracing a segv, see:

https://cygwin.com/pipermail/cygwin/2025-May/258144.html

Future work:

The 'debugging' static in exception::handle(), which makes us replay the
exception the next half a million times it's hit seems like cruft, maybe
we should look at if it's possible to remove that?

Fixes: 9145737 ("Cygwin: Make 'ulimit -c' control writing a coredump")
Reported-by: Christian Franke <[email protected]>
Signed-off-by: Jon Turney <[email protected]>
(cherry picked from commit b39b510)

Author: jon-turney

winsup/cygwin/exceptions.cc

@@ -597,7 +597,7 @@ try_to_debug ()
     {
       extern void break_here ();
       break_here ();
-      return 1;
+      return 0;
     }
 
   /* Otherwise, invoke the JIT debugger, if set */
@@ -812,6 +812,8 @@ exception::handle (EXCEPTION_RECORD *e, exception_list *frame, CONTEXT *in,
   else if (try_to_debug ())
     {
       debugging = 1;
+      /* If a JIT debugger just attached, replay the exception for the benefit
+	 of that */
       return ExceptionContinueExecution;
     }
 

winsup/cygwin/release/3.6.2

@@ -25,3 +25,6 @@ Fixes:
 
 - Fix deadlock for opening both side of a fifo in a process.
   Addresses: https://cygwin.com/pipermail/cygwin/2025-May/258138.html
+
+- Fix infinite exception loop on segmentation fault when strace-ing
+  Addresses: https://cygwin.com/pipermail/cygwin/2025-May/258144.html