Add more diagnostics to the deferred loading implementation.

sdk/lib/_internal/js_runtime/lib/js_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/js_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_helper.dart b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
index bf21636123cc218295666994b8b50fb01ddad721..77058e95a9a73eab3bf4d6d7dd55cbff8f491f8d 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -3647,6 +3647,9 @@ LoadLibraryFunctionType _loadLibraryWrapper(String loadId) {
 final Map<String, Future<Null>> _loadingLibraries = <String, Future<Null>>{};
 final Set<String> _loadedLibraries = new Set<String>();
 
+/// Events used to diagnose failures from deferred loading requests.
+final List<String> _eventLog = <String>[];

[sra1, 2017/08/14 18:24:08]

How do we get hold of this in a crash log?

[Siggi Cherem (dart-lang), 2017/08/14 18:59:43]

I'm attaching it on the exceptions that we throw explicitly, are you suggesting
we make it more easily accessible as well (e.g. make it an embedded global or
something that is not renamed and can be accessed from JS?)

[sra1, 2017/08/14 19:38:29]

That is fine.

+
 typedef void DeferredLoadCallback();
 
 // Function that will be called every time a new deferred import is loaded.
@@ -3674,15 +3677,30 @@ Future<Null> loadDeferredLibrary(String loadId) {
     // Now all hunks have been loaded, we run the needed initializers.
     var isHunkInitialized = JS_EMBEDDED_GLOBAL('', IS_HUNK_INITIALIZED);
     var initializer = JS_EMBEDDED_GLOBAL('', INITIALIZE_LOADED_HUNK);
-    for (String hash in hashes) {
+    for (int i = 0; i < hashes.length; ++i) {
       // It is possible for a hash to be repeated. This happens when two
       // different parts both end up empty. Checking in the loop rather than
       // pre-filtering prevents duplicate hashes leading to duplicated
       // initializations.
       // TODO(29572): Merge small parts.
       // TODO(29635): Remove duplicate parts from tables and output files.
-      if (JS('bool', '#(#)', isHunkInitialized, hash)) continue;
-      JS('void', '#(#)', initializer, hash);
+      var uri = uris[i];
+      var hash = hashes[i];
+      if (JS('bool', '#(#)', isHunkInitialized, hash)) {
+        _eventLog.add(' - already initialized: $uri ($hash)');
+        continue;
+      }
+      // On strange scenarios, e.g. if js encounters parse errors, we might get
+      // an "success" callback on the script load but the hunk will be null.
+      if (JS('bool', '#(#)', isHunkLoaded, hash)) {
+        _eventLog.add(' - initialize: $uri ($hash)');
+        JS('void', '#(#)', initializer, hash);
+      } else {
+        _eventLog.add(' - missing hunk: $uri ($hash)');
+        throw new DeferredLoadException("Loading ${uris[i]} failed: "
+            "the code with hash '${hash}' was not loaded.\n"
+            "event log:\n${_eventLog.join("\n")}\n");
+      }
     }
     bool updated = _loadedLibraries.add(loadId);
     if (updated && deferredLoadHook != null) {
@@ -3693,7 +3711,9 @@ Future<Null> loadDeferredLibrary(String loadId) {
 
 Future<Null> _loadHunk(String hunkName) {
   Future<Null> future = _loadingLibraries[hunkName];
+  _eventLog.add(' - _loadHunk: $hunkName');
   if (future != null) {
+    _eventLog.add('reuse: $hunkName');
     return future.then((_) => null);
   }
 
@@ -3701,23 +3721,29 @@ Future<Null> _loadHunk(String hunkName) {
 
   int index = uri.lastIndexOf('/');
   uri = '${uri.substring(0, index + 1)}$hunkName';
+  _eventLog.add(' - download: $hunkName from $uri');
 
   var deferredLibraryLoader = JS('', 'self.dartDeferredLibraryLoader');
   Completer<Null> completer = new Completer<Null>();
 
   void success() {
+    _eventLog.add(' - download success: $hunkName');
     completer.complete(null);
   }
 
-  void failure([error, StackTrace stackTrace]) {
+  void failure(error, String context, StackTrace stackTrace) {
+    _eventLog.add(' - download failed: $hunkName (context: $context)');
     _loadingLibraries[hunkName] = null;
-    completer.completeError(
-        new DeferredLoadException('Loading $uri failed: $error'), stackTrace);
+    stackTrace ??= StackTrace.current;
+    completer.completeError(new DeferredLoadException(
+          'Loading $uri failed: $error\n'
+          'event log:\n${_eventLog.join("\n")}\n'), stackTrace);
   }
 
   var jsSuccess = convertDartClosureToJS(success, 0);
   var jsFailure = convertDartClosureToJS((error) {
-    failure(unwrapException(error), getTraceFromException(error));
+    failure(unwrapException(error), 'js-failure-wrapper',
+        getTraceFromException(error));
   }, 1);
 
   if (JS('bool', 'typeof # === "function"', deferredLibraryLoader)) {
@@ -3725,7 +3751,7 @@ Future<Null> _loadHunk(String hunkName) {
       JS('void', '#(#, #, #)', deferredLibraryLoader, uri, jsSuccess,
           jsFailure);
     } catch (error, stackTrace) {
-      failure(error, stackTrace);
+      failure(error, "invoking dartDeferredLibraryLoader hook", stackTrace);
     }
   } else if (isWorker()) {
     // We are in a web worker. Load the code with an XMLHttpRequest.
@@ -3745,7 +3771,7 @@ Future<Null> _loadHunk(String hunkName) {
         convertDartClosureToJS((event) {
           int status = JS('int', '#.status', xhr);
           if (status != 200) {
-            failure('Request status: $status');
+            failure('Request status: $status', 'worker xhr', null);
           }
           String code = JS('String', '#.responseText', xhr);
           try {
@@ -3754,12 +3780,16 @@ Future<Null> _loadHunk(String hunkName) {
             JS('void', '(new Function(#))()', code);
             success();
           } catch (error, stackTrace) {
-            failure(error, stackTrace);
+            failure(error, 'evaluating the code in worker xhr', stackTrace);
           }
         }, 1));
 
-    JS('void', '#.addEventListener("error", #, false)', xhr, failure);
-    JS('void', '#.addEventListener("abort", #, false)', xhr, failure);
+    JS('void', '#.addEventListener("error", #, false)', xhr, (e) {
+      failure(e, 'xhr error handler', null);
+    });
+    JS('void', '#.addEventListener("abort", #, false)', xhr, (e) {
+      failure(e, 'xhr abort handler', null);
+    });
     JS('void', '#.send()', xhr);
   } else {
     // We are in a dom-context.