SkOnce in is_lcd_supported instead of hand rolled double-checked locking.

src/ports/SkFontHost_FreeType.cpp

Index: src/ports/SkFontHost_FreeType.cpp
diff --git a/src/ports/SkFontHost_FreeType.cpp b/src/ports/SkFontHost_FreeType.cpp
index fe32ebb42a7fe33c7e7758738cc40f26f7afaa8d..9aa9283f801f6bf477a96e50531669b608206241 100644
--- a/src/ports/SkFontHost_FreeType.cpp
+++ b/src/ports/SkFontHost_FreeType.cpp
@@ -6,6 +6,7 @@
  * found in the LICENSE file.
  */
 
+#include "SkAdvancedTypefaceMetrics.h"
 #include "SkBitmap.h"
 #include "SkCanvas.h"
 #include "SkColorPriv.h"
@@ -18,7 +19,7 @@
 #include "SkMask.h"
 #include "SkMaskGamma.h"
 #include "SkOTUtils.h"
-#include "SkAdvancedTypefaceMetrics.h"
+#include "SkOnce.h"
 #include "SkScalerContext.h"
 #include "SkStream.h"
 #include "SkString.h"
@@ -160,16 +161,22 @@ static bool InitFreetype() {
     return true;
 }
 
+static void determine_lcd_support(int) {
+    SkAutoMutexAcquire lock(gFTMutex);
+    if (!gLCDSupportValid) {
+        // This will determine LCD support as a side effect.
+        InitFreetype();
+        FT_Done_FreeType(gFTLibrary);
+    }
+    SkASSERT(gLCDSupportValid);
+}
+
 // Lazy, once, wrapper to ask the FreeType Library if it can support LCD text
 static bool is_lcd_supported() {
-    if (!gLCDSupportValid) {
-        SkAutoMutexAcquire  ac(gFTMutex);
+    SK_DECLARE_STATIC_ONCE(once);
+    SkOnce(&once, determine_lcd_support, 0);
 
-        if (!gLCDSupportValid) {
-            InitFreetype();
-            FT_Done_FreeType(gFTLibrary);
-        }
-    }
+    SkAutoMutexAcquire lock(gFTMutex);

[bungeman-skia, 2014/02/05 18:02:04]

Why are we still taking this lock, just to return a global?

[mtklein, 2014/02/05 18:10:26]

You know, that's a thought provoking question.

I was going to say, for consistency with the rest of the locking discipline
around these freetype globals.  gLCDSupport, gLCDExtra, and gLCDSupportValid are
weird near-consts that are the exception to the normal locking discipline rather
than the norm.

But now I'm looking at it, and clearly this is more complicated but no different
than just writing

static bool is_lcd_supported() {
  SkAutoMutexAcquire lock(gFTMutex);
  if (!gLCDSupportValid) { ... }
  return gLCDSupport;
}

It bothers me to have two locks involved here.  Going back to drawing board.

     return gLCDSupport;
 }