Eliminate RwLock around LanguageServer's outbound message channel

We observed a deadlock when quitting zed. The main thread was attempting
to acquire a write lock to this outbound message sender. We weren't able
to understand exactly how this occurred, but we removed the use of a
lock there, so this shouldn't happen anymore.

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

crates/lsp/src/lsp.rs

@@ -33,7 +33,7 @@ type ResponseHandler = Box<dyn Send + FnOnce(Result<&str, Error>)>;
 
 pub struct LanguageServer {
     next_id: AtomicUsize,
-    outbound_tx: RwLock<Option<channel::Sender<Vec<u8>>>>,
+    outbound_tx: channel::Sender<Vec<u8>>,
     capabilities: watch::Receiver<Option<ServerCapabilities>>,
     notification_handlers: Arc<RwLock<HashMap<&'static str, NotificationHandler>>>,
     response_handlers: Arc<Mutex<HashMap<usize, ResponseHandler>>>,
@@ -213,7 +213,7 @@ impl LanguageServer {
             response_handlers,
             capabilities: capabilities_rx,
             next_id: Default::default(),
-            outbound_tx: RwLock::new(Some(outbound_tx)),
+            outbound_tx,
             executor: executor.clone(),
             io_tasks: Mutex::new(Some((input_task, output_task))),
             initialized: initialized_rx,
@@ -296,37 +296,41 @@ impl LanguageServer {
         let request = Self::request_internal::<request::Initialize>(
             &this.next_id,
             &this.response_handlers,
-            this.outbound_tx.read().as_ref(),
+            &this.outbound_tx,
             params,
         );
         let response = request.await?;
         Self::notify_internal::<notification::Initialized>(
-            this.outbound_tx.read().as_ref(),
+            &this.outbound_tx,
             InitializedParams {},
         )?;
         Ok(response.capabilities)
     }
 
-    pub fn shutdown(&self) -> Option<impl 'static + Send + Future<Output = Result<()>>> {
+    pub fn shutdown(&self) -> Option<impl 'static + Send + Future<Output = Option<()>>> {
         if let Some(tasks) = self.io_tasks.lock().take() {
             let response_handlers = self.response_handlers.clone();
-            let outbound_tx = self.outbound_tx.write().take();
             let next_id = AtomicUsize::new(self.next_id.load(SeqCst));
+            let outbound_tx = self.outbound_tx.clone();
             let mut output_done = self.output_done_rx.lock().take().unwrap();
-            Some(async move {
+            let shutdown_request = Self::request_internal::<request::Shutdown>(
-                Self::request_internal::<request::Shutdown>(
+                &next_id,
-                    &next_id,
+                &response_handlers,
-                    &response_handlers,
+                &outbound_tx,
-                    outbound_tx.as_ref(),
+                (),
-                    (),
+            );
-                )
+            let exit = Self::notify_internal::<notification::Exit>(&outbound_tx, ());
-                .await?;
+            outbound_tx.close();
-                Self::notify_internal::<notification::Exit>(outbound_tx.as_ref(), ())?;
+            Some(
-                drop(outbound_tx);
+                async move {
-                output_done.recv().await;
+                    shutdown_request.await?;
-                drop(tasks);
+                    exit?;
-                Ok(())
+                    output_done.recv().await;
-            })
+                    drop(tasks);
+                    Ok(())
+                }
+                .log_err(),
+            )
         } else {
             None
         }
@@ -375,7 +379,7 @@ impl LanguageServer {
             Self::request_internal::<T>(
                 &this.next_id,
                 &this.response_handlers,
-                this.outbound_tx.read().as_ref(),
+                &this.outbound_tx,
                 params,
             )
             .await
@@ -385,7 +389,7 @@ impl LanguageServer {
     fn request_internal<T: request::Request>(
         next_id: &AtomicUsize,
         response_handlers: &Mutex<HashMap<usize, ResponseHandler>>,
-        outbound_tx: Option<&channel::Sender<Vec<u8>>>,
+        outbound_tx: &channel::Sender<Vec<u8>>,
         params: T::Params,
     ) -> impl 'static + Future<Output = Result<T::Result>>
     where
@@ -415,16 +419,8 @@ impl LanguageServer {
         );
 
         let send = outbound_tx
-            .as_ref()
+            .try_send(message)
-            .ok_or_else(|| {
+            .context("failed to write to language server's stdin");
-                anyhow!("tried to send a request to a language server that has been shut down")
-            })
-            .and_then(|outbound_tx| {
-                outbound_tx
-                    .try_send(message)
-                    .context("failed to write to language server's stdin")?;
-                Ok(())
-            });
         async move {
             send?;
             rx.recv().await.unwrap()
@@ -438,13 +434,13 @@ impl LanguageServer {
         let this = self.clone();
         async move {
             this.initialized.clone().recv().await;
-            Self::notify_internal::<T>(this.outbound_tx.read().as_ref(), params)?;
+            Self::notify_internal::<T>(&this.outbound_tx, params)?;
             Ok(())
         }
     }
 
     fn notify_internal<T: notification::Notification>(
-        outbound_tx: Option<&channel::Sender<Vec<u8>>>,
+        outbound_tx: &channel::Sender<Vec<u8>>,
         params: T::Params,
     ) -> Result<()> {
         let message = serde_json::to_vec(&Notification {
@@ -453,9 +449,6 @@ impl LanguageServer {
             params,
         })
         .unwrap();
-        let outbound_tx = outbound_tx
-            .as_ref()
-            .ok_or_else(|| anyhow!("tried to notify a language server that has been shut down"))?;
         outbound_tx.try_send(message)?;
         Ok(())
     }