Tracebacks for async coroutines now provide a 'best guess' for where the coroutine was at the time that an exception was thrown, by showing the frame at which the generator's last continuation callback was created.

Added a 'location' property to RequestException, analogous to the 'location' property of wrapped nsIExceptions, so that tracebacks can be made for the exceptions if needed.

services/sync/modules/async.js

@@ -115,6 +115,7 @@ function Generator(thisArg, method, onComplete, args) {
   this._id = gCurrentId++;
   this.onComplete = onComplete;
   this._args = args;
+  this._stackAtLastCallbackGen = null;
 
   gOutstandingGenerators.add(this);
 
@@ -132,6 +133,7 @@ Generator.prototype = {
     let caller = Components.stack.caller;
     let cbId = gCurrentCbId++;
     this._outstandingCbs++;
+    this._stackAtLastCallbackGen = caller;
     this._log.trace(this.name + ": cb-" + cbId + " generated at:\n" +
                     formatAsyncFrame(caller));
     let self = this;
@@ -172,7 +174,11 @@ Generator.prototype = {
   },
 
   get asyncStack() {
-    return ("unknown (async) :: " + this.name + "\n" +
+    let cbGenText = "";
+    if (this._stackAtLastCallbackGen)
+      cbGenText = (" (last self.cb generated at " +
+                   formatAsyncFrame(this._stackAtLastCallbackGen) + ")");
+    return ("unknown (async) :: " + this.name + cbGenText + "\n" +
             traceAsyncFrame(this._initFrame));
   },
 

services/sync/modules/remote.js

@@ -58,6 +58,7 @@ function RequestException(resource, action, request) {
   this._resource = resource;
   this._action = action;
   this._request = request;
+  this.location = Components.stack.caller;
 }
 RequestException.prototype = {
   get resource() { return this._resource; },