flow::Semaphore for simple platform specific unnamed semaphores. (#2937)

flow/BUILD.gn

@@ -46,6 +46,7 @@ source_set("flow") {
   deps = [
     "//flutter/glue",
     "//flutter/skia",
+    "//flutter/synchronization",
     "//lib/ftl",
     "//mojo/services/gfx/composition/interfaces",
   ]

synchronization/BUILD.gn

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+source_set("synchronization") {
+  sources = [
+    "semaphore.cc",
+    "semaphore.h",
+  ]
+
+  deps = [
+    "//lib/ftl",
+  ]
+}

synchronization/semaphore.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/synchronization/semaphore.h"
+#include "lib/ftl/build_config.h"
+#include "lib/ftl/logging.h"
+
+#if OS_MACOSX
+
+#include <dispatch/dispatch.h>
+
+namespace flutter {
+
+class PlatformSemaphore {
+ public:
+  explicit PlatformSemaphore(uint32_t count)
+      : _sem(dispatch_semaphore_create(count)) {}
+
+  ~PlatformSemaphore() {
+    if (_sem != nullptr) {
+      dispatch_release(reinterpret_cast<dispatch_object_t>(_sem));
+      _sem = nullptr;
+    }
+  }
+
+  bool IsValid() const { return _sem != nullptr; }
+
+  bool TryWait() {
+    if (_sem == nullptr) {
+      return false;
+    }
+
+    return dispatch_semaphore_wait(_sem, DISPATCH_TIME_NOW) == 0;
+  }
+
+  void Signal() {
+    if (_sem != nullptr) {
+      dispatch_semaphore_signal(_sem);
+    }
+  }
+
+ private:
+  dispatch_semaphore_t _sem;
+
+  FTL_DISALLOW_COPY_AND_ASSIGN(PlatformSemaphore);
+};
+
+}  // namespace flutter
+
+#else  // OS_MACOSX
+
+#include <semaphore.h>
+#include "lib/ftl/files/eintr_wrapper.h"
+
+namespace flutter {
+
+class PlatformSemaphore {
+ public:
+  explicit PlatformSemaphore(uint32_t count)
+      : valid_(::sem_init(&sem_, 0 /* not shared */, count) == 0) {}
+
+  ~PlatformSemaphore() {
+    if (valid_) {
+      int result = ::sem_destroy(&sem_);
+      // Can only be EINVAL which should not be possible since we checked for
+      // validity.
+      FTL_DCHECK(result == 0);
+    }
+  }
+
+  bool IsValid() const { return valid_; }
+
+  bool TryWait() {
+    if (!valid_) {
+      return false;
+    }
+
+    return HANDLE_EINTR(::sem_trywait(&sem_)) == 0;
+  }
+
+  void Signal() {
+    if (!valid_) {
+      return;
+    }
+
+    ::sem_post(&sem_);
+
+    return;
+  }
+
+ private:
+  bool valid_;
+  sem_t sem_;
+
+  FTL_DISALLOW_COPY_AND_ASSIGN(PlatformSemaphore);
+};
+
+}  // namespace flutter
+
+#endif
+
+namespace flutter {
+
+Semaphore::Semaphore(uint32_t count) : _impl(new PlatformSemaphore(count)) {}
+
+Semaphore::~Semaphore() = default;
+
+bool Semaphore::IsValid() const {
+  return _impl->IsValid();
+}
+
+bool Semaphore::TryWait() {
+  return _impl->TryWait();
+}
+
+void Semaphore::Signal() {
+  return _impl->Signal();
+}
+
+}  // namespace flutter

synchronization/semaphore.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef SYNCHRONIZATION_SEMAPHORE_H_
+#define SYNCHRONIZATION_SEMAPHORE_H_
+
+#include <memory>
+
+#include "lib/ftl/macros.h"
+#include "lib/ftl/time/time_delta.h"
+#include "lib/ftl/compiler_specific.h"
+
+namespace flutter {
+
+class PlatformSemaphore;
+
+class Semaphore {
+ public:
+  explicit Semaphore(uint32_t count);
+
+  ~Semaphore();
+
+  bool IsValid() const;
+
+  FTL_WARN_UNUSED_RESULT
+  bool TryWait();
+
+  void Signal();
+
+ private:
+  std::unique_ptr<PlatformSemaphore> _impl;
+
+  FTL_DISALLOW_COPY_AND_ASSIGN(Semaphore);
+};
+
+}  // namespace flutter
+
+#endif  // SYNCHRONIZATION_SEMAPHORE_H_