Bug 1663499 - fix ending webbrowserpersist downloading so it only happens once, r=valentin

This fixes a few causes of firing EndDownload twice:

1. firing it from OnDataAvailable if the download was already canceled when the
   method was initially invoked, rather than if we are ourselves wanting to
   cancel the download because we encountered issues inside OnDataAvailable.
2. firing it from FinishDownload, dispatched from SerializeNextFile, after the
   download has already been ended elsewhere.
3. calling Cancel() multiple times.

It also adds some code to avoid the re-entrancy on the Promise code that the
bug was originally filed for, and a diagnostic assert to check if there are any
other cases of repeated EndDownload calling that we've missed.

As a driveby, it adds a few thread assertions to help with code clarity.

Differential Revision: https://phabricator.services.mozilla.com/D90186

dom/webbrowserpersist/nsWebBrowserPersist.cpp

@@ -290,6 +290,7 @@ nsWebBrowserPersist::nsWebBrowserPersist()
       mFirstAndOnlyUse(true),
       mSavingDocument(false),
       mCancel(false),
+      mEndCalled(false),
       mCompleted(false),
       mStartSaving(false),
       mReplaceExisting(true),
@@ -518,6 +519,10 @@ NS_IMETHODIMP nsWebBrowserPersist::SaveDocument(nsISupports* aDocument,
 }
 
 NS_IMETHODIMP nsWebBrowserPersist::Cancel(nsresult aReason) {
+  // No point cancelling if we're already complete.
+  if (mEndCalled) {
+    return NS_OK;
+  }
   mCancel = true;
   EndDownload(aReason);
   return NS_OK;
@@ -957,6 +962,8 @@ NS_IMETHODIMP
 nsWebBrowserPersist::OnDataAvailable(nsIRequest* request,
                                      nsIInputStream* aIStream, uint64_t aOffset,
                                      uint32_t aLength) {
+  // MOZ_ASSERT(!NS_IsMainThread()); // no guarantees, but it's likely.
+
   bool cancel = mCancel;
   if (!cancel) {
     nsresult rv = NS_OK;
@@ -1069,16 +1076,14 @@ nsWebBrowserPersist::OnDataAvailable(nsIRequest* request,
             self->SendErrorStatusChange(readError, rv, req, file);
           });
       NS_DispatchToMainThread(errorOnMainThread);
-    }
-  }
 
-  // Cancel reading?
-  if (cancel) {
+      // And end the download on the main thread.
       nsCOMPtr<nsIRunnable> endOnMainThread = NewRunnableMethod<nsresult>(
           "nsWebBrowserPersist::EndDownload", this,
           &nsWebBrowserPersist::EndDownload, NS_BINDING_ABORTED);
       NS_DispatchToMainThread(endOnMainThread);
     }
+  }
 
   return cancel ? NS_BINDING_ABORTED : NS_OK;
 }
@@ -2294,13 +2299,34 @@ nsresult nsWebBrowserPersist::MakeOutputStreamFromURI(
   return NS_OK;
 }
 
-void nsWebBrowserPersist::FinishDownload() { EndDownload(NS_OK); }
+void nsWebBrowserPersist::FinishDownload() {
+  // We call FinishDownload when we run out of things to download for this
+  // persist operation, by dispatching this method to the main thread. By now,
+  // it's possible that we have been canceled or encountered an error earlier
+  // in the download, or something else called EndDownload. In that case, don't
+  // re-run EndDownload.
+  if (mEndCalled) {
+    return;
+  }
+  EndDownload(NS_OK);
+}
 
 void nsWebBrowserPersist::EndDownload(nsresult aResult) {
+  MOZ_ASSERT(NS_IsMainThread(), "Should end download on the main thread.");
+
+  MOZ_DIAGNOSTIC_ASSERT(!mEndCalled, "Should only end the download once.");
+  // Really this should just never happen, but if it does, at least avoid
+  // no-op notifications or pretending we succeeded if we already failed.
+  if (mEndCalled && (NS_SUCCEEDED(aResult) || mPersistResult == aResult)) {
+    return;
+  }
+  mEndCalled = true;
+
   // Store the error code in the result if it is an error
   if (NS_SUCCEEDED(mPersistResult) && NS_FAILED(aResult)) {
     mPersistResult = aResult;
   }
+
   ClosePromise::All(GetCurrentSerialEventTarget(), mFileClosePromises)
       ->Then(GetCurrentSerialEventTarget(), __func__,
              [self = RefPtr{this}, aResult]() {

dom/webbrowserpersist/nsWebBrowserPersist.h

@@ -168,6 +168,7 @@ class nsWebBrowserPersist final : public nsIInterfaceRequestor,
   // mCancel is used from both the main thread, and (inside OnDataAvailable)
   // from a background thread.
   mozilla::Atomic<bool> mCancel;
+  bool mEndCalled;
   bool mCompleted;
   bool mStartSaving;
   bool mReplaceExisting;