Bug 1584143 - Don't try to reinitialize platform font list from GetDefaultFont() if we're not on the main thread. r=jwatt

This can happen if we need to use gfxFontGroup::GetDefaultFont() during stylo traversal,
but we initially failed to create the required font because the font list is stale.
In this case, use a "last-resort" default font entry as a stopgap until the font list
update is completed.

Differential Revision: https://phabricator.services.mozilla.com/D47637

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

gfx/thebes/gfxPlatformFontList.cpp

@@ -386,6 +386,9 @@ bool gfxPlatformFontList::AddWithLegacyFamilyName(const nsACString& aLegacyName,
 }
 
 nsresult gfxPlatformFontList::InitFontList() {
+  // This shouldn't be called from stylo threads!
+  MOZ_ASSERT(NS_IsMainThread());
+
   mFontlistInitCount++;
 
   if (LOG_FONTINIT_ENABLED()) {
@@ -483,6 +486,21 @@ nsresult gfxPlatformFontList::InitFontList() {
     ApplyWhitelist();
   }
 
+  // Set up mDefaultFontEntry as a "last resort" default that we can use
+  // to avoid crashing if the font list is otherwise unusable.
+  gfxFontStyle defStyle;
+  FontFamily fam = GetDefaultFont(&defStyle);
+  if (fam.mIsShared) {
+    auto face = fam.mShared->FindFaceForStyle(SharedFontList(), defStyle);
+    if (!face) {
+      mDefaultFontEntry = nullptr;
+    } else {
+      mDefaultFontEntry = GetOrCreateFontEntry(face, fam.mShared);
+    }
+  } else {
+    mDefaultFontEntry = fam.mUnshared->FindFontForStyle(defStyle);
+  }
+
   return NS_OK;
 }
 

gfx/thebes/gfxPlatformFontList.h

@@ -303,6 +303,10 @@ class gfxPlatformFontList : public gfxFontInfoLoader {
   // get the system default font family
   FontFamily GetDefaultFont(const gfxFontStyle* aStyle);
 
+  // get the "ultimate" default font, for use if the font list is otherwise
+  // unusable (e.g. in the middle of being updated)
+  gfxFontEntry* GetDefaultFontEntry() { return mDefaultFontEntry.get(); }
+
   /**
    * Look up a font by name on the host platform.
    *
@@ -797,6 +801,8 @@ class gfxPlatformFontList : public gfxFontInfoLoader {
   nsRefPtrHashtable<nsPtrHashKey<mozilla::fontlist::Face>, gfxFontEntry>
       mFontEntries;
 
+  RefPtr<gfxFontEntry> mDefaultFontEntry;
+
   bool mFontFamilyWhitelistActive;
 };
 

gfx/thebes/gfxTextRun.cpp

@@ -1992,10 +1992,19 @@ gfxFont* gfxFontGroup::GetDefaultFont() {
     // If we're a content process, it's possible this is failing because the
     // chrome process has just updated the shared font list and we haven't yet
     // refreshed our reference to it. If that's the case, update and retry.
-    uint32_t oldGeneration = pfl->SharedFontList()->GetGeneration();
-    pfl->UpdateFontList();
-    if (pfl->SharedFontList()->GetGeneration() != oldGeneration) {
-      return GetDefaultFont();
+    // But if we're not on the main thread, we can't do this, so just use
+    // the platform default font directly.
+    if (NS_IsMainThread()) {
+      uint32_t oldGeneration = pfl->SharedFontList()->GetGeneration();
+      pfl->UpdateFontList();
+      if (pfl->SharedFontList()->GetGeneration() != oldGeneration) {
+        return GetDefaultFont();
+      }
+    } else {
+      gfxFontEntry* fe = pfl->GetDefaultFontEntry();
+      if (fe) {
+        return fe->FindOrMakeFont(&mStyle);
+      }
     }
   }