EW-9632 Increase max WebSocket message size limit to 32MiB

See code comments for rationale.

This change will be deployed to the Cloudflare network via an autogate first. Then, once that stabilizes, we'll remove the autogate, making the new limit the default for both production and local development.

Author: harrishancock

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

@@ -13,6 +13,7 @@
 #include <workerd/io/worker.h>
 #include <workerd/jsg/jsg.h>
 #include <workerd/jsg/ser.h>
+#include <workerd/util/autogate.h>
 #include <workerd/util/sentry.h>
 
 #include <kj/compat/url.h>
@@ -475,10 +476,24 @@ WebSocket::Accepted::~Accepted() noexcept(false) {
   }
 }
 
+// Default max WebSocket message size limit. Note that kj-http's own default is 1MiB
+// (`kj::WebSocket::SUGGESTED_MAX_MESSAGE_SIZE`). We've found this to be too small for many commmon
+// use cases, such as proxying Chrome Devtools Protocol messages.
+//
+// JS-RPC messages are size-limited to 32MiB, and it seems to be working well, so we're setting the
+// WebSocket default max message size to match that.
+//
+// TODO(soon): This is currently autogated, so that we can enable the change in production before
+//   enabling it by default in local development. This is so that people do not inadvertently deploy
+//   something that works locally but breaks when deployed.
+static constexpr size_t WEBSOCKET_MAX_MESSAGE_SIZE = 32u << 20;
+
 void WebSocket::startReadLoop(jsg::Lock& js, kj::Maybe<kj::Own<InputGate::CriticalSection>> cs) {
   size_t maxMessageSize = kj::WebSocket::SUGGESTED_MAX_MESSAGE_SIZE;
   if (FeatureFlags::get(js).getIncreaseWebsocketMessageSize()) {
     maxMessageSize = 128u << 20;
+  } else if (util::Autogate::isEnabled(util::AutogateKey::WEBSOCKET_MAX_MESSAGE_SIZE_32M)) {
+    maxMessageSize = WEBSOCKET_MAX_MESSAGE_SIZE;
   }
 
   // If the kj::WebSocket happens to be an AbortableWebSocket (see util/abortable.h), then

src/workerd/util/autogate.c++

@@ -25,6 +25,8 @@ kj::StringPtr KJ_STRINGIFY(AutogateKey key) {
       return "streaming-tail-worker"_kj;
     case AutogateKey::TAIL_STREAM_REFACTOR:
       return "tail-stream-refactor"_kj;
+    case AutogateKey::WEBSOCKET_MAX_MESSAGE_SIZE_32M:
+      return "websocket-max-message-size-32m"_kj;
     case AutogateKey::NumOfKeys:
       KJ_FAIL_ASSERT("NumOfKeys should not be used in getName");
   }

src/workerd/util/autogate.h

@@ -20,6 +20,8 @@ enum class AutogateKey {
   STREAMING_TAIL_WORKER,
   // Enable refactor used to consolidate the different tail worker stream implementations.
   TAIL_STREAM_REFACTOR,
+  // Increase max WebSocket message size to 32MiB.
+  WEBSOCKET_MAX_MESSAGE_SIZE_32M,
   NumOfKeys  // Reserved for iteration.
 };