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/process_manager.rs

@@ -0,0 +1,68 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use std::process::Child;
+
+use crossbeam_channel::{Receiver, Select};
+use log::{debug, warn};
+
+pub enum Process {
+    Unsandboxed(Child),
+    Sandboxed(u32),
+}
+
+impl Process {
+    fn pid(&self) -> u32 {
+        match self {
+            Self::Unsandboxed(child) => child.id(),
+            Self::Sandboxed(pid) => *pid,
+        }
+    }
+
+    fn wait(&mut self) {
+        match self {
+            Self::Unsandboxed(child) => {
+                let _ = child.wait();
+            },
+            Self::Sandboxed(_pid) => {
+                // TODO: use nix::waitpid() on supported platforms.
+                warn!("wait() is not yet implemented for sandboxed processes.");
+            },
+        }
+    }
+}
+
+type ProcessReceiver = Receiver<Result<(), ipc_channel::Error>>;
+
+pub(crate) struct ProcessManager {
+    processes: Vec<(Process, ProcessReceiver)>,
+}
+
+impl ProcessManager {
+    pub fn new() -> Self {
+        Self { processes: vec![] }
+    }
+
+    pub fn add(&mut self, receiver: ProcessReceiver, process: Process) {
+        debug!("Adding process pid={}", process.pid());
+        self.processes.push((process, receiver));
+    }
+
+    pub fn register<'a>(&'a self, select: &mut Select<'a>) {
+        for (_, receiver) in &self.processes {
+            select.recv(receiver);
+        }
+    }
+
+    pub fn receiver_at(&self, index: usize) -> &ProcessReceiver {
+        let (_, receiver) = &self.processes[index];
+        receiver
+    }
+
+    pub fn remove(&mut self, index: usize) {
+        let (mut process, _) = self.processes.swap_remove(index);
+        debug!("Removing process pid={}", process.pid());
+        process.wait();
+    }
+}