Bug 1649728 - Use WeakPtr in SharedStyleSheetCache::mLoadingDatas. r=…

…firefox-style-system-reviewers,jwatt

This is safer in case Necko fails to notify us. The only repro we have
is fixed by bug 1651661, but this should hopefully be uncontroversial as
well and prevents crashing in release builds.

Differential Revision: https://phabricator.services.mozilla.com/D83642

layout/style/SharedStyleSheetCache.cpp

@@ -504,12 +504,14 @@ void SharedStyleSheetCache::CancelLoadsForLoader(css::Loader& aLoader) {
   // We can't stop in-progress loads because some other loader may care about
   // them.
   for (auto iter = mLoadingDatas.Iter(); !iter.Done(); iter.Next()) {
-    SheetLoadData* data = iter.Data();
-    do {
+    MOZ_DIAGNOSTIC_ASSERT(iter.Data(),
+                          "We weren't properly notified and the load was "
+                          "incorrectly dropped on the floor");
+    for (SheetLoadData* data = iter.Data(); data; data = data->mNext) {
       if (data->mLoader == &aLoader) {
         data->mIsCancelled = true;
       }
-    } while ((data = data->mNext));
+    }
   }
 }
 

layout/style/SharedStyleSheetCache.h

@@ -124,11 +124,12 @@ class SharedStyleSheetCache final : public nsIMemoryReporter {
 
   nsDataHashtable<SheetLoadDataHashKey, CompleteSheet> mCompleteSheets;
   nsRefPtrHashtable<SheetLoadDataHashKey, css::SheetLoadData> mPendingDatas;
-  // The SheetLoadData pointers in mLoadingDatas below are weak references.
+  // The SheetLoadData pointers in mLoadingDatas below are weak references that
+  // get cleaned up when StreamLoader::OnStopRequest gets called.
   //
   // Note that we hold on to all sheet loads, even if in the end they happen not
   // to be cacheable.
-  nsDataHashtable<SheetLoadDataHashKey, css::SheetLoadData*> mLoadingDatas;
+  nsDataHashtable<SheetLoadDataHashKey, WeakPtr<css::SheetLoadData>> mLoadingDatas;
 
   // An origin-to-number-of-registered-documents count, in order to manage cache
   // eviction as described in RegisterLoader / UnregisterLoader.

layout/style/SheetLoadData.h

@@ -39,6 +39,8 @@ static_assert(eAuthorSheetFeatures == 0 && eUserSheetFeatures == 1 &&
               "in SheetLoadData::mParsingMode");
 
 class SheetLoadData final : public PreloaderBase,
+                            // FIXME(bug 1653011): This is a bit unfortunate.
+                            public SupportsWeakPtr<SheetLoadData>,
                             public nsIRunnable,
                             public nsIThreadObserver {
   using MediaMatched = dom::LinkStyle::MediaMatched;