Add and use SkSemaphore

src/core/SkSemaphore.cpp

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkSemaphore.h"
+
+#if defined(SK_BUILD_FOR_MAC) || defined(SK_BUILD_FOR_IOS)
+    #include <mach/mach.h>
+    struct SkSemaphore::OSSemaphore {
+        semaphore_t fSemaphore;
+
+        OSSemaphore()  {
+            semaphore_create(mach_task_self(), &fSemaphore, SYNC_POLICY_FIFO, 0/*initial count*/);
+        }
+        ~OSSemaphore() { semaphore_destroy(mach_task_self(), fSemaphore); }
+
+        void signal(int n) { while (n --> 0) { semaphore_signal(fSemaphore); } }
+        void wait() { semaphore_wait(fSemaphore); }
+    };
+#elif defined(SK_BUILD_FOR_WIN32)
+    struct SkSemaphore::OSSemaphore {
+        HANDLE fSemaphore;
+
+        OSSemaphore()  {
+            fSemaphore = CreateSemaphore(NULL    /*security attributes, optional*/,
+                                         0       /*initial count*/,
+                                         MAXLONG /*max count*/,
+                                         NULL    /*name, optional*/);
+        }
+        ~OSSemaphore() { CloseHandle(fSemaphore); }
+
+        void signal(int n) {
+            ReleaseSemaphore(fSemaphore, n, NULL/*returns previous count, optional*/);
+        }
+        void wait() { WaitForSingleObject(fSemaphore, INFINITE/*timeout in ms*/); }
+    };
+#else
+    // It's important we test for Mach before this.  This code will compile but not work there.
+    #include <errno.h>
+    #include <semaphore.h>
+    struct SkSemaphore::OSSemaphore {
+        sem_t fSemaphore;
+
+        OSSemaphore()  { sem_init(&fSemaphore, 0/*cross process?*/, 0/*initial count*/); }
+        ~OSSemaphore() { sem_destroy(&fSemaphore); }
+
+        void signal(int n) { while (n --> 0) { sem_post(&fSemaphore); } }
+        void wait() {
+            // Try until we're not interrupted.
+            while(sem_wait(&fSemaphore) == -1 && errno == EINTR);
+        }
+    };
+#endif
+
+SkSemaphore::SkSemaphore() : fCount(0), fOSSemaphore(SkNEW(OSSemaphore)) {}
+SkSemaphore::~SkSemaphore() { SkDELETE(fOSSemaphore); }
+
+void SkSemaphore::signal(int n) {
+    SkASSERT(n >= 0);
+
+    // We only want to call the OS semaphore when our logical count crosses
+    // from <= 0 to >0 (when we need to wake sleeping threads).
+    //
+    // This is easiest to think about with specific examples of prev and n.
+    // If n == 5 and prev == -3, there are 3 threads sleeping and we signal
+    // SkTMin(-(-3), 5) == 3 times on the OS semaphore, leaving the count at 2.
+    //
+    // If prev >= 0, no threads are waiting, SkTMin(-prev, n) is always <= 0,
+    // so we don't call the OS semaphore, leaving the count at (prev + n).
+    int prev = fCount.fetch_add(n, sk_memory_order_release);
+    int toSignal = SkTMin(-prev, n);
+    if (toSignal > 0) {
+        fOSSemaphore->signal(toSignal);
+    }
+}
+
+void SkSemaphore::wait() {
+    // We only wait() on the OS semaphore if the count drops <= 0,
+    // i.e. when we need to make this thread sleep to wait for it to go back up.
+    if (fCount.fetch_add(-1, sk_memory_order_acquire) <= 0) {
+        fOSSemaphore->wait();
+    }
+}