Reset WebSocket outgoing message pump status in a single continuation

Author: bcaimano

src/workerd/api/web-socket.c++

@@ -564,20 +564,19 @@ void WebSocket::dispatchOpen(jsg::Lock& js) {
 void WebSocket::ensurePumping(jsg::Lock& js) {
   auto& native = *farNative;
   if (!native.isPumping) {
-    native.isPumping = true;
     auto& context = IoContext::current();
     auto& accepted = KJ_ASSERT_NONNULL(native.state.tryGet<Accepted>());
     auto promise = kj::evalNow([&]() {
-      return accepted.canceler.wrap(pump(context, *outgoingMessages, *accepted.ws));
+      return accepted.canceler.wrap(pump(context, *outgoingMessages, *accepted.ws, native));
     });
+
     // TODO(cleanup): We use awaitIoLegacy() here because we don't want this to count as a pending
     //   event if this is a WebSocketPair with the other end being handled in the same isolate.
     //   In that case, the pump can hang if accept() is never called on the other end. Ideally,
     //   this scenario would be handled in-isolate using jsg::Promise, but that would take some
     //   refactoring.
     context.awaitIoLegacy(kj::mv(promise)).then(js, [this, thisHandle = JSG_THIS](jsg::Lock& js) {
       auto& native = *farNative;
-      native.isPumping = false;
       if (native.outgoingAborted) {
         // Apparently, the peer stopped accepting messages (probably, disconnected entirely), but
         // this didn't cause our writes to fail, maybe due to timing. Let's set the error now.
@@ -588,26 +587,11 @@ void WebSocket::ensurePumping(jsg::Lock& js) {
         native.state.init<Released>();
       }
     }, [this](jsg::Lock& js, jsg::Value&& exception) mutable {
-      farNative->isPumping = false;
-      outgoingMessages->clear();
       reportError(js, kj::mv(exception));
     });
   }
 }
 
-kj::Promise<void> WebSocket::pump(
-    IoContext& context, OutgoingMessagesMap& outgoingMessages, kj::WebSocket& ws) {
-  if (outgoingMessages.size() == 0) {
-    return kj::READY_NOW;
-  } else KJ_IF_MAYBE(promise, outgoingMessages.ordered().begin()->outputLock) {
-    return promise->then([&context, &outgoingMessages, &ws]() mutable {
-      return pumpAfterFrontOutputLock(context, outgoingMessages, ws);
-    });
-  } else {
-    return pumpAfterFrontOutputLock(context, outgoingMessages, ws);
-  }
-}
-
 namespace {
 size_t countBytesFromMessage(const kj::WebSocket::Message& message) {
   // This does not count the extra data of the RPC frame or the savings from any compression.
@@ -633,37 +617,48 @@ size_t countBytesFromMessage(const kj::WebSocket::Message& message) {
 }
 }
 
-kj::Promise<void> WebSocket::pumpAfterFrontOutputLock(
-    IoContext& context, OutgoingMessagesMap& outgoingMessages, kj::WebSocket& ws) {
-  GatedMessage gatedMessage =
-      outgoingMessages.release(*outgoingMessages.ordered().begin());
-  auto size = countBytesFromMessage(gatedMessage.message);
+kj::Promise<void> WebSocket::pump(
+    IoContext& context, OutgoingMessagesMap& outgoingMessages, kj::WebSocket& ws, Native& native) {
+  KJ_ASSERT(!native.isPumping);
+  native.isPumping = true;
+  KJ_DEFER({
+    // We use a KJ_DEFER to set native.isPumping = false to ensure that it happens -- we had a bug
+    // in the past where this was handled by the caller of WebSocket::pump() and it allowed for
+    // messages to get stuck in `outgoingMessages` until the pump task was restarted.
+    native.isPumping = false;
+
+    // Either we were already through all our outgoing messages or we experienced failure/
+    // cancellation and cannot send these anyway.
+    outgoingMessages.clear();
+  });
+
+  while (outgoingMessages.size() > 0) {
+    GatedMessage gatedMessage = outgoingMessages.release(*outgoingMessages.ordered().begin());
+    KJ_IF_MAYBE(promise, gatedMessage.outputLock) {
+      co_await *promise;
+    }
+
+    auto size = countBytesFromMessage(gatedMessage.message);
 
-  kj::Promise<void> promise = nullptr;
-  {
     KJ_SWITCH_ONEOF(gatedMessage.message) {
       KJ_CASE_ONEOF(text, kj::String) {
-        promise = ws.send(text);
+        co_await ws.send(text);
         break;
       }
       KJ_CASE_ONEOF(data, kj::Array<byte>) {
-        promise = ws.send(data);
+        co_await ws.send(data);
         break;
       }
       KJ_CASE_ONEOF(close, kj::WebSocket::Close) {
-        promise = ws.close(close.code, close.reason);
+        co_await ws.close(close.code, close.reason);
         break;
       }
     }
-  }
 
-  return promise.attach(kj::mv(gatedMessage.message))
-      .then([&context, &outgoingMessages, &ws, size]() {
     KJ_IF_MAYBE(a, context.getActor()) {
       a->getMetrics().sentWebSocketMessage(size);
     }
-    return pump(context, outgoingMessages, ws);
-  });
+  }
 }
 
 kj::Promise<void> WebSocket::readLoop(kj::WebSocket& ws) {

src/workerd/api/web-socket.h

@@ -346,9 +346,7 @@ class WebSocket: public EventTarget {
   void ensurePumping(jsg::Lock& js);
 
   static kj::Promise<void> pump(
-      IoContext& context, OutgoingMessagesMap& outgoingMessages, kj::WebSocket& ws);
-  static kj::Promise<void> pumpAfterFrontOutputLock(
-      IoContext& context, OutgoingMessagesMap& outgoingMessages, kj::WebSocket& ws);
+      IoContext& context, OutgoingMessagesMap& outgoingMessages, kj::WebSocket& ws, Native& native);
   // Write messages from `outgoingMessages` into `ws`.
   //
   // These are not necessarily called under isolate lock, but they are called on the given