Add and use SkSemaphore

src/core/SkSemaphore.h

Index: src/core/SkSemaphore.h
diff --git a/src/core/SkSemaphore.h b/src/core/SkSemaphore.h
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+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkSemaphore_DEFINED
+#define SkSemaphore_DEFINED
+
+#include "SkTypes.h"
+#include "SkAtomics.h"
+
+/**
+ * SkSemaphore is a fast mostly-user-space semaphore.
+ *
+ * A semaphore is logically an atomic integer with a few special properties:
+ *   - The integer always starts at 0.
+ *   - You can only increment or decrement it, never read or write it.
+ *   - Increment is spelled 'signal()'; decrement is spelled 'wait()'.
+ *   - If a call to wait() decrements the counter to <= 0,
+ *     the calling thread sleeps until another thread signal()s it back above 0.
+ */
+class SkSemaphore : SkNoncopyable {
+public:
+    // Initializes the counter to 0.
+    // (Though all current implementations could start from an arbitrary value.)
+    SkSemaphore();
+    ~SkSemaphore();
+
+    // Increment the counter N times.
+    // Generally it's better to call signal(N) instead of signal() N times.
+    void signal(int N = 1);
+
+    // Decrement the counter by 1,
+    // then if the counter is <= 0, sleep this thread until the counter is > 0.
+    void wait();
+
+private:
+    // This implementation follows the general strategy of
+    //     'A Lightweight Semaphore with Partial Spinning'
+    // found here
+    //     http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
+    // That article (and entire blog) are very much worth reading.
+    //
+    // We wrap an OS-provided semaphore with a user-space atomic counter that
+    // lets us avoid interacting with the OS semaphore unless strictly required:
+    // moving the count from >0 to <=0 or vice-versa, i.e. sleeping or waking threads.
+    struct OSSemaphore;
+
+    SkAtomic<int>  fCount;
+    OSSemaphore*   fOSSemaphore;
+};
+
+#endif//SkSemaphore_DEFINED