Bug 1287666 - Avoid a round trip to the X server in nsShmImage::CreateDrawTarget. r=lsalzman

MozReview-Commit-ID: LpvsntOCMab

--HG--
extra : rebase_source : e13ad6f33b875bcae59110347b696ea640085bbb

widget/nsShmImage.cpp

@@ -32,18 +32,14 @@ nsShmImage::nsShmImage(Display* aDisplay,
   , mSize(0, 0)
   , mPixmap(XCB_NONE)
   , mGC(XCB_NONE)
+  , mRequestPending(false)
   , mShmSeg(XCB_NONE)
   , mShmId(-1)
   , mShmAddr(nullptr)
 {
   mConnection = XGetXCBConnection(aDisplay);
-  mozilla::PodZero(&mLastRequest);
-  if (aDisplay == mozilla::DefaultXDisplay()) {
-    // If another thread spins the X event loop during a checked call,
-    // an error that should've been checked by XCB may be handled by the Xlib
-    // error handler. See bug 1287463.
-    NS_WARNING("Main thread X display used with nsShmImage!");
-  }
+  mozilla::PodZero(&mPutRequest);
+  mozilla::PodZero(&mSyncRequest);
 }
 
 nsShmImage::~nsShmImage()
@@ -250,13 +246,19 @@ nsShmImage::CreateDrawTarget(const mozilla::LayoutDeviceIntRegion& aRegion)
   // Wait for any in-flight requests to complete.
   // Typically X clients would wait for a XShmCompletionEvent to be received,
   // but this works as it's sent immediately after the request is processed.
-  xcb_generic_error_t* error;
-  if (mLastRequest.sequence != XCB_NONE &&
-      (error = xcb_request_check(mConnection, mLastRequest)))
-  {
-    gShmAvailable = false;
-    free(error);
-    return nullptr;
+  if (mRequestPending) {
+    xcb_get_input_focus_reply_t* reply;
+    if ((reply = xcb_get_input_focus_reply(mConnection, mSyncRequest, nullptr))) {
+      free(reply);
+    }
+    mRequestPending = false;
+
+    xcb_generic_error_t* error;
+    if ((error = xcb_request_check(mConnection, mPutRequest))) {
+      gShmAvailable = false;
+      free(error);
+      return nullptr;
+    }
   }
 
   // Due to bug 1205045, we must avoid making GTK calls off the main thread to query window size.
@@ -301,17 +303,22 @@ nsShmImage::Put(const mozilla::LayoutDeviceIntRegion& aRegion)
                           xrects.Length(), xrects.Elements());
 
   if (mPixmap != XCB_NONE) {
-    mLastRequest = xcb_copy_area_checked(mConnection, mPixmap, mWindow, mGC,
-                                         0, 0, 0, 0, mSize.width, mSize.height);
+    mPutRequest = xcb_copy_area_checked(mConnection, mPixmap, mWindow, mGC,
+                                        0, 0, 0, 0, mSize.width, mSize.height);
   } else {
-    mLastRequest = xcb_shm_put_image_checked(mConnection, mWindow, mGC,
-                                             mSize.width, mSize.height,
-                                             0, 0, mSize.width, mSize.height,
-                                             0, 0, mDepth,
-                                             XCB_IMAGE_FORMAT_Z_PIXMAP, 0,
-                                             mShmSeg, 0);
+    mPutRequest = xcb_shm_put_image_checked(mConnection, mWindow, mGC,
+                                            mSize.width, mSize.height,
+                                            0, 0, mSize.width, mSize.height,
+                                            0, 0, mDepth,
+                                            XCB_IMAGE_FORMAT_Z_PIXMAP, 0,
+                                            mShmSeg, 0);
   }
 
+  // Send a request that returns a response so that we don't have to start a
+  // sync in nsShmImage::CreateDrawTarget to retrieve the result of mPutRequest.
+  mSyncRequest = xcb_get_input_focus(mConnection);
+  mRequestPending = true;
+
   xcb_flush(mConnection);
 }
 

widget/nsShmImage.h

@@ -58,7 +58,9 @@ private:
 
   xcb_pixmap_t                 mPixmap;
   xcb_gcontext_t               mGC;
-  xcb_void_cookie_t            mLastRequest;
+  xcb_void_cookie_t            mPutRequest;
+  xcb_get_input_focus_cookie_t mSyncRequest;
+  bool                         mRequestPending;
 
   xcb_shm_seg_t                mShmSeg;
   int                          mShmId;