Fixed stack trace for isolates without any Dart frames.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
===================================================================
--- runtime/vm/isolate.cc	(revision 21220)
+++ runtime/vm/isolate.cc	(working copy)
@@ -884,18 +884,33 @@
   if (status_sync == NULL) {
     status_sync = new Monitor();
   }
+  bool is_main_isolate = (strstr(name(), "$main-") != NULL);
   ScheduleInterrupts(Isolate::kVmStatusInterrupt);
   {
     MonitorLocker ml(status_sync);
     if (stacktrace_ == NULL) {  // It may already be available.
-      ml.Wait();
+      if (is_main_isolate) {
+        // Main isolate may have finished, but waiting for other isolates
+        // to finish. If so, it won't respond to interrupts.
+        ml.Wait(500);

[Ivan Posva, 2013/04/12 14:37:36]

This is quite unfortunate. There must be a way to figure out whether an isolate
will ever run again.

+      } else {
+        ml.Wait();
+      }
     }
   }
   SetVmStatsCallback(NULL);
-  ASSERT(stacktrace_ != NULL);
-  // result is freed by VmStats::WebServer().
   char* result = stacktrace_;
   stacktrace_ = NULL;
+  if (is_main_isolate && result == NULL) {

[Ivan Posva, 2013/04/12 14:37:36]

Why do we only care about the main isolate here? Any isolate can have an empty
stack?

[Tom Ball, 2013/04/16 18:11:02]

Replaced main-only test, with test for whether isolate is runnable.

+    // Return empty stack.
+    TextBuffer buffer(256);
+    buffer.Printf("{ \"handle\": \"0x%"Px64"\", \"stacktrace\": []}",
+                  reinterpret_cast<int64_t>(this));
+
+    result = strndup(buffer.buf(), buffer.length());
+  }
+  ASSERT(result != NULL);
+  // result is freed by VmStats::WebServer().
   return result;
 }