Prevent zombie processes in multi-process mode. (#36329)

This introduces a process manager that holds for each process a
"lifeline": this is the receiving end of a ipc channel that is not used
to send anything, but only to monitor the process presence. We turn that
ipc receiver into a crossbeam one to integrate the monitoring into the
constellation run loop. The sender side is made part of the initial
"UnprivilegedContent" data structure sent to the new process, both for
content and for service worker processes.
When a process dies we currently wait() on it to let the OS do a clean
shutdown.

Signed-off-by: webbeef <me@webbeef.org>

components/constellation/pipeline.rs

@@ -46,6 +46,7 @@ use webrender_api::DocumentId;
 use webrender_traits::CrossProcessCompositorApi;
 
 use crate::event_loop::EventLoop;
+use crate::process_manager::Process;
 use crate::sandboxing::{UnprivilegedContent, spawn_multiprocess};
 
 /// A `Pipeline` is the constellation's view of a `Window`. Each pipeline has an event loop
@@ -201,6 +202,7 @@ pub struct InitialPipelineState {
 pub struct NewPipeline {
     pub pipeline: Pipeline,
     pub bhm_control_chan: Option<IpcSender<BackgroundHangMonitorControlMsg>>,
+    pub lifeline: Option<(IpcReceiver<()>, Process)>,
 }
 
 impl Pipeline {
@@ -210,7 +212,7 @@ impl Pipeline {
     ) -> Result<NewPipeline, Error> {
         // Note: we allow channel creation to panic, since recovering from this
         // probably requires a general low-memory strategy.
-        let (script_chan, bhm_control_chan) = match state.event_loop {
+        let (script_chan, (bhm_control_chan, lifeline)) = match state.event_loop {
             Some(script_chan) => {
                 let new_layout_info = NewLayoutInfo {
                     parent_info: state.parent_pipeline_id,
@@ -226,7 +228,7 @@ impl Pipeline {
                 {
                     warn!("Sending to script during pipeline creation failed ({})", e);
                 }
-                (script_chan, None)
+                (script_chan, (None, None))
             },
             None => {
                 let (script_chan, script_port) = ipc::channel().expect("Pipeline script chan");
@@ -292,17 +294,21 @@ impl Pipeline {
                     player_context: state.player_context,
                     rippy_data: state.rippy_data,
                     user_content_manager: state.user_content_manager,
+                    lifeline_sender: None,
                 };
 
                 // Spawn the child process.
                 //
                 // Yes, that's all there is to it!
-                let bhm_control_chan = if opts::get().multiprocess {
+                let multiprocess_data = if opts::get().multiprocess {
                     let (bhm_control_chan, bhm_control_port) =
                         ipc::channel().expect("Sampler chan");
                     unprivileged_pipeline_content.bhm_control_port = Some(bhm_control_port);
-                    unprivileged_pipeline_content.spawn_multiprocess()?;
-                    Some(bhm_control_chan)
+                    let (sender, receiver) =
+                        ipc::channel().expect("Failed to create lifeline channel");
+                    unprivileged_pipeline_content.lifeline_sender = Some(sender);
+                    let process = unprivileged_pipeline_content.spawn_multiprocess()?;
+                    (Some(bhm_control_chan), Some((receiver, process)))
                 } else {
                     // Should not be None in single-process mode.
                     let register = state
@@ -313,10 +319,10 @@ impl Pipeline {
                         state.layout_factory,
                         register,
                     );
-                    None
+                    (None, None)
                 };
 
-                (EventLoop::new(script_chan), bhm_control_chan)
+                (EventLoop::new(script_chan), multiprocess_data)
             },
         };
 
@@ -333,6 +339,7 @@ impl Pipeline {
         Ok(NewPipeline {
             pipeline,
             bhm_control_chan,
+            lifeline,
         })
     }
 
@@ -498,6 +505,7 @@ pub struct UnprivilegedPipelineContent {
     player_context: WindowGLContext,
     rippy_data: Vec<u8>,
     user_content_manager: UserContentManager,
+    lifeline_sender: Option<IpcSender<()>>,
 }
 
 impl UnprivilegedPipelineContent {
@@ -558,7 +566,7 @@ impl UnprivilegedPipelineContent {
         }
     }
 
-    pub fn spawn_multiprocess(self) -> Result<(), Error> {
+    pub fn spawn_multiprocess(self) -> Result<Process, Error> {
         spawn_multiprocess(UnprivilegedContent::Pipeline(self))
     }