Bug 799595 - Add nsIMessageLoop::PostIdleTask and use it to take screenshots. r=cjones

dom/browser-element/BrowserElementChild.js

@@ -453,29 +453,54 @@ BrowserElementChild.prototype = {
   _recvGetScreenshot: function(data) {
     debug("Received getScreenshot message: (" + data.json.id + ")");
 
+    let self = this;
+    let maxWidth = data.json.args.width;
+    let maxHeight = data.json.args.height;
+    let domRequestID = data.json.id;
+
+    let takeScreenshotClosure = function() {
+      self._takeScreenshot(maxWidth, maxHeight, domRequestID);
+    };
+
+    let maxDelayMS = 2000;
+    try {
+      maxDelayMS = Services.prefs.getIntPref('dom.browserElement.maxScreenshotDelayMS');
+    }
+    catch(e) {}
+
+    // Try to wait for the event loop to go idle before we take the screenshot,
+    // but once we've waited maxDelayMS milliseconds, go ahead and take it
+    // anyway.
+    Cc['@mozilla.org/message-loop;1'].getService(Ci.nsIMessageLoop).postIdleTask(
+      takeScreenshotClosure, maxDelayMS);
+  },
+
+  /**
+   * Actually take a screenshot and foward the result up to our parent, given
+   * the desired maxWidth and maxHeight, and given the DOMRequest ID associated
+   * with the request from the parent.
+   */
+  _takeScreenshot: function(maxWidth, maxHeight, domRequestID) {
     // You can think of the screenshotting algorithm as carrying out the
     // following steps:
     //
-    // - Let max-width be data.json.args.width, and let max-height be
-    //   data.json.args.height.
-    //
-    // - Let scale-width be the factor by which we'd need to downscale the
-    //   viewport so it would fit within max-width.  (If the viewport's width
-    //   is less than max-width, let scale-width be 1.) Compute scale-height
+    // - Let scaleWidth be the factor by which we'd need to downscale the
+    //   viewport so it would fit within maxWidth.  (If the viewport's width
+    //   is less than maxWidth, let scaleWidth be 1.) Compute scaleHeight
     //   the same way.
     //
-    // - Scale the viewport by max(scale-width, scale-height).  Now either the
-    //   viewport's width is no larger than max-width, the viewport's height is
-    //   no larger than max-height, or both.
+    // - Scale the viewport by max(scaleWidth, scaleHeight).  Now either the
+    //   viewport's width is no larger than maxWidth, the viewport's height is
+    //   no larger than maxHeight, or both.
     //
-    // - Crop the viewport so its width is no larger than max-width and its
-    //   height is no larger than max-height.
+    // - Crop the viewport so its width is no larger than maxWidth and its
+    //   height is no larger than maxHeight.
     //
     // - Return a screenshot of the page's viewport scaled and cropped per
     //   above.
-
-    let maxWidth = data.json.args.width;
-    let maxHeight = data.json.args.height;
+    debug("Taking a screenshot: maxWidth=" + maxWidth +
+          ", maxHeight=" + maxHeight +
+          ", domRequestID=" + domRequestID + ".");
 
     let scaleWidth = Math.min(1, maxWidth / content.innerWidth);
     let scaleHeight = Math.min(1, maxHeight / content.innerHeight);
@@ -497,10 +522,11 @@ BrowserElementChild.prototype = {
                    "rgb(255,255,255)");
 
     sendAsyncMsg('got-screenshot', {
-      id: data.json.id,
-      // Hack around the fact that we can't specify opaque PNG, this requires
-      // us to unpremultiply the alpha channel which is expensive on ARM
-      // processors because they lack a hardware integer division instruction.
+      id: domRequestID,
+      // Use JPEG to hack around the fact that we can't specify opaque PNG.
+      // This requires us to unpremultiply the alpha channel, which is
+      // expensive on ARM processors because they lack a hardware integer
+      // division instruction.
       successRv: canvas.toDataURL("image/jpeg")
     });
   },

modules/libpref/src/init/all.js

@@ -3811,3 +3811,7 @@ pref("dom.mozApps.maxLocalId", 1000);
 // they are handled separately. This pref is only read once at startup:
 // a restart is required to enable a new value.
 pref("network.activity.blipIntervalMilliseconds", 0);
+
+// When we're asked to take a screenshot, don't wait more than 2000ms for the
+// event loop to become idle before actually taking the screenshot.
+pref("dom.browserElement.maxScreenshotDelayMS", 2000);

xpcom/base/Makefile.in

@@ -42,6 +42,7 @@ CPPSRCS		= \
 		nsErrorAsserts.cpp \
 		nsGZFileWriter.cpp \
 		MemoryInfoDumper.cpp \
+		nsMessageLoop.cpp \
 		$(NULL)
 
 ifeq ($(OS_ARCH),Linux)
@@ -137,6 +138,7 @@ XPIDLSRCS	= \
 		nsIMutable.idl \
 		nsIMemoryReporter.idl \
 		nsIGZFileWriter.idl \
+		nsIMessageLoop.idl \
 		$(NULL)
 
 ifeq (cocoa,$(MOZ_WIDGET_TOOLKIT))

xpcom/base/nsIMessageLoop.idl

@@ -0,0 +1,36 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsIRunnable;
+
+/**
+ * This service allows access to the current thread's Chromium MessageLoop
+ * instance, with some extra sugar added.  If you're calling from C++, it may
+ * or may not make sense for you to use this interface.  If you're calling from
+ * JS, you don't have a choice!
+ *
+ * Right now, you can only call PostIdleTask(), but nothing is stopping you
+ * from adding other methods.
+ *
+ * nsIMessageLoop's contractid is "@mozilla.org/message-loop;1".
+ */
+[scriptable, uuid(3E8C58E8-E52C-43E0-8E66-669CA788FF5F)]
+interface nsIMessageLoop : nsISupports
+{
+  /**
+   * Posts a task to be run when this thread's message loop is idle, or after
+   * ensureRunsAfterMS milliseconds have elapsed.  (That is, the task is
+   * guaranteed to run /eventually/.)
+   *
+   * Note that if the event loop is busy, we will hold a reference to the task
+   * until ensureRunsAfterMS milliseconds have elapsed.  Be careful when
+   * specifying long timeouts and tasks which hold references to windows or
+   * other large objects, because you can leak memory in a difficult-to-detect
+   * way!
+   */
+  void postIdleTask(in nsIRunnable task, in uint32_t ensureRunsAfterMS);
+};

xpcom/base/nsMessageLoop.cpp

@@ -0,0 +1,159 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsMessageLoop.h"
+#include "mozilla/WeakPtr.h"
+#include "base/message_loop.h"
+#include "base/task.h"
+#include "nsIRunnable.h"
+#include "nsITimer.h"
+#include "nsCOMPtr.h"
+#include "nsAutoPtr.h"
+#include "nsComponentManagerUtils.h"
+#include "nsThreadUtils.h"
+
+using namespace mozilla;
+
+namespace {
+
+/**
+ * This Task runs its nsIRunnable when Run() is called, or after
+ * aEnsureRunsAfterMS milliseconds have elapsed since the object was
+ * constructed.
+ *
+ * Note that the MessageLoop owns this object and will delete it after it calls
+ * Run().  Tread lightly.
+ */
+class MessageLoopIdleTask
+  : public Task
+  , public SupportsWeakPtr<MessageLoopIdleTask>
+{
+public:
+  MessageLoopIdleTask(nsIRunnable* aTask, uint32_t aEnsureRunsAfterMS);
+  virtual ~MessageLoopIdleTask() {}
+  virtual void Run();
+
+private:
+  nsresult Init(uint32_t aEnsureRunsAfterMS);
+
+  nsCOMPtr<nsIRunnable> mTask;
+  nsCOMPtr<nsITimer> mTimer;
+};
+
+/**
+ * This timer callback calls MessageLoopIdleTask::Run() when its timer fires.
+ * (The timer can't call back into MessageLoopIdleTask directly since that's
+ * not a refcounted object; it's owned by the MessageLoop.)
+ *
+ * We keep a weak reference to the MessageLoopIdleTask, although a raw pointer
+ * should in theory suffice: When the MessageLoopIdleTask runs (right before
+ * the MessageLoop deletes it), it cancels its timer.  But the weak pointer
+ * saves us from worrying about an edge case somehow messing us up here.
+ */
+class MessageLoopTimerCallback
+  : public nsITimerCallback
+{
+public:
+  MessageLoopTimerCallback(MessageLoopIdleTask* aTask);
+  virtual ~MessageLoopTimerCallback() {};
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSITIMERCALLBACK
+
+private:
+  WeakPtr<MessageLoopIdleTask> mTask;
+};
+
+MessageLoopIdleTask::MessageLoopIdleTask(nsIRunnable* aTask,
+                                         uint32_t aEnsureRunsAfterMS)
+  : mTask(aTask)
+{
+  // Init() really shouldn't fail, but if it does, we schedule our runnable
+  // immediately, because it's more important to guarantee that we run the task
+  // eventually than it is to run the task when we're idle.
+  nsresult rv = Init(aEnsureRunsAfterMS);
+  if (NS_FAILED(rv)) {
+    NS_WARNING("Running idle task early because we couldn't initialize our timer.");
+    NS_DispatchToCurrentThread(mTask);
+
+    mTask = nullptr;
+    mTimer = nullptr;
+  }
+}
+
+nsresult
+MessageLoopIdleTask::Init(uint32_t aEnsureRunsAfterMS)
+{
+  mTimer = do_CreateInstance("@mozilla.org/timer;1");
+  NS_ENSURE_STATE(mTimer);
+
+  nsRefPtr<MessageLoopTimerCallback> callback =
+    new MessageLoopTimerCallback(this);
+
+  return mTimer->InitWithCallback(callback, aEnsureRunsAfterMS,
+                                  nsITimer::TYPE_ONE_SHOT);
+}
+
+/* virtual */ void
+MessageLoopIdleTask::Run()
+{
+  // Null out our pointers because if Run() was called by the timer, this
+  // object will be kept alive by the MessageLoop until the MessageLoop calls
+  // Run().
+
+  if (mTimer) {
+    mTimer->Cancel();
+    mTimer = nullptr;
+  }
+
+  if (mTask) {
+    mTask->Run();
+    mTask = nullptr;
+  }
+}
+
+MessageLoopTimerCallback::MessageLoopTimerCallback(MessageLoopIdleTask* aTask)
+  : mTask(aTask->asWeakPtr())
+{}
+
+NS_IMETHODIMP
+MessageLoopTimerCallback::Notify(nsITimer* aTimer)
+{
+  // We don't expect to hit the case when the timer fires but mTask has been
+  // deleted, because mTask should cancel the timer before the mTask is
+  // deleted.  But you never know...
+  NS_WARN_IF_FALSE(mTask, "This timer shouldn't have fired.");
+
+  if (mTask) {
+    mTask->Run();
+  }
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS1(MessageLoopTimerCallback, nsITimerCallback)
+
+} // anonymous namespace
+
+NS_IMPL_ISUPPORTS1(nsMessageLoop, nsIMessageLoop)
+
+NS_IMETHODIMP
+nsMessageLoop::PostIdleTask(nsIRunnable* aTask, uint32_t aEnsureRunsAfterMS)
+{
+  // The message loop owns MessageLoopIdleTask and deletes it after calling
+  // Run().  Be careful...
+  MessageLoop::current()->PostIdleTask(FROM_HERE,
+    new MessageLoopIdleTask(aTask, aEnsureRunsAfterMS));
+  return NS_OK;
+}
+
+nsresult
+nsMessageLoopConstructor(nsISupports* aOuter,
+                         const nsIID& aIID,
+                         void** aInstancePtr)
+{
+  NS_ENSURE_FALSE(aOuter, NS_ERROR_NO_AGGREGATION);
+  nsISupports* messageLoop = new nsMessageLoop();
+  return messageLoop->QueryInterface(aIID, aInstancePtr);
+}

xpcom/base/nsMessageLoop.h

@@ -0,0 +1,27 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsIMessageLoop.h"
+
+/*
+ * nsMessageLoop implements nsIMessageLoop, which wraps Chromium's MessageLoop
+ * class and adds a bit of sugar.
+ */
+class nsMessageLoop : public nsIMessageLoop
+{
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIMESSAGELOOP
+
+  virtual ~nsMessageLoop() {}
+};
+
+#define NS_MESSAGE_LOOP_CID \
+{0x67b3ac0c, 0xd806, 0x4d48, \
+{0x93, 0x9e, 0x6a, 0x81, 0x9e, 0x6c, 0x24, 0x8f}}
+
+extern nsresult
+nsMessageLoopConstructor(nsISupports* outer,
+                         const nsIID& aIID,
+                         void* *aInstancePtr);

xpcom/build/XPCOMModule.inc

@@ -78,3 +78,4 @@
     COMPONENT(MEMORY_REPORTER_MANAGER, nsMemoryReporterManagerConstructor)
     COMPONENT(IOUTIL, nsIOUtilConstructor)
     COMPONENT(CYCLE_COLLECTOR_LOGGER, nsCycleCollectorLoggerConstructor)
+    COMPONENT(MESSAGE_LOOP, nsMessageLoopConstructor)

xpcom/build/nsXPCOMCID.h

@@ -75,6 +75,11 @@
  */
 #define NS_CYCLE_COLLECTOR_LOGGER_CONTRACTID "@mozilla.org/cycle-collector-logger;1"
 
+/**
+ * nsMessageLoop contract id
+ */
+#define NS_MESSAGE_LOOP_CONTRACTID "@mozilla.org/message-loop;1"
+
 /**
  * The following are the CIDs and Contract IDs of the nsISupports wrappers for 
  * primative types.  

xpcom/build/nsXPComInit.cpp

@@ -96,6 +96,7 @@ extern nsresult nsStringInputStreamConstructor(nsISupports *, REFNSIID, void **)
 
 #include "nsSystemInfo.h"
 #include "nsMemoryReporterManager.h"
+#include "nsMessageLoop.h"
 
 #include <locale.h>
 #include "mozilla/Services.h"