unwind: better debugging of exceptions.

Once upon a time when we didn't have continuable warning
exceptions, it was easy to debug internal error throws in TXR:
just set a breakpoint on uw_throw and catch it in the
debugger. This approach became unworkable with uw_throw being
executed numerous times as part of the ordinary program
control flow. With this change, we get that debugging
experience back.

* unwind.c (uw_break_on_error): New static variable.
(uw_throw): If the break-on-error variable is set and we
are processing an exception derived from error, then
call the breakpt function.

1 files changed, 9 insertions, 0 deletions

diff --git a/unwind.c b/unwind.c
index c1e5ba26..0032493d 100644
--- a/unwind.c
+++ b/unwind.c
@@ -65,6 +65,10 @@ static val frame_type, catch_frame_type, handle_frame_type;
 
 static val deferred_warnings, tentative_defs;
 
+#if CONFIG_EXTRA_DEBUGGING
+static int uw_break_on_error;
+#endif
+
 /* C99 inline instantiations. */
 #if __STDC_VERSION__ >= 199901L
 val uw_block_return(val tag, val result);
@@ -555,6 +559,11 @@ val uw_throw(val sym, val args)
     abort();
   }
 
+#if CONFIG_EXTRA_DEBUGGING
+  if (uw_break_on_error && uw_exception_subtype_p(sym, error_s))
+    breakpt();
+#endif
+
   if (!listp(args))
     args = cons(args, nil);