Bug 1522417 - Add diagnostics. r=jwatt

I haven't managed to find a way stuff should unexpectedly die around these
functions, so add a few diagnostics to catch this hopefully, before papering
over the bug / backing out / adding a kungfudeathgrip.

Differential Revision: https://phabricator.services.mozilla.com/D17514

--HG--
extra : moz-landing-system : lando

layout/style/nsFontFaceLoader.cpp

@@ -12,6 +12,7 @@
 #include "nsFontFaceLoader.h"
 
 #include "nsError.h"
+#include "mozilla/AutoRestore.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/StaticPrefs.h"
 #include "mozilla/Telemetry.h"
@@ -59,6 +60,8 @@ nsFontFaceLoader::nsFontFaceLoader(gfxUserFontEntry* aUserFontEntry,
 }
 
 nsFontFaceLoader::~nsFontFaceLoader() {
+  MOZ_DIAGNOSTIC_ASSERT(!mInLoadTimerCallback);
+  MOZ_DIAGNOSTIC_ASSERT(!mInStreamComplete);
   if (mUserFontEntry) {
     mUserFontEntry->mLoader = nullptr;
   }
@@ -96,6 +99,11 @@ void nsFontFaceLoader::StartedLoading(nsIStreamLoader* aStreamLoader) {
                                                       void* aClosure) {
   nsFontFaceLoader* loader = static_cast<nsFontFaceLoader*>(aClosure);
 
+  MOZ_DIAGNOSTIC_ASSERT(!loader->mInLoadTimerCallback);
+  MOZ_DIAGNOSTIC_ASSERT(!loader->mInStreamComplete);
+  AutoRestore<bool> scope { loader->mInLoadTimerCallback };
+  loader->mInLoadTimerCallback = true;
+
   if (!loader->mFontFaceSet) {
     // We've been canceled
     return;
@@ -190,6 +198,11 @@ nsFontFaceLoader::OnStreamComplete(nsIStreamLoader* aLoader,
                                    uint32_t aStringLen,
                                    const uint8_t* aString) {
   MOZ_ASSERT(NS_IsMainThread());
+  MOZ_DIAGNOSTIC_ASSERT(!mInLoadTimerCallback);
+  MOZ_DIAGNOSTIC_ASSERT(!mInStreamComplete);
+
+  AutoRestore<bool> scope { mInStreamComplete };
+  mInStreamComplete = true;
 
   DropChannel();
 
@@ -198,8 +211,6 @@ nsFontFaceLoader::OnStreamComplete(nsIStreamLoader* aLoader,
     return aStatus;
   }
 
-  mFontFaceSet->RemoveLoader(this);
-
   TimeStamp doneTime = TimeStamp::Now();
   TimeDuration downloadTime = doneTime - mStartTime;
   uint32_t downloadTimeMS = uint32_t(downloadTime.ToMilliseconds());
@@ -272,6 +283,8 @@ nsFontFaceLoader::OnStreamComplete(nsIStreamLoader* aLoader,
     }
   }
 
+  MOZ_DIAGNOSTIC_ASSERT(mFontFaceSet);
+  mFontFaceSet->RemoveLoader(this);
   // done with font set
   mFontFaceSet = nullptr;
   if (mLoadTimer) {
@@ -305,6 +318,10 @@ nsFontFaceLoader::OnStopRequest(nsIRequest* aRequest, nsISupports* aContext,
 }
 
 void nsFontFaceLoader::Cancel() {
+  MOZ_DIAGNOSTIC_ASSERT(!mInLoadTimerCallback);
+  MOZ_DIAGNOSTIC_ASSERT(!mInStreamComplete);
+  MOZ_DIAGNOSTIC_ASSERT(mFontFaceSet);
+
   mUserFontEntry->LoadCanceled();
   mFontFaceSet = nullptr;
   if (mLoadTimer) {

layout/style/nsFontFaceLoader.h

@@ -61,6 +61,8 @@ class nsFontFaceLoader final : public nsIStreamLoaderObserver,
   nsCOMPtr<nsITimer> mLoadTimer;
   mozilla::TimeStamp mStartTime;
   nsIStreamLoader* mStreamLoader;
+  bool mInStreamComplete = false;
+  bool mInLoadTimerCallback = false;
 };
 
 #endif /* !defined(nsFontFaceLoader_h_) */