Add a BarrierClosure.

Chromium frequently needs to execute a "final" closure once a given
number of other callbacks have run. BarrierClosure encapsulates that
logic while staying as light-weight as possible.

TBR=gbillock@chromium.org, willchan@chromium.org
BUG=none

Review URL: https://chromiumcodereview.appspot.com/22859056

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@219446 0039d316-1c4b-4281-b951-d872f2087c98

5 files changed, 121 insertions, 0 deletions

diff --git a/base/barrier_closure.cc b/base/barrier_closure.cc
new file mode 100644
index 0000000..65e9c086
--- /dev/null
+++ b/base/barrier_closure.cc
@@ -0,0 +1,52 @@
+// Copyright 2013 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 "base/barrier_closure.h"
+
+#include "base/atomic_ref_count.h"
+#include "base/bind.h"
+
+namespace {
+
+// Maintains state for a BarrierClosure.
+class BarrierInfo {
+ public:
+  BarrierInfo(int num_callbacks_left, const base::Closure& done_closure);
+  void Run();
+
+ private:
+  base::AtomicRefCount num_callbacks_left_;
+  base::Closure done_closure_;
+};
+
+BarrierInfo::BarrierInfo(int num_callbacks, const base::Closure& done_closure)
+    : num_callbacks_left_(num_callbacks),
+      done_closure_(done_closure) {
+}
+
+void BarrierInfo::Run() {
+  DCHECK(!base::AtomicRefCountIsZero(&num_callbacks_left_));
+  if (!base::AtomicRefCountDec(&num_callbacks_left_)) {
+    done_closure_.Run();
+    done_closure_.Reset();
+  }
+}
+
+}  // namespace
+
+namespace base {
+
+base::Closure BarrierClosure(int num_callbacks_left,
+                             const base::Closure& done_closure) {
+  DCHECK(num_callbacks_left >= 0);
+
+  if (num_callbacks_left == 0)
+    done_closure.Run();
+
+  return base::Bind(&BarrierInfo::Run,
+                    base::Owned(
+                        new BarrierInfo(num_callbacks_left, done_closure)));
+}
+
+}  // namespace base
\ No newline at end of file
diff --git a/base/barrier_closure.h b/base/barrier_closure.h
new file mode 100644
index 0000000..2fb870f
--- /dev/null
+++ b/base/barrier_closure.h
@@ -0,0 +1,30 @@
+// Copyright 2013 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 BASE_BARRIER_CLOSURE_H_
+#define BASE_BARRIER_CLOSURE_H_
+
+#include "base/base_export.h"
+#include "base/callback_forward.h"
+
+namespace base {
+
+// BarrierClosure executes |done_closure| after it has been invoked
+// |num_closures| times.
+//
+// If |num_closures| is 0, |done_closure| is executed immediately.
+//
+// BarrierClosure is thread-safe - the count of remaining closures is
+// maintained as a base::AtomicRefCount. |done_closure| will be run on
+// the thread that calls the final Run() on the returned closures.
+//
+// |done_closure| is also Reset() on the final calling thread but due to the
+// refcounted nature of callbacks, it is hard to know what thread resources
+// will be released on.
+BASE_EXPORT base::Closure BarrierClosure(int num_closures,
+                                         const base::Closure& done_closure);
+
+}  // namespace base
+
+#endif  // BASE_BARRIER_CLOSURE_H_
\ No newline at end of file
diff --git a/base/barrier_closure_unittest.cc b/base/barrier_closure_unittest.cc
new file mode 100644
index 0000000..ab05cb8
--- /dev/null
+++ b/base/barrier_closure_unittest.cc
@@ -0,0 +1,36 @@
+// Copyright 2013 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 "base/barrier_closure.h"
+
+#include "base/bind.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace {
+
+void Increment(int* count) { (*count)++; }
+
+TEST(BarrierClosureTest, RunImmediatelyForZeroClosures) {
+  int count = 0;
+  base::Closure doneClosure(base::Bind(&Increment, base::Unretained(&count)));
+
+  base::Closure barrierClosure = base::BarrierClosure(0, doneClosure);
+  EXPECT_EQ(1, count);
+}
+
+TEST(BarrierClosureTest, RunAfterNumClosures) {
+  int count = 0;
+  base::Closure doneClosure(base::Bind(&Increment, base::Unretained(&count)));
+
+  base::Closure barrierClosure = base::BarrierClosure(2, doneClosure);
+  EXPECT_EQ(0, count);
+
+  barrierClosure.Run();
+  EXPECT_EQ(0, count);
+
+  barrierClosure.Run();
+  EXPECT_EQ(1, count);
+}
+
+}  // namespace
diff --git a/base/base.gyp b/base/base.gyp
index 2600754..39e9f33 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -453,6 +453,7 @@
         'async_socket_io_handler_unittest.cc',
         'at_exit_unittest.cc',
         'atomicops_unittest.cc',
+        'barrier_closure_unittest.cc',
         'base64_unittest.cc',
         'bind_helpers_unittest.cc',
         'bind_unittest.cc',
diff --git a/base/base.gypi b/base/base.gypi
index e22aa2e..fce37f6 100644
--- a/base/base.gypi
+++ b/base/base.gypi
@@ -78,6 +78,8 @@
           'atomicops_internals_x86_gcc.cc',
           'atomicops_internals_x86_gcc.h',
           'atomicops_internals_x86_msvc.h',
+          'barrier_closure.cc',
+          'barrier_closure.h',
           'base_export.h',
           'base_paths.cc',
           'base_paths.h',