Switch to crossbeam-queue for events

Use a lock-free queue to store the event list
because the event callbacks might be called from
inside a signal handler. Most system calls are not
safe in that context, so we just spin if the queue
is full.

analysis/runtime/Cargo.toml

@@ -17,3 +17,5 @@ bincode = "1.0.1"
 once_cell = "1"
 enum_dispatch = "0.3"
 fs-err = "2"
+crossbeam-queue = "0.3"
+crossbeam-utils = "0.8"

analysis/runtime/src/runtime/backend.rs

@@ -1,7 +1,10 @@
+use crossbeam_queue::ArrayQueue;
+use crossbeam_utils::Backoff;
 use enum_dispatch::enum_dispatch;
 use fs_err::{File, OpenOptions};
 use std::fmt::Debug;
 use std::io::{stderr, BufWriter, Write};
+use std::sync::Arc;
 
 use bincode;
 
@@ -80,18 +83,32 @@ pub enum Backend {
 }
 
 impl Backend {
-    fn write_all(&mut self, events: impl IntoIterator<Item = Event>) {
-        for event in events {
+    fn write_all(&mut self, events: Arc<ArrayQueue<Event>>) {
+        let backoff = Backoff::new();
+        loop {
+            let event = match events.pop() {
+                Some(event) => event,
+                None => {
+                    // We can't use anything with a futex here since
+                    // the event sender might run inside a signal handler
+                    backoff.snooze();
+                    continue;
+                }
+            };
+
             let done = matches!(event.kind, EventKind::Done);
             self.write(event);
             if done {
                 break;
             }
+
+            // Reset the backoff timer since we got an event
+            backoff.reset();
         }
         self.flush();
     }
 
-    pub fn run(&mut self, events: impl IntoIterator<Item = Event>) {
+    pub fn run(&mut self, events: Arc<ArrayQueue<Event>>) {
         let (lock, cvar) = &*FINISHED;
         let mut finished = lock.lock().unwrap();
         self.write_all(events);

analysis/runtime/src/runtime/scoped_runtime.rs

@@ -1,11 +1,10 @@
 use std::{
-    sync::{
-        mpsc::{self, SyncSender},
-        Mutex,
-    },
+    sync::{Arc, Mutex},
     thread,
 };
 
+use crossbeam_queue::ArrayQueue;
+use crossbeam_utils::Backoff;
 use enum_dispatch::enum_dispatch;
 use once_cell::sync::OnceCell;
 
@@ -117,16 +116,32 @@ impl Runtime for MainThreadRuntime {
 }
 
 pub struct BackgroundThreadRuntime {
-    tx: SyncSender<Event>,
+    tx: Arc<ArrayQueue<Event>>,
     finalized: OnceCell<()>,
 }
 
+impl BackgroundThreadRuntime {
+    fn push_event(&self, mut event: Event, can_sleep: bool) {
+        let backoff = Backoff::new();
+        while let Err(event_back) = self.tx.push(event) {
+            if can_sleep {
+                backoff.snooze();
+            } else {
+                // We have no choice but to spin here because
+                // we might be inside a signal handler
+                backoff.spin();
+            }
+            event = event_back;
+        }
+    }
+}
+
 impl ExistingRuntime for BackgroundThreadRuntime {
     fn finalize(&self) {
         // Only run the finalizer once.
         self.finalized.get_or_init(|| {
             // Notify the backend that we're done.
-            self.tx.send(Event::done()).unwrap();
+            self.push_event(Event::done(), true);
 
             // Wait for the backend thread to finish.
             let (lock, cvar) = &*FINISHED;
@@ -147,9 +162,7 @@ impl ExistingRuntime for BackgroundThreadRuntime {
     fn send_event(&self, event: Event) {
         match self.finalized.get() {
             None => {
-                // `.unwrap()` as we're in no place to handle an error here,
-                // unless we should silently drop the [`Event`] instead.
-                self.tx.send(event).unwrap();
+                self.push_event(event, false);
             }
             Some(()) => {
                 // Silently drop the [`Event`] as the [`BackgroundThreadRuntime`] has already been [`BackgroundThreadRuntime::finalize`]d.
@@ -172,7 +185,8 @@ impl Runtime for BackgroundThreadRuntime {
     /// Initialize the [`BackgroundThreadRuntime`], which includes [`thread::spawn`]ing,
     /// so it must be run post-`main`.
     fn try_init(mut backend: Backend) -> Result<Self, AnyError> {
-        let (tx, rx) = mpsc::sync_channel(1024);
+        let tx = Arc::new(ArrayQueue::new(1 << 20));
+        let rx = Arc::clone(&tx);
         thread::spawn(move || backend.run(rx));
         Ok(Self {
             tx,