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

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