auto merge of #4536 : mrobinson/servo/pixmap, r=pcwalton

It is possible for a PaintTask to start exiting soon after sending new
buffers to the compositor. In that case, the compositor should return
the now unnecessary buffers to the PaintTask so that it can properly
free them.

To accomplish this, the compositor now keeps a hash map of paint task
channels per pipeline id. When a PaintTask exists, the constellation
informs the compositor that it can forget about it. Additionally, the
PaintTask should not wait for any buffers when the engine is doing a
complete shutdown. In that case, the compositor is already halted and
has simply let all buffers leak. We pipe through the shutdown type when
destroying the pipeline to make this decision.

Fixes #2641.

13 files changed, 180 insertions, 103 deletions

diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index 0e5294ae2f3..71a37ddbc4f 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -129,6 +129,9 @@ pub struct IOCompositor<Window: WindowMethods> {
 
     /// Pending scroll events.
     pending_scroll_events: Vec<ScrollEvent>,
+
+    /// PaintTask channels that we know about, used to return unused buffers.
+    paint_channels: HashMap<PipelineId, PaintChan>,
 }
 
 pub struct ScrollEvent {
@@ -198,6 +201,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             fragment_point: None,
             outstanding_paint_msgs: 0,
             last_composite_time: 0,
+            paint_channels: HashMap::new(),
         }
     }
 
@@ -341,6 +345,12 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 self.window.set_cursor(cursor)
             }
 
+            (Msg::PaintTaskExited(pipeline_id), ShutdownState::NotShuttingDown) => {
+                if self.paint_channels.remove(&pipeline_id).is_none() {
+                    panic!("Saw PaintTaskExited message from an unknown pipeline!");
+                }
+            }
+
             // When we are shutting_down, we need to avoid performing operations
             // such as Paint that may crash because we have begun tearing down
             // the rest of our resources.
@@ -456,6 +466,38 @@ impl<Window: WindowMethods> IOCompositor<Window> {
         self.composite_if_necessary();
     }
 
+    fn create_root_layer_for_pipeline_and_rect(&mut self,
+                                               pipeline: &CompositionPipeline,
+                                               frame_rect: Option<TypedRect<PagePx, f32>>)
+                                               -> Rc<Layer<CompositorData>> {
+        let layer_properties = LayerProperties {
+            pipeline_id: pipeline.id,
+            epoch: Epoch(0),
+            id: LayerId::null(),
+            rect: Rect::zero(),
+            background_color: azure_hl::Color::new(0., 0., 0., 0.),
+            scroll_policy: Scrollable,
+        };
+
+        let root_layer = CompositorData::new_layer(pipeline.clone(),
+                                                   layer_properties,
+                                                   WantsScrollEventsFlag::WantsScrollEvents,
+                                                   opts::get().tile_size);
+        if !self.paint_channels.contains_key(&pipeline.id) {
+            self.paint_channels.insert(pipeline.id, pipeline.paint_chan.clone());
+        }
+
+        // All root layers mask to bounds.
+        *root_layer.masks_to_bounds.borrow_mut() = true;
+
+        if let Some(ref frame_rect) = frame_rect {
+            let frame_rect = frame_rect.to_untyped();
+            *root_layer.bounds.borrow_mut() = Rect::from_untyped(&frame_rect);
+        }
+
+        return root_layer;
+    }
+
     fn create_frame_tree_root_layers(&mut self,
                                      frame_tree: &SendableFrameTree,
                                      frame_rect: Option<TypedRect<PagePx, f32>>)
@@ -464,7 +506,8 @@ impl<Window: WindowMethods> IOCompositor<Window> {
         self.ready_states.insert(frame_tree.pipeline.id, Blank);
         self.paint_states.insert(frame_tree.pipeline.id, PaintingPaintState);
 
-        let root_layer = create_root_layer_for_pipeline_and_rect(&frame_tree.pipeline, frame_rect);
+        let root_layer = self.create_root_layer_for_pipeline_and_rect(&frame_tree.pipeline,
+                                                                      frame_rect);
         for kid in frame_tree.children.iter() {
             root_layer.add_child(self.create_frame_tree_root_layers(&kid.frame_tree, kid.rect));
         }
@@ -474,8 +517,8 @@ impl<Window: WindowMethods> IOCompositor<Window> {
     fn update_frame_tree(&mut self, frame_tree_diff: &FrameTreeDiff) {
         let parent_layer = self.find_pipeline_root_layer(frame_tree_diff.parent_pipeline.id);
         parent_layer.add_child(
-            create_root_layer_for_pipeline_and_rect(&frame_tree_diff.pipeline,
-                                                    frame_tree_diff.rect));
+            self.create_root_layer_for_pipeline_and_rect(&frame_tree_diff.pipeline,
+                                                         frame_tree_diff.rect));
     }
 
     fn find_pipeline_root_layer(&self, pipeline_id: PipelineId) -> Rc<Layer<CompositorData>> {
@@ -614,6 +657,24 @@ impl<Window: WindowMethods> IOCompositor<Window> {
              layer_id: LayerId,
              new_layer_buffer_set: Box<LayerBufferSet>,
              epoch: Epoch) {
+        match self.find_layer_with_pipeline_and_layer_id(pipeline_id, layer_id) {
+            Some(layer) => self.paint_to_layer(layer, new_layer_buffer_set, epoch),
+            None => {
+                match self.paint_channels.entry(pipeline_id) {
+                    Occupied(entry) => {
+                        let message = UnusedBufferMsg(new_layer_buffer_set.buffers);
+                        let _ = entry.get().send_opt(message);
+                    },
+                    Vacant(_) => panic!("Received a buffer from an unknown pipeline!"),
+                }
+            }
+        }
+    }
+
+    fn paint_to_layer(&mut self,
+                      layer: Rc<Layer<CompositorData>>,
+                      new_layer_buffer_set: Box<LayerBufferSet>,
+                      epoch: Epoch) {
         debug!("compositor received new frame at size {}x{}",
                self.window_size.width.get(),
                self.window_size.height.get());
@@ -622,20 +683,10 @@ impl<Window: WindowMethods> IOCompositor<Window> {
         let mut new_layer_buffer_set = new_layer_buffer_set;
         new_layer_buffer_set.mark_will_leak();
 
-        match self.find_layer_with_pipeline_and_layer_id(pipeline_id, layer_id) {
-            Some(ref layer) => {
-                // FIXME(pcwalton): This is going to cause problems with inconsistent frames since
-                // we only composite one layer at a time.
-                assert!(layer.add_buffers(new_layer_buffer_set, epoch));
-                self.composite_if_necessary();
-            }
-            None => {
-                // FIXME: This may potentially be triggered by a race condition where a
-                // buffers are being painted but the layer is removed before painting
-                // completes.
-                panic!("compositor given paint command for non-existent layer");
-            }
-        }
+        // FIXME(pcwalton): This is going to cause problems with inconsistent frames since
+        // we only composite one layer at a time.
+        assert!(layer.add_buffers(new_layer_buffer_set, epoch));
+        self.composite_if_necessary();
     }
 
     fn scroll_fragment_to_point(&mut self,
@@ -1196,37 +1247,6 @@ fn find_layer_with_pipeline_and_layer_id_for_layer(layer: Rc<Layer<CompositorDat
     return None;
 }
 
-fn create_root_layer_for_pipeline_and_rect(pipeline: &CompositionPipeline,
-                                           frame_rect: Option<TypedRect<PagePx, f32>>)
-                                           -> Rc<Layer<CompositorData>> {
-    let layer_properties = LayerProperties {
-        pipeline_id: pipeline.id,
-        epoch: Epoch(0),
-        id: LayerId::null(),
-        rect: Rect::zero(),
-        background_color: azure_hl::Color::new(0., 0., 0., 0.),
-        scroll_policy: Scrollable,
-    };
-
-    let root_layer = CompositorData::new_layer(pipeline.clone(),
-                                               layer_properties,
-                                               WantsScrollEventsFlag::WantsScrollEvents,
-                                               opts::get().tile_size);
-
-    // All root layers mask to bounds.
-    *root_layer.masks_to_bounds.borrow_mut() = true;
-
-    match frame_rect {
-        Some(ref frame_rect) => {
-            let frame_rect = frame_rect.to_untyped();
-            *root_layer.bounds.borrow_mut() = Rect::from_untyped(&frame_rect);
-        }
-        None => {}
-    }
-
-    return root_layer;
-}
-
 impl<Window> CompositorEventListener for IOCompositor<Window> where Window: WindowMethods {
     fn handle_event(&mut self, msg: WindowEvent) -> bool {
         // Check for new messages coming from the other tasks in the system.
diff --git a/components/compositing/compositor_task.rs b/components/compositing/compositor_task.rs
index c8e8932b956..147bbe4b211 100644
--- a/components/compositing/compositor_task.rs
+++ b/components/compositing/compositor_task.rs
@@ -216,6 +216,8 @@ pub enum Msg {
     KeyEvent(Key, KeyModifiers),
     /// Changes the cursor.
     SetCursor(Cursor),
+    /// Informs the compositor that the paint task for the given pipeline has exited.
+    PaintTaskExited(PipelineId),
 }
 
 impl Show for Msg {
@@ -240,6 +242,7 @@ impl Show for Msg {
             Msg::ScrollTimeout(..) => write!(f, "ScrollTimeout"),
             Msg::KeyEvent(..) => write!(f, "KeyEvent"),
             Msg::SetCursor(..) => write!(f, "SetCursor"),
+            Msg::PaintTaskExited(..) => write!(f, "PaintTaskExited"),
         }
     }
 }
diff --git a/components/compositing/constellation.rs b/components/compositing/constellation.rs
index 118f33bc3f3..6ec526d1b90 100644
--- a/components/compositing/constellation.rs
+++ b/components/compositing/constellation.rs
@@ -11,11 +11,10 @@ use devtools_traits::DevtoolsControlChan;
 use geom::rect::{Rect, TypedRect};
 use geom::scale_factor::ScaleFactor;
 use gfx::font_cache_task::FontCacheTask;
-use gfx::paint_task;
 use layers::geometry::DevicePixel;
-use layout_traits::{LayoutControlChan, LayoutTaskFactory, ExitNowMsg};
+use layout_traits::LayoutTaskFactory;
 use libc;
-use script_traits::{mod, GetTitleMsg, ResizeMsg, ResizeInactiveMsg, ExitPipelineMsg, SendEventMsg};
+use script_traits::{mod, GetTitleMsg, ResizeMsg, ResizeInactiveMsg, SendEventMsg};
 use script_traits::{ScriptControlChan, ScriptTaskFactory};
 use servo_msg::compositor_msg::LayerId;
 use servo_msg::constellation_msg::{mod, ConstellationChan, ExitMsg, FailureMsg, Failure};
@@ -23,7 +22,7 @@ use servo_msg::constellation_msg::{FrameRectMsg, GetPipelineTitleMsg};
 use servo_msg::constellation_msg::{IFrameSandboxState, IFrameUnsandboxed, InitLoadUrlMsg};
 use servo_msg::constellation_msg::{KeyEvent, Key, KeyState, KeyModifiers, LoadCompleteMsg};
 use servo_msg::constellation_msg::{LoadData, LoadUrlMsg, NavigateMsg, NavigationType};
-use servo_msg::constellation_msg::{PainterReadyMsg, PipelineId, ResizedWindowMsg};
+use servo_msg::constellation_msg::{PainterReadyMsg, PipelineExitType, PipelineId, ResizedWindowMsg};
 use servo_msg::constellation_msg::{ScriptLoadedURLInIFrameMsg, SetCursorMsg, SubpageId};
 use servo_msg::constellation_msg::{WindowSizeData};
 use servo_msg::constellation_msg::Msg as ConstellationMsg;
@@ -515,7 +514,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
 
     fn handle_exit(&mut self) {
         for (_id, ref pipeline) in self.pipelines.iter() {
-            pipeline.exit();
+            pipeline.exit(PipelineExitType::Complete);
         }
         self.image_cache_task.exit();
         self.resource_task.send(resource_task::Exit);
@@ -547,14 +546,8 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
             Some(pipeline) => pipeline.clone()
         };
 
-        fn force_pipeline_exit(old_pipeline: &Rc<Pipeline>) {
-            let ScriptControlChan(ref old_script) = old_pipeline.script_chan;
-            let _ = old_script.send_opt(ExitPipelineMsg(old_pipeline.id));
-            let _ = old_pipeline.paint_chan.send_opt(paint_task::ExitMsg(None));
-            let LayoutControlChan(ref old_layout) = old_pipeline.layout_chan;
-            let _ = old_layout.send_opt(ExitNowMsg);
-        }
-        force_pipeline_exit(&old_pipeline);
+        old_pipeline.force_exit();
+        self.compositor_proxy.send(CompositorMsg::PaintTaskExited(old_pipeline.id));
         self.pipelines.remove(&pipeline_id);
 
         loop {
@@ -564,7 +557,8 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
             match idx {
                 Some(idx) => {
                     debug!("removing pending frame change for failed pipeline");
-                    force_pipeline_exit(&self.pending_frames[idx].after.pipeline);
+                    self.pending_frames[idx].after.pipeline.force_exit();
+                    self.compositor_proxy.send(CompositorMsg::PaintTaskExited(old_pipeline.id));
                     self.pending_frames.remove(idx);
                 },
                 None => break,
@@ -987,7 +981,8 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         // TODO(tkuehn): should only exit once per unique script task,
         // and then that script task will handle sub-exits
         for frame_tree in frame_tree.iter() {
-            frame_tree.pipeline.exit();
+            frame_tree.pipeline.exit(PipelineExitType::PipelineOnly);
+            self.compositor_proxy.send(CompositorMsg::PaintTaskExited(frame_tree.pipeline.id));
             self.pipelines.remove(&frame_tree.pipeline.id);
         }
     }
diff --git a/components/compositing/headless.rs b/components/compositing/headless.rs
index 9651ec7de78..e2ed207b274 100644
--- a/components/compositing/headless.rs
+++ b/components/compositing/headless.rs
@@ -111,6 +111,7 @@ impl CompositorEventListener for NullCompositor {
             Msg::ChangePageLoadData(..) |
             Msg::KeyEvent(..) |
             Msg::SetCursor(..) => {}
+            Msg::PaintTaskExited(..) => {}
         }
         true
     }
diff --git a/components/compositing/lib.rs b/components/compositing/lib.rs
index 4e906b83133..ea0405a3045 100644
--- a/components/compositing/lib.rs
+++ b/components/compositing/lib.rs
@@ -5,7 +5,7 @@
 #![comment = "The Servo Parallel Browser Project"]
 #![license = "MPL"]
 
-#![feature(globs, phase, macro_rules)]
+#![feature(globs, phase, macro_rules, if_let)]
 
 #![deny(unused_imports)]
 #![deny(unused_variables)]
diff --git a/components/compositing/pipeline.rs b/components/compositing/pipeline.rs
index 6dd9dabb0bf..e90bf221e1d 100644
--- a/components/compositing/pipeline.rs
+++ b/components/compositing/pipeline.rs
@@ -3,15 +3,16 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use CompositorProxy;
-use layout_traits::{LayoutTaskFactory, LayoutControlChan};
+use layout_traits::{ExitNowMsg, LayoutTaskFactory, LayoutControlChan};
 use script_traits::{ScriptControlChan, ScriptTaskFactory};
 use script_traits::{AttachLayoutMsg, LoadMsg, NewLayoutInfo, ExitPipelineMsg};
 
 use devtools_traits::DevtoolsControlChan;
+use gfx::paint_task;
 use gfx::paint_task::{PaintPermissionGranted, PaintPermissionRevoked};
 use gfx::paint_task::{PaintChan, PaintTask};
-use servo_msg::constellation_msg::{ConstellationChan, Failure, PipelineId, SubpageId};
-use servo_msg::constellation_msg::{LoadData, WindowSizeData};
+use servo_msg::constellation_msg::{ConstellationChan, ExitMsg, Failure, PipelineId, SubpageId};
+use servo_msg::constellation_msg::{LoadData, WindowSizeData, PipelineExitType};
 use servo_net::image_cache_task::ImageCacheTask;
 use gfx::font_cache_task::FontCacheTask;
 use servo_net::resource_task::ResourceTask;
@@ -173,18 +174,27 @@ impl Pipeline {
         let _ = self.paint_chan.send_opt(PaintPermissionRevoked);
     }
 
-    pub fn exit(&self) {
+    pub fn exit(&self, exit_type: PipelineExitType) {
         debug!("pipeline {} exiting", self.id);
 
         // Script task handles shutting down layout, and layout handles shutting down the painter.
         // For now, if the script task has failed, we give up on clean shutdown.
         let ScriptControlChan(ref chan) = self.script_chan;
-        if chan.send_opt(ExitPipelineMsg(self.id)).is_ok() {
+        if chan.send_opt(ExitPipelineMsg(self.id, exit_type)).is_ok() {
             // Wait until all slave tasks have terminated and run destructors
             // NOTE: We don't wait for script task as we don't always own it
             let _ = self.paint_shutdown_port.recv_opt();
             let _ = self.layout_shutdown_port.recv_opt();
         }
+
+    }
+
+    pub fn force_exit(&self) {
+        let ScriptControlChan(ref script_channel) = self.script_chan;
+        let _ = script_channel.send_opt( ExitPipelineMsg(self.id, PipelineExitType::PipelineOnly));
+        let _ = self.paint_chan.send_opt(paint_task::ExitMsg(None, PipelineExitType::PipelineOnly));
+        let LayoutControlChan(ref layout_channel) = self.layout_chan;
+        let _ = layout_channel.send_opt(ExitNowMsg(PipelineExitType::PipelineOnly));
     }
 
     pub fn to_sendable(&self) -> CompositionPipeline {
diff --git a/components/gfx/paint_task.rs b/components/gfx/paint_task.rs
index d5008bf355d..676e0ec5d6b 100644
--- a/components/gfx/paint_task.rs
+++ b/components/gfx/paint_task.rs
@@ -23,8 +23,8 @@ use layers;
 use native::task::NativeTaskBuilder;
 use servo_msg::compositor_msg::{Epoch, IdlePaintState, LayerId};
 use servo_msg::compositor_msg::{LayerMetadata, PaintListener, PaintingPaintState, ScrollPolicy};
-use servo_msg::constellation_msg::{ConstellationChan, Failure, FailureMsg, PipelineId};
-use servo_msg::constellation_msg::{PainterReadyMsg};
+use servo_msg::constellation_msg::{ConstellationChan, Failure, FailureMsg, PipelineExitType};
+use servo_msg::constellation_msg::{PipelineId, PainterReadyMsg};
 use servo_msg::platform::surface::NativeSurfaceAzureMethods;
 use servo_util::geometry::{Au, ZERO_POINT};
 use servo_util::opts;
@@ -73,7 +73,7 @@ pub enum Msg {
     UnusedBufferMsg(Vec<Box<LayerBuffer>>),
     PaintPermissionGranted,
     PaintPermissionRevoked,
-    ExitMsg(Option<Sender<()>>),
+    ExitMsg(Option<Sender<()>>, PipelineExitType),
 }
 
 #[deriving(Clone)]
@@ -122,6 +122,10 @@ pub struct PaintTask<C> {
 
     /// Communication handles to each of the worker threads.
     worker_threads: Vec<WorkerThreadProxy>,
+
+    /// Tracks the number of buffers that the compositor currently owns. The
+    /// PaintTask waits to exit until all buffers are returned.
+    used_buffer_count: uint,
 }
 
 // If we implement this as a function, we get borrowck errors from borrowing
@@ -201,6 +205,7 @@ impl<C> PaintTask<C> where C: PaintListener + Send {
                     epoch: Epoch(0),
                     buffer_map: BufferMap::new(10000000),
                     worker_threads: worker_threads,
+                    used_buffer_count: 0,
                 };
 
                 paint_task.start();
@@ -223,8 +228,10 @@ impl<C> PaintTask<C> where C: PaintListener + Send {
     }
 
     fn start(&mut self) {
-        debug!("paint_task: beginning painting loop");
+        debug!("PaintTask: beginning painting loop");
 
+        let mut exit_response_channel : Option<Sender<()>> = None;
+        let mut waiting_for_compositor_buffers_to_exit = false;
         loop {
             match self.port.recv() {
                 Msg::PaintInitMsg(stacking_context) => {
@@ -232,7 +239,7 @@ impl<C> PaintTask<C> where C: PaintListener + Send {
                     self.root_stacking_context = Some(stacking_context.clone());
 
                     if !self.paint_permission {
-                        debug!("paint_task: paint ready msg");
+                        debug!("PaintTask: paint ready msg");
                         let ConstellationChan(ref mut c) = self.constellation_chan;
                         c.send(PainterReadyMsg(self.id));
                         continue;
@@ -245,7 +252,7 @@ impl<C> PaintTask<C> where C: PaintListener + Send {
                 }
                 Msg::PaintMsg(requests) => {
                     if !self.paint_permission {
-                        debug!("paint_task: paint ready msg");
+                        debug!("PaintTask: paint ready msg");
                         let ConstellationChan(ref mut c) = self.constellation_chan;
                         c.send(PainterReadyMsg(self.id));
                         self.compositor.paint_msg_discarded();
@@ -265,13 +272,27 @@ impl<C> PaintTask<C> where C: PaintListener + Send {
 
                     self.compositor.set_paint_state(self.id, IdlePaintState);
 
-                    debug!("paint_task: returning surfaces");
+                    for reply in replies.iter() {
+                        let &(_, ref buffer_set) = reply;
+                        self.used_buffer_count += (*buffer_set).buffers.len();
+                    }
+
+                    debug!("PaintTask: returning surfaces");
                     self.compositor.paint(self.id, self.epoch, replies);
                 }
                 Msg::UnusedBufferMsg(unused_buffers) => {
+                    debug!("PaintTask: Received {} unused buffers", unused_buffers.len());
+                    self.used_buffer_count -= unused_buffers.len();
+
                     for buffer in unused_buffers.into_iter().rev() {
                         self.buffer_map.insert(native_graphics_context!(self), buffer);
                     }
+
+                    if waiting_for_compositor_buffers_to_exit && self.used_buffer_count == 0 {
+                        debug!("PaintTask: Received all loaned buffers, exiting.");
+                        exit_response_channel.map(|channel| channel.send(()));
+                        break;
+                    }
                 }
                 Msg::PaintPermissionGranted => {
                     self.paint_permission = true;
@@ -290,10 +311,24 @@ impl<C> PaintTask<C> where C: PaintListener + Send {
                 Msg::PaintPermissionRevoked => {
                     self.paint_permission = false;
                 }
-                Msg::ExitMsg(response_ch) => {
-                    debug!("paint_task: exitmsg response send");
-                    response_ch.map(|ch| ch.send(()));
-                    break;
+                Msg::ExitMsg(response_channel, exit_type) => {
+                    let should_wait_for_compositor_buffers = match exit_type {
+                        PipelineExitType::Complete => false,
+                        PipelineExitType::PipelineOnly => self.used_buffer_count != 0
+                    };
+
+                    if !should_wait_for_compositor_buffers {
+                        debug!("PaintTask: Exiting without waiting for compositor buffers.");
+                        response_channel.map(|channel| channel.send(()));
+                        break;
+                    }
+
+                    // If we own buffers in the compositor and we are not exiting completely, wait
+                    // for the compositor to return buffers, so that we can release them properly.
+                    // When doing a complete exit, the compositor lets all buffers leak.
+                    println!("PaintTask: Saw ExitMsg, {} buffers in use", self.used_buffer_count);
+                    waiting_for_compositor_buffers_to_exit = true;
+                    exit_response_channel = response_channel;
                 }
             }
         }
diff --git a/components/layout/layout_task.rs b/components/layout/layout_task.rs
index 2eeeb31409f..fae8dbb070a 100644
--- a/components/layout/layout_task.rs
+++ b/components/layout/layout_task.rs
@@ -42,8 +42,8 @@ use script::layout_interface::{Reflow, ReflowGoal, ScriptLayoutChan, TrustedNode
 use script_traits::{SendEventMsg, ReflowEvent, ReflowCompleteMsg, OpaqueScriptLayoutChannel};
 use script_traits::{ScriptControlChan, UntrustedNodeAddress};
 use servo_msg::compositor_msg::Scrollable;
-use servo_msg::constellation_msg::{ConstellationChan, PipelineId, Failure, FailureMsg};
-use servo_msg::constellation_msg::{SetCursorMsg};
+use servo_msg::constellation_msg::{ConstellationChan, Failure, FailureMsg, PipelineExitType};
+use servo_msg::constellation_msg::{PipelineId, SetCursorMsg};
 use servo_net::image_cache_task::{ImageCacheTask, ImageResponseMsg};
 use servo_net::local_image_cache::{ImageResponder, LocalImageCache};
 use servo_net::resource_task::{ResourceTask, load_bytes_iter};
@@ -343,8 +343,8 @@ impl LayoutTask {
         match port_to_read {
             PortToRead::Pipeline => {
                 match self.pipeline_port.recv() {
-                    layout_traits::ExitNowMsg => {
-                        self.handle_script_request(Msg::ExitNow, possibly_locked_rw_data)
+                    layout_traits::ExitNowMsg(exit_type) => {
+                        self.handle_script_request(Msg::ExitNow(exit_type), possibly_locked_rw_data)
                     }
                 }
             },
@@ -411,9 +411,9 @@ impl LayoutTask {
                 self.prepare_to_exit(response_chan, possibly_locked_rw_data);
                 return false
             },
-            Msg::ExitNow => {
+            Msg::ExitNow(exit_type) => {
                 debug!("layout: ExitNowMsg received");
-                self.exit_now(possibly_locked_rw_data);
+                self.exit_now(possibly_locked_rw_data, exit_type);
                 return false
             }
         }
@@ -435,9 +435,9 @@ impl LayoutTask {
                         LayoutTask::handle_reap_layout_data(dead_layout_data)
                     }
                 }
-                Msg::ExitNow => {
+                Msg::ExitNow(exit_type) => {
                     debug!("layout task is exiting...");
-                    self.exit_now(possibly_locked_rw_data);
+                    self.exit_now(possibly_locked_rw_data, exit_type);
                     break
                 }
                 _ => {
@@ -450,7 +450,9 @@ impl LayoutTask {
 
     /// Shuts down the layout task now. If there are any DOM nodes left, layout will now (safely)
     /// crash.
-    fn exit_now<'a>(&'a self, possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>) {
+    fn exit_now<'a>(&'a self,
+                    possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>,
+                    exit_type: PipelineExitType) {
         let (response_chan, response_port) = channel();
 
         {
@@ -462,7 +464,7 @@ impl LayoutTask {
             LayoutTask::return_rw_data(possibly_locked_rw_data, rw_data);
         }
 
-        self.paint_chan.send(paint_task::ExitMsg(Some(response_chan)));
+        self.paint_chan.send(paint_task::ExitMsg(Some(response_chan), exit_type));
         response_port.recv()
     }
 
diff --git a/components/layout_traits/lib.rs b/components/layout_traits/lib.rs
index 229791ff928..acc5587ab5a 100644
--- a/components/layout_traits/lib.rs
+++ b/components/layout_traits/lib.rs
@@ -21,8 +21,7 @@ extern crate "util" as servo_util;
 
 use gfx::font_cache_task::FontCacheTask;
 use gfx::paint_task::PaintChan;
-use servo_msg::constellation_msg::{ConstellationChan, PipelineId};
-use servo_msg::constellation_msg::Failure;
+use servo_msg::constellation_msg::{ConstellationChan, Failure, PipelineId, PipelineExitType};
 use servo_net::image_cache_task::ImageCacheTask;
 use servo_net::resource_task::ResourceTask;
 use servo_util::time::TimeProfilerChan;
@@ -31,7 +30,7 @@ use std::comm::Sender;
 
 /// Messages sent to the layout task from the constellation
 pub enum LayoutControlMsg {
-    ExitNowMsg,
+    ExitNowMsg(PipelineExitType),
 }
 
 /// A channel wrapper for constellation messages
diff --git a/components/msg/constellation_msg.rs b/components/msg/constellation_msg.rs
index 78cda641aef..e9e69c669c7 100644
--- a/components/msg/constellation_msg.rs
+++ b/components/msg/constellation_msg.rs
@@ -253,3 +253,10 @@ pub struct PipelineId(pub uint);
 
 #[deriving(Clone, PartialEq, Eq, Hash, Show)]
 pub struct SubpageId(pub uint);
+
+// The type of pipeline exit. During complete shutdowns, pipelines do not have to
+// release resources automatically released on process termination.
+pub enum PipelineExitType {
+    PipelineOnly,
+    Complete,
+}
diff --git a/components/script/layout_interface.rs b/components/script/layout_interface.rs
index b19e789a4ed..2856930b073 100644
--- a/components/script/layout_interface.rs
+++ b/components/script/layout_interface.rs
@@ -11,7 +11,7 @@ use dom::node::LayoutDataRef;
 use geom::point::Point2D;
 use geom::rect::Rect;
 use script_traits::{ScriptControlChan, OpaqueScriptLayoutChannel, UntrustedNodeAddress};
-use servo_msg::constellation_msg::WindowSizeData;
+use servo_msg::constellation_msg::{PipelineExitType, WindowSizeData};
 use servo_util::geometry::Au;
 use std::any::{Any, AnyRefExt};
 use std::comm::{channel, Receiver, Sender};
@@ -50,7 +50,7 @@ pub enum Msg {
 
     /// Requests that the layout task immediately shut down. There must be no more nodes left after
     /// this, or layout will crash.
-    ExitNow,
+    ExitNow(PipelineExitType),
 }
 
 /// Synchronous messages that script can send to layout.
diff --git a/components/script/script_task.rs b/components/script/script_task.rs
index 918d776c59f..4840dab2a3b 100644
--- a/components/script/script_task.rs
+++ b/components/script/script_task.rs
@@ -50,6 +50,7 @@ use servo_msg::constellation_msg::{LoadData, LoadUrlMsg, NavigationDirection, Pi
 use servo_msg::constellation_msg::{Failure, FailureMsg, WindowSizeData, Key, KeyState};
 use servo_msg::constellation_msg::{KeyModifiers, SUPER, SHIFT, CONTROL, ALT, Repeated, Pressed};
 use servo_msg::constellation_msg::{Released};
+use servo_msg::constellation_msg::{PipelineExitType};
 use servo_msg::constellation_msg;
 use servo_net::image_cache_task::ImageCacheTask;
 use servo_net::resource_task::{ResourceTask, Load};
@@ -545,8 +546,11 @@ impl ScriptTask {
         // Process the gathered events.
         for msg in sequential.into_iter() {
             match msg {
-                MixedMessage::FromConstellation(ExitPipelineMsg(id)) =>
-                    if self.handle_exit_pipeline_msg(id) { return false },
+                MixedMessage::FromConstellation(ExitPipelineMsg(id, exit_type)) => {
+                    if self.handle_exit_pipeline_msg(id, exit_type) {
+                        return false
+                    }
+                },
                 MixedMessage::FromConstellation(inner_msg) => self.handle_msg_from_constellation(inner_msg),
                 MixedMessage::FromScript(inner_msg) => self.handle_msg_from_script(inner_msg),
                 MixedMessage::FromDevtools(inner_msg) => self.handle_msg_from_devtools(inner_msg),
@@ -713,20 +717,20 @@ impl ScriptTask {
 
     /// Handles a request to exit the script task and shut down layout.
     /// Returns true if the script task should shut down and false otherwise.
-    fn handle_exit_pipeline_msg(&self, id: PipelineId) -> bool {
+    fn handle_exit_pipeline_msg(&self, id: PipelineId, exit_type: PipelineExitType) -> bool {
         // If root is being exited, shut down all pages
         let page = self.page.borrow_mut();
         if page.id == id {
             debug!("shutting down layout for root page {}", id);
             *self.js_context.borrow_mut() = None;
-            shut_down_layout(&*page, (*self.js_runtime).ptr);
+            shut_down_layout(&*page, (*self.js_runtime).ptr, exit_type);
             return true
         }
 
         // otherwise find just the matching page and exit all sub-pages
         match page.remove(id) {
             Some(ref mut page) => {
-                shut_down_layout(&*page, (*self.js_runtime).ptr);
+                shut_down_layout(&*page, (*self.js_runtime).ptr, exit_type);
                 false
             }
             // TODO(tkuehn): pipeline closing is currently duplicated across
@@ -1261,7 +1265,7 @@ impl ScriptTask {
 }
 
 /// Shuts down layout for the given page tree.
-fn shut_down_layout(page_tree: &Rc<Page>, rt: *mut JSRuntime) {
+fn shut_down_layout(page_tree: &Rc<Page>, rt: *mut JSRuntime, exit_type: PipelineExitType) {
     for page in page_tree.iter() {
         // Tell the layout task to begin shutting down, and wait until it
         // processed this message.
@@ -1290,7 +1294,7 @@ fn shut_down_layout(page_tree: &Rc<Page>, rt: *mut JSRuntime) {
     // Destroy the layout task. If there were node leaks, layout will now crash safely.
     for page in page_tree.iter() {
         let LayoutChan(ref chan) = page.layout_chan;
-        chan.send(layout_interface::Msg::ExitNow);
+        chan.send(layout_interface::Msg::ExitNow(exit_type));
     }
 }
 
diff --git a/components/script_traits/lib.rs b/components/script_traits/lib.rs
index 59a1447a75a..ada4ba7cfbf 100644
--- a/components/script_traits/lib.rs
+++ b/components/script_traits/lib.rs
@@ -26,6 +26,7 @@ use devtools_traits::DevtoolsControlChan;
 use libc::c_void;
 use servo_msg::constellation_msg::{ConstellationChan, PipelineId, Failure, WindowSizeData};
 use servo_msg::constellation_msg::{LoadData, SubpageId, Key, KeyState, KeyModifiers};
+use servo_msg::constellation_msg::PipelineExitType;
 use servo_msg::compositor_msg::ScriptListener;
 use servo_net::image_cache_task::ImageCacheTask;
 use servo_net::resource_task::ResourceTask;
@@ -60,7 +61,7 @@ pub enum ConstellationControlMsg {
     /// Notifies script that window has been resized but to not take immediate action.
     ResizeInactiveMsg(PipelineId, WindowSizeData),
     /// Notifies the script that a pipeline should be closed.
-    ExitPipelineMsg(PipelineId),
+    ExitPipelineMsg(PipelineId, PipelineExitType),
     /// Sends a DOM event.
     SendEventMsg(PipelineId, CompositorEvent),
     /// Notifies script that reflow is finished.