Bug 1292275 - Stylo: Fix crash after failed stylesheet load. r=heycam

Fixes a crash ServoStyleSet::AddDocStyleSheet caused by
ServoStyleSheet::RawSheet returning null.

MozReview-Commit-ID: BdDosompqTv

--HG--
extra : rebase_source : 0b46c9f9327eec0bac3bb2ca697198887948e2e6

layout/style/Loader.cpp

@@ -1815,6 +1815,10 @@ Loader::SheetComplete(SheetLoadData* aLoadData, nsresult aStatus)
 {
   LOG(("css::Loader::SheetComplete"));
 
+  if (aLoadData->mSheet->IsServo() && NS_FAILED(aStatus)) {
+    aLoadData->mSheet->AsServo()->LoadFailed();
+  }
+
   // 8 is probably big enough for all our common cases.  It's not likely that
   // imports will nest more than 8 deep, and multiple sheets with the same URI
   // are rare.

layout/style/ServoBindingList.h

@@ -22,6 +22,8 @@
 SERVO_BINDING_FUNC(Servo_Node_ClearNodeData, void, RawGeckoNodeBorrowed node)
 
 // Styleset and Stylesheet management
+SERVO_BINDING_FUNC(Servo_StyleSheet_Empty, RawServoStyleSheetStrong,
+                   mozilla::css::SheetParsingMode parsing_mode)
 SERVO_BINDING_FUNC(Servo_StyleSheet_FromUTF8Bytes, RawServoStyleSheetStrong,
                    const nsACString* data,
                    mozilla::css::SheetParsingMode parsing_mode,

layout/style/ServoStyleSheet.cpp

@@ -81,6 +81,12 @@ ServoStyleSheet::ParseSheet(const nsAString& aInput,
   return NS_OK;
 }
 
+void
+ServoStyleSheet::LoadFailed()
+{
+  mSheet = Servo_StyleSheet_Empty(mParsingMode).Consume();
+}
+
 void
 ServoStyleSheet::DropSheet()
 {

layout/style/ServoStyleSheet.h

@@ -40,6 +40,13 @@ public:
                                    nsIPrincipal* aSheetPrincipal,
                                    uint32_t aLineNumber);
 
+  /**
+   * Called instead of ParseSheet to initialize the Servo stylesheet object
+   * for a failed load. Either ParseSheet or LoadFailed must be called before
+   * adding a ServoStyleSheet to a ServoStyleSet.
+   */
+  void LoadFailed();
+
   size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const;
 
 #ifdef DEBUG