Sketch of SK_ONCE

tests/OnceTest.cpp

+#include "SkOnce.h"
+#include "SkRunnable.h"
+#include "SkThreadPool.h"
+#include "Test.h"
+#include "TestClassDef.h"
+
+DEF_SK_ONCE(add_five, int* x) {
+    *x += 5;
+}
+
+DEF_TEST(SkOnce_Singlethreaded, r) {
+    int x = 0;
+
+    // No matter how many times we do this, x will be 5.
+    SK_ONCE(add_five, &x);
+    SK_ONCE(add_five, &x);
+    SK_ONCE(add_five, &x);
+    SK_ONCE(add_five, &x);
+    SK_ONCE(add_five, &x);
+
+    REPORTER_ASSERT(r, 5 == x);
+}
+
+
+DEF_SK_ONCE(add_six, int* x) {
+    *x += 6;
+}
+
+DEF_TEST(SkOnce_Multithreaded, r) {
+    const int kTasks = 16, kThreads = 4;
+
+    // Make a bunch of tasks that will race to be the first to add six.
+    struct : public SkRunnable {
+        int* ptr;
+        virtual void run() SK_OVERRIDE {
+            SK_ONCE(add_six, ptr);
+        }
+    } racers[kTasks];
+
+    // Point them at x.
+    int x = 0;
+    for (int i = 0; i < kTasks; i++) {
+        racers[i].ptr = &x;
+    }
+
+    // Let them race.
+    SkAutoTDelete<SkThreadPool> pool(new SkThreadPool(kThreads));
+    for (int i = 0; i < kTasks; i++) {
+        pool->add(&racers[i]);
+    }
+    pool.free();  // Blocks until all threads are done.
+
+    // Only one should have done the +=.
+    REPORTER_ASSERT(r, 6 == x);
+}