Bug 1749103 - Avoid thrashing gfxDWriteFont's ScaledFonts between canvas and content. r=jfkthame

This is essentially a partial revert of a cleanup done in bug 1742896. If canvas and content
are using the same underlying gfxDWriteFont and simultaneously retrieving a ScaledFont, this
may cause the ScaledFont to get repeatedly destroyed and re-instantiated every time we switch
from one context to the other. If this destruction happens at an inopportune time while WR
is still using a blob image with the font, it can potentially trigger undesirable assertions.

For now, duplicate the part of Bas' original patch from bug 1730722 that separatedly cached
the GDI and non-GDI versions of the ScaledFont. This way, both will exist for the lifetime
of the underlying gfxDWriteFont and thus avoid the thrashing.

Differential Revision: https://phabricator.services.mozilla.com/D135442

gfx/thebes/gfxDWriteFonts.cpp

@@ -84,8 +84,7 @@ gfxDWriteFont::gfxDWriteFont(const RefPtr<UnscaledFontDWrite>& aUnscaledFont,
       mMetrics(nullptr),
       mUseSubpixelPositions(false),
       mAllowManualShowGlyphs(true),
-      mAzureScaledFontUsedClearType(false),
+      mAzureScaledFontUsedClearType(false) {
-      mAzureScaledFontForcedGDI(false) {
   // If the IDWriteFontFace1 interface is available, we can use that for
   // faster glyph width retrieval.
   mFontFace->QueryInterface(__uuidof(IDWriteFontFace1),
@@ -754,12 +753,14 @@ void gfxDWriteFont::AddSizeOfIncludingThis(MallocSizeOf aMallocSizeOf,
 
 already_AddRefed<ScaledFont> gfxDWriteFont::GetScaledFont(
     const TextRunDrawParams& aRunParams) {
-  bool forceGDI = aRunParams.allowGDI && GetForceGDIClassic();
+  if (mAzureScaledFontUsedClearType != UsingClearType()) {
-  if (mAzureScaledFontUsedClearType != UsingClearType() ||
-      mAzureScaledFontForcedGDI != forceGDI) {
     mAzureScaledFont = nullptr;
+    mAzureScaledFontGDI = nullptr;
   }
-  if (!mAzureScaledFont) {
+  bool forceGDI = aRunParams.allowGDI && GetForceGDIClassic();
+  RefPtr<ScaledFont>& azureScaledFont =
+      forceGDI ? mAzureScaledFontGDI : mAzureScaledFont;
+  if (!azureScaledFont) {
     gfxDWriteFontEntry* fe = static_cast<gfxDWriteFontEntry*>(mFontEntry.get());
     bool useEmbeddedBitmap =
         (gfxVars::SystemTextRenderingMode() == DWRITE_RENDERING_MODE_DEFAULT ||
@@ -767,19 +768,17 @@ already_AddRefed<ScaledFont> gfxDWriteFont::GetScaledFont(
         fe->IsCJKFont() && HasBitmapStrikeForSize(NS_lround(mAdjustedSize));
 
     const gfxFontStyle* fontStyle = GetStyle();
-    mAzureScaledFont = Factory::CreateScaledFontForDWriteFont(
+    azureScaledFont = Factory::CreateScaledFontForDWriteFont(
         mFontFace, fontStyle, GetUnscaledFont(), GetAdjustedSize(),
         useEmbeddedBitmap, forceGDI);
-    if (!mAzureScaledFont) {
+    if (!azureScaledFont) {
       return nullptr;
     }
-    InitializeScaledFont();
+    InitializeScaledFont(azureScaledFont);
     mAzureScaledFontUsedClearType = UsingClearType();
-    mAzureScaledFontForcedGDI = forceGDI;
   }
 
-  RefPtr<ScaledFont> scaledFont(mAzureScaledFont);
+  return do_AddRef(azureScaledFont);
-  return scaledFont.forget();
 }
 
 bool gfxDWriteFont::ShouldRoundXOffset(cairo_t* aCairo) const {

gfx/thebes/gfxDWriteFonts.h

@@ -103,7 +103,11 @@ class gfxDWriteFont final : public gfxFont {
   // Used to record the sUseClearType setting at the time mAzureScaledFont
   // was set up, so we can tell if it's stale and needs to be re-created.
   bool mAzureScaledFontUsedClearType;
-  bool mAzureScaledFontForcedGDI;
+
+  // Cache the GDI version of the ScaledFont so that font keys and other
+  // meta-data can remain stable even if there is thrashing between GDI and
+  // non-GDI usage.
+  RefPtr<mozilla::gfx::ScaledFont> mAzureScaledFontGDI;
 
   bool UsingClearType() {
     return mozilla::gfx::gfxVars::SystemTextQuality() == CLEARTYPE_QUALITY;

gfx/thebes/gfxFont.cpp

@@ -1552,14 +1552,15 @@ already_AddRefed<mozilla::gfx::ScaledFont> gfxFont::GetScaledFont(
   return GetScaledFont(params);
 }
 
-void gfxFont::InitializeScaledFont() {
+void gfxFont::InitializeScaledFont(
-  if (!mAzureScaledFont) {
+    const RefPtr<mozilla::gfx::ScaledFont>& aScaledFont) {
+  if (!aScaledFont) {
     return;
   }
 
   float angle = AngleForSyntheticOblique();
   if (angle != 0.0f) {
-    mAzureScaledFont->SetSyntheticObliqueAngle(angle);
+    aScaledFont->SetSyntheticObliqueAngle(angle);
   }
 }
 

gfx/thebes/gfxFont.h

@@ -1901,7 +1901,12 @@ class gfxFont {
   already_AddRefed<mozilla::gfx::ScaledFont> GetScaledFont(
       mozilla::gfx::DrawTarget* aDrawTarget);
 
-  void InitializeScaledFont();
+  // gfxFont implementations may cache ScaledFont versions other than the
+  // default, so InitializeScaledFont must support explicitly specifying
+  // other ScaledFonts than the default to initialize.
+  void InitializeScaledFont(
+      const RefPtr<mozilla::gfx::ScaledFont>& aScaledFont);
+  void InitializeScaledFont() { InitializeScaledFont(mAzureScaledFont); }
 
   bool KerningDisabled() { return mKerningSet && !mKerningEnabled; }