[Mac] Maximise timer slack for background tabs

base/message_loop/message_pump_mac.mm

 // Copyright (c) 2012 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.
 
 #import "base/message_loop/message_pump_mac.h"
 
 #import <Foundation/Foundation.h>
 
+#include <dlfcn.h>
 #include <limits>
 #include <stack>
 
 #include "base/format_macros.h"
 #include "base/logging.h"
 #include "base/mac/scoped_cftyperef.h"
 #include "base/metrics/histogram.h"
+#include "base/message_loop/timer_slack.h"
 #include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
 #include "base/time/time.h"
 
 #if !defined(OS_IOS)
 #import <AppKit/AppKit.h>
 #endif  // !defined(OS_IOS)
 
 namespace {
 
 void NoOp(void* info) {
 }
 
 const CFTimeInterval kCFTimeIntervalMax =
     std::numeric_limits<CFTimeInterval>::max();
 
 #if !defined(OS_IOS)
 // Set to true if MessagePumpMac::Create() is called before NSApp is
 // initialized.  Only accessed from the main thread.
 bool g_not_using_cr_app = false;
 #endif
 
+// Call through to CFRunLoopTimerSetTolerance(), which is only available on
+// OS X 10.9.
+void SetTimerTolerance(CFRunLoopTimerRef timer, CFTimeInterval tolerance) {
+  typedef void (*CFRunLoopTimerSetTolerancePtr)(CFRunLoopTimerRef timer,
+      CFTimeInterval tolerance);
+
+  static CFRunLoopTimerSetTolerancePtr settimertolerance_function_ptr = NULL;
+  static bool function_lookup_error = false;
+
+  if (function_lookup_error)
+    return;
+
+  if (!settimertolerance_function_ptr) {
+    settimertolerance_function_ptr =
+        reinterpret_cast<CFRunLoopTimerSetTolerancePtr>(
+            dlsym(RTLD_DEFAULT, "CFRunLoopTimerSetTolerance"));
+    if (!settimertolerance_function_ptr) {
+        function_lookup_error = true;
+        return;
+    }
+  }
+
+  settimertolerance_function_ptr(timer, tolerance);
+}
+
 }  // namespace
 
 namespace base {
 
 // A scoper for autorelease pools created from message pump run loops.
 // Avoids dirtying up the ScopedNSAutoreleasePool interface for the rare
 // case where an autorelease pool needs to be passed in.
 class MessagePumpScopedAutoreleasePool {
  public:
   explicit MessagePumpScopedAutoreleasePool(MessagePumpCFRunLoopBase* pump) :
@@ skipping 383 matching lines @@
   CFRunLoopSourceSignal(work_source_);
   CFRunLoopWakeUp(run_loop_);
 }
 
 // Must be called on the run loop thread.
 void MessagePumpCFRunLoopBase::ScheduleDelayedWork(
     const TimeTicks& delayed_work_time) {
   TimeDelta delta = delayed_work_time - TimeTicks::Now();
   delayed_work_fire_time_ = CFAbsoluteTimeGetCurrent() + delta.InSecondsF();
   CFRunLoopTimerSetNextFireDate(delayed_work_timer_, delayed_work_fire_time_);
+  if (GetTimerSlack() == TIMER_SLACK_MAXIMUM) {
+    SetTimerTolerance(delayed_work_timer_, delta.InSecondsF() * 0.5);
+  } else {
+    SetTimerTolerance(delayed_work_timer_, 0);
+  }
 }
 
 // Called from the run loop.
 // static
 void MessagePumpCFRunLoopBase::RunDelayedWorkTimer(CFRunLoopTimerRef timer,
                                                    void* info) {
   MessagePumpCFRunLoopBase* self = static_cast<MessagePumpCFRunLoopBase*>(info);
 
   // The timer won't fire again until it's reset.
   self->delayed_work_fire_time_ = kCFTimeIntervalMax;
@@ skipping 503 matching lines @@
     [NSApplication sharedApplication];
     g_not_using_cr_app = true;
     return new MessagePumpNSApplication;
 #endif
   }
 
   return new MessagePumpNSRunLoop;
 }
 
 }  // namespace base