2 files changed, 69 insertions, 20 deletions

diff --git a/cli/tests/unit/websocket_test.ts b/cli/tests/unit/websocket_test.ts
index 6737edc11..948e2add2 100644
--- a/cli/tests/unit/websocket_test.ts
+++ b/cli/tests/unit/websocket_test.ts
@@ -36,3 +36,49 @@ Deno.test(async function websocketPingPong() {
   await promise;
   ws.close();
 });
+
+// https://github.com/denoland/deno/issues/18700
+Deno.test(
+  { sanitizeOps: false, sanitizeResources: false },
+  async function websocketWriteLock() {
+    const ac = new AbortController();
+    const listeningPromise = deferred();
+
+    const server = Deno.serve({
+      handler: (req) => {
+        const { socket, response } = Deno.upgradeWebSocket(req);
+        socket.onopen = function () {
+          setTimeout(() => socket.send("Hello"), 500);
+        };
+        socket.onmessage = function (e) {
+          assertEquals(e.data, "Hello");
+          ac.abort();
+        };
+        return response;
+      },
+      signal: ac.signal,
+      onListen: () => listeningPromise.resolve(),
+      hostname: "localhost",
+      port: 4246,
+    });
+
+    await listeningPromise;
+    const promise = deferred();
+    const ws = new WebSocket("ws://localhost:4246/");
+    assertEquals(ws.url, "ws://localhost:4246/");
+    ws.onerror = () => fail();
+    ws.onmessage = (e) => {
+      assertEquals(e.data, "Hello");
+      setTimeout(() => {
+        ws.send(e.data);
+      }, 1000);
+      promise.resolve();
+    };
+    ws.onclose = () => {
+      promise.resolve();
+    };
+
+    await Promise.all([promise, server]);
+    ws.close();
+  },
+);
diff --git a/ext/websocket/server.rs b/ext/websocket/server.rs
index eb8737b19..44bc07e59 100644
--- a/ext/websocket/server.rs
+++ b/ext/websocket/server.rs
@@ -28,6 +28,23 @@ pub struct ServerWebSocket {
   ws: AsyncRefCell<FragmentCollector<Pin<Box<dyn Upgraded>>>>,
 }
 
+impl ServerWebSocket {
+  #[inline]
+  pub async fn write_frame(
+    self: Rc<Self>,
+    frame: Frame,
+  ) -> Result<(), AnyError> {
+    // SAFETY: fastwebsockets only needs a mutable reference to the WebSocket
+    // to populate the write buffer. We encounter an await point when writing
+    // to the socket after the frame has already been written to the buffer.
+    let ws = unsafe { &mut *self.ws.as_ptr() };
+    ws.write_frame(frame)
+      .await
+      .map_err(|err| type_error(err.to_string()))?;
+    Ok(())
+  }
+}
+
 impl Resource for ServerWebSocket {
   fn name(&self) -> Cow<str> {
     "serverWebSocket".into()
@@ -61,12 +78,9 @@ pub async fn op_server_ws_send_binary(
     .borrow_mut()
     .resource_table
     .get::<ServerWebSocket>(rid)?;
-
-  let mut ws = RcRef::map(&resource, |r| &r.ws).borrow_mut().await;
-  ws.write_frame(Frame::new(true, OpCode::Binary, None, data.to_vec()))
+  resource
+    .write_frame(Frame::new(true, OpCode::Binary, None, data.to_vec()))
     .await
-    .map_err(|err| type_error(err.to_string()))?;
-  Ok(())
 }
 
 #[op]
@@ -79,11 +93,9 @@ pub async fn op_server_ws_send_text(
     .borrow_mut()
     .resource_table
     .get::<ServerWebSocket>(rid)?;
-  let mut ws = RcRef::map(&resource, |r| &r.ws).borrow_mut().await;
-  ws.write_frame(Frame::new(true, OpCode::Text, None, data.into_bytes()))
+  resource
+    .write_frame(Frame::new(true, OpCode::Text, None, data.into_bytes()))
     .await
-    .map_err(|err| type_error(err.to_string()))?;
-  Ok(())
 }
 
 #[op]
@@ -107,12 +119,7 @@ pub async fn op_server_ws_send(
     .borrow_mut()
     .resource_table
     .get::<ServerWebSocket>(rid)?;
-  let mut ws = RcRef::map(&resource, |r| &r.ws).borrow_mut().await;
-
-  ws.write_frame(msg)
-    .await
-    .map_err(|err| type_error(err.to_string()))?;
-  Ok(())
+  resource.write_frame(msg).await
 }
 
 #[op(deferred)]
@@ -126,14 +133,10 @@ pub async fn op_server_ws_close(
     .borrow_mut()
     .resource_table
     .get::<ServerWebSocket>(rid)?;
-  let mut ws = RcRef::map(&resource, |r| &r.ws).borrow_mut().await;
   let frame = reason
     .map(|reason| Frame::close(code.unwrap_or(1005), reason.as_bytes()))
     .unwrap_or_else(|| Frame::close_raw(vec![]));
-  ws.write_frame(frame)
-    .await
-    .map_err(|err| type_error(err.to_string()))?;
-  Ok(())
+  resource.write_frame(frame).await
 }
 
 #[op(deferred)]