Stall PaintTask exit until it can release all buffers

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.

1 files changed, 11 insertions, 7 deletions

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));
     }
 }