Changes the Constellation shutdown procedure to a message and response

from the Compositor instead of a message with an immediate callback.

The problem was that the old model introduced a potential deadlock. If
the Constellation had a pending RenderReadyMsg in its queue when the
ExitMsg arrived from the Compositor, the Compositor would exit its
message loop and wait for the response from the Constellation. But,
the Constellation would send a SetIds message to the Compositor, which
also required a response - resulting in deadlock.

src/components/main/compositing/compositor.rs

@@ -32,7 +32,7 @@ use layers::scene::Scene;
 use opengles::gl2;
 use png;
 use servo_msg::compositor_msg::{Epoch, IdleRenderState, LayerBufferSet, RenderState};
-use servo_msg::constellation_msg::{ConstellationChan, NavigateMsg, ResizedWindowMsg, LoadUrlMsg, PipelineId};
+use servo_msg::constellation_msg::{ConstellationChan, ExitMsg, NavigateMsg, ResizedWindowMsg, LoadUrlMsg, PipelineId};
 use servo_msg::constellation_msg;
 use servo_util::time::{profile, ProfilerChan, Timer};
 use servo_util::{time, url};
@@ -129,7 +129,7 @@ impl IOCompositor {
             zoom_action: false,
             zoom_time: 0f64,
             compositor_layer: None,
-            constellation_chan: constellation_chan.clone(),
+            constellation_chan: constellation_chan,
             profiler_chan: profiler_chan,
             fragment_point: None
         }
@@ -202,10 +202,16 @@ impl IOCompositor {
                 None => break,
 
                 Some(Exit(chan)) => {
-                    self.done = true;
+                    debug!("shutting down the constellation");
+                    self.constellation_chan.send(ExitMsg);
                     chan.send(());
                 }
 
+                Some(ShutdownComplete) => {
+                    debug!("constellation completed shutdown");
+                    self.done = true;
+                }
+
                 Some(ChangeReadyState(ready_state)) => {
                     self.window.set_ready_state(ready_state);
                 }
@@ -496,12 +502,14 @@ impl IOCompositor {
             FinishedWindowEvent => {
                 let exit = self.opts.exit_after_load;
                 if exit {
-                    self.done = true;
+                    debug!("shutting down the constellation for FinishedWindowEvent");
+                    self.constellation_chan.send(ExitMsg);
                 }
             }
 
             QuitWindowEvent => {
-                self.done = true;
+                debug!("shutting down the constellation for QuitWindowEvent");
+                self.constellation_chan.send(ExitMsg);
             }
         }
     }

src/components/main/compositing/compositor_task.rs

@@ -16,7 +16,7 @@ use gfx::opts::Opts;
 use layers::platform::surface::{NativeCompositingGraphicsContext, NativeGraphicsMetadata};
 use servo_msg::compositor_msg::{Epoch, RenderListener, LayerBufferSet, RenderState, ReadyState};
 use servo_msg::compositor_msg::{ScriptListener, Tile};
-use servo_msg::constellation_msg::{ConstellationChan, PipelineId, ExitMsg};
+use servo_msg::constellation_msg::{ConstellationChan, PipelineId};
 use servo_util::time::ProfilerChan;
 use std::comm::{Chan, SharedChan, Port};
 use std::num::Orderable;
@@ -112,11 +112,16 @@ impl CompositorChan {
         self.chan.send(msg);
     }
 }
-
 /// Messages from the painting task and the constellation task to the compositor task.
 pub enum Msg {
     /// Requests that the compositor shut down.
     Exit(Chan<()>),
+
+    /// Informs the compositor that the constellation has completed shutdown.
+    /// Required because the constellation can have pending calls to make (e.g. SetIds)
+    /// at the time that we send it an ExitMsg.
+    ShutdownComplete,
+
     /// Requests the compositor's graphics metadata. Graphics metadata is what the renderer needs
     /// to create surfaces that the compositor can see. On Linux this is the X display; on Mac this
     /// is the pixel format.
@@ -185,9 +190,7 @@ impl CompositorTask {
     pub fn create(opts: Opts,
                   port: Port<Msg>,
                   constellation_chan: ConstellationChan,
-                  profiler_chan: ProfilerChan,
-                  exit_chan: Chan<()>,
-                  exit_response_from_constellation: Port<()>) {
+                  profiler_chan: ProfilerChan) {
 
         let compositor = CompositorTask::new(opts.headless);
 
@@ -197,18 +200,12 @@ impl CompositorTask {
                                                  opts,
                                                  port,
                                                  constellation_chan.clone(),
-                                                 profiler_chan);
+                                                 profiler_chan)
             }
             Headless => {
                 headless::NullCompositor::create(port,
-                                                 constellation_chan.clone());
+                                                 constellation_chan.clone())
             }
-        }
-
-        // Constellation has to be shut down before the compositor goes out of
-        // scope, as the compositor manages setup/teardown of global subsystems
-        debug!("shutting down the constellation");
-        constellation_chan.send(ExitMsg(exit_chan));
-        exit_response_from_constellation.recv();
+        };
     }
 }

src/components/main/compositing/headless.rs

@@ -5,7 +5,7 @@
 use compositing::*;
 
 use geom::size::Size2D;
-use servo_msg::constellation_msg::{ConstellationChan, ResizedWindowMsg};
+use servo_msg::constellation_msg::{ConstellationChan, ExitMsg, ResizedWindowMsg};
 use std::comm::Port;
 
 
@@ -32,14 +32,20 @@ impl NullCompositor {
 
         // Tell the constellation about the initial fake size.
         constellation_chan.send(ResizedWindowMsg(Size2D(640u, 480u)));
-        compositor.handle_message();
+        compositor.handle_message(constellation_chan);
     }
 
-    fn handle_message(&self) {
+    fn handle_message(&self, constellation_chan: ConstellationChan) {
         loop {
             match self.port.recv() {
                 Exit(chan) => {
+                    debug!("shutting down the constellation");
+                    constellation_chan.send(ExitMsg);
                     chan.send(());
+                }
+
+                ShutdownComplete => {
+                    debug!("constellation completed shutdown");
                     break
                 }
 

src/components/main/constellation.rs

@@ -2,7 +2,7 @@
  * 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/. */
 
-use compositing::{CompositorChan, SetIds, SetLayerClipRect};
+use compositing::{CompositorChan, SetIds, SetLayerClipRect, ShutdownComplete};
 
 use extra::url::Url;
 use geom::rect::Rect;
@@ -315,9 +315,9 @@ impl Constellation {
     /// Handles loading pages, navigation, and granting access to the compositor
     fn handle_request(&mut self, request: Msg) -> bool {
         match request {
-            ExitMsg(sender) => {
+            ExitMsg => {
                 debug!("constellation exiting");
-                self.handle_exit(sender);
+                self.handle_exit();
                 return false;
             }
             FailureMsg(pipeline_id, subpage_id) => {
@@ -363,14 +363,13 @@ impl Constellation {
         true
     }
 
-    fn handle_exit(&self, sender: Chan<()>) {
+    fn handle_exit(&self) {
         for (_id, ref pipeline) in self.pipelines.iter() {
             pipeline.exit();
         }
         self.image_cache_task.exit();
         self.resource_task.send(resource_task::Exit);
-
-        sender.send(());
+        self.compositor_chan.send(ShutdownComplete);
     }
 
     fn handle_failure_msg(&mut self, pipeline_id: PipelineId, subpage_id: Option<SubpageId>) {
@@ -804,6 +803,7 @@ impl Constellation {
 
     fn set_ids(&self, frame_tree: @mut FrameTree) {
         let (port, chan) = Chan::new();
+        debug!("Constellation sending SetIds");
         self.compositor_chan.send(SetIds(frame_tree.to_sendable(), chan, self.chan.clone()));
         match port.recv_opt() {
             Some(()) => {

src/components/main/pipeline.rs

@@ -181,6 +181,7 @@ impl Pipeline {
     }
 
     pub fn revoke_paint_permission(&self) {
+        debug!("pipeline revoking render channel paint permission");
         self.render_chan.send(PaintPermissionRevoked);
     }
 

src/components/main/servo.rc

@@ -135,7 +135,6 @@ pub extern "C" fn android_start(argc: int, argv: **u8) -> int {
 fn run(opts: Opts) {
     let mut pool = green::SchedPool::new(green::PoolConfig::new());
 
-    let (exit_response_from_constellation, exit_chan) = Chan::new();
     let (compositor_port, compositor_chan) = CompositorChan::new();
     let profiler_chan = Profiler::create(opts.profiler_period);
 
@@ -154,13 +153,13 @@ fn run(opts: Opts) {
                                                       image_cache_task,
                                                       profiler_chan_clone);
 
-        // Send the constallation Chan as the result
-        result_chan.send(constellation_chan.clone());
-
         // Send the URL command to the constellation.
         for filename in opts.urls.iter() {
             constellation_chan.send(InitLoadUrlMsg(make_url(filename.clone(), None)))
         }
+
+        // Send the constallation Chan as the result
+        result_chan.send(constellation_chan);
     });
 
     let constellation_chan = result_port.recv();
@@ -169,9 +168,7 @@ fn run(opts: Opts) {
     CompositorTask::create(opts,
                            compositor_port,
                            constellation_chan,
-                           profiler_chan,
-                           exit_chan,
-                           exit_response_from_constellation);
+                           profiler_chan);
 
     pool.shutdown();
 }

src/components/msg/constellation_msg.rs

@@ -8,7 +8,7 @@
 use extra::url::Url;
 use geom::rect::Rect;
 use geom::size::Size2D;
-use std::comm::{Chan, SharedChan};
+use std::comm::SharedChan;
 
 #[deriving(Clone)]
 pub struct ConstellationChan(SharedChan<Msg>);
@@ -28,7 +28,7 @@ pub enum IFrameSandboxState {
 
 /// Messages from the compositor to the constellation.
 pub enum Msg {
-    ExitMsg(Chan<()>),
+    ExitMsg,
     FailureMsg(PipelineId, Option<SubpageId>),
     InitLoadUrlMsg(Url),
     FrameRectMsg(PipelineId, SubpageId, Rect<f32>),