runtime: if "panic during panic"'s stacktrace fails, don't recurse.

R=golang-codereviews, iant, khr, dvyukov
CC=golang-codereviews
https://golang.org/cl/54160043

diff --git a/src/pkg/runtime/panic.c b/src/pkg/runtime/panic.c
index 7bd408aea8b64592c42c15bcbf8d9a1c730ab7e4..73185273cbd911d88d6c2b0ee8967590c0d1a2f0 100644 (file)
--- a/src/pkg/runtime/panic.c
+++ b/src/pkg/runtime/panic.c
@@ -355,19 +355,34 @@ runtime·startpanic(void)
                m->mallocing = 1; // tell rest of panic not to try to malloc
        } else if(m->mcache == nil) // can happen if called from signal handler or throw
                m->mcache = runtime·allocmcache();
-       if(m->dying) {
+       switch(m->dying) {
+       case 0:
+               m->dying = 1;
+               if(g != nil)
+                       g->writebuf = nil;
+               runtime·xadd(&runtime·panicking, 1);
+               runtime·lock(&paniclk);
+               if(runtime·debug.schedtrace > 0 || runtime·debug.scheddetail > 0)
+                       runtime·schedtrace(true);
+               runtime·freezetheworld();
+               return;
+       case 1:
+               // Something failed while panicing, probably the print of the
+               // argument to panic().  Just print a stack trace and exit.
+               m->dying = 2;
                runtime·printf("panic during panic\n");
                runtime·dopanic(0);
-               runtime·exit(3); // not reached
+               runtime·exit(3);
+       case 2:
+               // This is a genuine bug in the runtime, we couldn't even
+               // print the stack trace successfully.
+               m->dying = 3;
+               runtime·printf("stack trace unavailable\n");
+               runtime·exit(4);
+       default:
+               // Can't even print!  Just exit.
+               runtime·exit(5);
        }
-       m->dying = 1;
-       if(g != nil)
-               g->writebuf = nil;
-       runtime·xadd(&runtime·panicking, 1);
-       runtime·lock(&paniclk);
-       if(runtime·debug.schedtrace > 0 || runtime·debug.scheddetail > 0)
-               runtime·schedtrace(true);
-       runtime·freezetheworld();
 }
 
 void