Bug 1664151 - Drop FC_CHARSET element from fontconfig patterns for TrueType/OpenType fonts, as we will read the cmap directly anyhow. r=lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D117423

gfx/thebes/SharedFontList-impl.h

@@ -287,7 +287,12 @@ class FontList {
   // systems, and memory is more constrained, so use a smaller default block
   // size.
   static constexpr uint32_t SHM_BLOCK_SIZE = 64 * 1024;
+#elif XP_LINUX
+  // On Linux, font face descriptors are rather large (serialized FcPatterns),
+  // so use a larger block size for efficiency.
+  static constexpr uint32_t SHM_BLOCK_SIZE = 1024 * 1024;
 #else
+  // Default block size for Windows and macOS.
   static constexpr uint32_t SHM_BLOCK_SIZE = 256 * 1024;
 #endif
   static_assert(SHM_BLOCK_SIZE <= (1 << Pointer::kBlockShift),

gfx/thebes/gfxFcPlatformFontList.cpp

@@ -1781,7 +1781,22 @@ void gfxFcPlatformFontList::InitSharedFontListForPlatform() {
       }
 #endif
 
-      addPattern(pattern, lastFamilyName, familyName, aAppFonts);
+      // If this is a TrueType or OpenType font, discard the FC_CHARSET object
+      // (which may be very large), because we'll read the 'cmap' directly.
+      // This substantially reduces the pressure on shared memory (bug 1664151)
+      // due to the large font descriptors (serialized patterns).
+      FcChar8* fontFormat;
+      if (FcPatternGetString(pattern, FC_FONTFORMAT, 0, &fontFormat) ==
+              FcResultMatch &&
+          (!FcStrCmp(fontFormat, (const FcChar8*)"TrueType") ||
+           !FcStrCmp(fontFormat, (const FcChar8*)"CFF"))) {
+        FcPattern* clone = FcPatternDuplicate(pattern);
+        FcPatternDel(clone, FC_CHARSET);
+        addPattern(clone, lastFamilyName, familyName, aAppFonts);
+        FcPatternDestroy(clone);
+      } else {
+        addPattern(pattern, lastFamilyName, familyName, aAppFonts);
+      }
     }
   };