quic/server: wire SharedThreadedPacketWriter into QuicServer and QuicServerWorker

quic/QuicConstants.h

@@ -805,6 +805,7 @@ enum class NoWriteReason {
   NO_FRAME,
   NO_BODY,
   SOCKET_FAILURE,
+  WRITER_BACKPRESSURE, // async writer SPSC queue full
 };
 
 enum class NoReadReason {

quic/api/CMakeLists.txt

@@ -5,6 +5,14 @@
 
 # Auto-generated by quic/facebook/generate_cmake.py - DO NOT EDIT MANUALLY
 
+mvfst_add_library(mvfst_api_quic_packet_writer
+  EXPORTED_DEPS
+    mvfst_exception
+    mvfst_constants
+    Folly::folly_io_iobuf
+    Folly::folly_network_address
+)
+
 mvfst_add_library(mvfst_api_quic_batch_writer
   SRCS
     QuicBatchWriter.cpp
@@ -112,6 +120,20 @@ mvfst_add_library(mvfst_api_transport_lite
     Folly::folly_maybe_managed_ptr
 )
 
+mvfst_add_library(mvfst_api_shared_threaded_packet_writer
+  SRCS
+    SharedThreadedPacketWriter.cpp
+  DEPS
+    mvfst_common_mvfst_logging
+  EXPORTED_DEPS
+    mvfst_api_quic_packet_writer
+    mvfst_codec_types
+    mvfst_common_event_fd_queue
+    mvfst_common_events_eventbase
+    Folly::folly
+    Folly::folly_io_async_async_udp_socket
+)
+
 mvfst_add_library(mvfst_api_ack_scheduler
   SRCS
     QuicAckScheduler.cpp
@@ -139,6 +161,7 @@ mvfst_add_library(mvfst_api_transport_helpers
   EXPORTED_DEPS
     mvfst_api_ack_scheduler
     mvfst_api_quic_batch_writer
+    mvfst_api_quic_packet_writer
     mvfst_client_state_and_handshake
     mvfst_codec
     mvfst_codec_pktbuilder

quic/api/IoBufQuicBatch.h

@@ -8,16 +8,12 @@
 #pragma once
 #include <quic/QuicException.h>
 #include <quic/api/QuicBatchWriter.h>
+#include <quic/api/QuicPacketWriter.h>
 #include <quic/client/state/ClientStateMachine.h>
 #include <quic/state/QuicTransportStatsCallback.h>
 
 namespace quic {
 
-struct BufQuicBatchResult {
-  uint64_t packetsSent{0};
-  uint64_t bytesSent{0};
-};
-
 class IOBufQuicBatch {
  public:
   IOBufQuicBatch(

quic/api/QuicPacketWriter.h

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <cstdint>
+
+#include <folly/SocketAddress.h>
+#include <folly/io/IOBuf.h>
+
+#include <quic/QuicException.h>
+#include <quic/common/Expected.h>
+
+namespace quic {
+
+// Defined here (not in IoBufQuicBatch.h) so StateData.h can include this
+// header without pulling in the IoBufQuicBatch → QuicBatchWriter → StateData
+// include cycle.
+struct BufQuicBatchResult {
+  uint64_t packetsSent{0};
+  uint64_t bytesSent{0};
+};
+
+/**
+ * Abstract interface for sending fully-built, encrypted QUIC packets.
+ *
+ * The default path (conn.packetWriter == nullptr) calls IOBufQuicBatch
+ * directly. When conn.packetWriter is set (ChainedMemory data path only),
+ * writeConnectionDataToSocket dispatches through this interface instead.
+ */
+class QuicPacketWriter {
+ public:
+  virtual ~QuicPacketWriter() = default;
+
+  // Called on the EventBase thread. Returns false → stop write loop
+  // (backpressure, not an error). Returns unexpected → close connection.
+  [[nodiscard]] virtual quic::Expected<bool, QuicError> write(
+      BufPtr&& buf,
+      size_t encodedSize,
+      const folly::SocketAddress& peerAddr) = 0;
+
+  [[nodiscard]] virtual quic::Expected<bool, QuicError> flush() = 0;
+
+  // packetsSent counts packets handed to this writer (enqueued or sent inline).
+  virtual BufQuicBatchResult getResult() const = 0;
+
+  // Last retriable errno (EAGAIN/ENOBUFS) seen. Always 0 for async writers —
+  // errno tracking is internal to their drain thread.
+  virtual int getLastRetryableErrno() const {
+    return 0;
+  }
+};
+
+} // namespace quic

quic/api/QuicTransportBaseLite.cpp

@@ -1103,6 +1103,15 @@ QuicTransportBaseLite::getStreamFlowControl(StreamId id) const {
       stream->flowControlState.advertisedMaxOffset);
 }
 
+void QuicTransportBaseLite::setPacketWriter(
+    std::unique_ptr<QuicPacketWriter> writer) {
+  conn_->packetWriter = std::move(writer);
+}
+
+void QuicTransportBaseLite::scheduleWrite() {
+  runOnEvbAsyncOp({.type = AsyncOpType::ConnectionWriteReady});
+}
+
 void QuicTransportBaseLite::runOnEvbAsyncOp(AsyncOpData data) {
   auto evb = getEventBase();
   evb->runInLoop(

quic/api/QuicTransportBaseLite.h

@@ -307,6 +307,13 @@ class QuicTransportBaseLite : virtual public QuicSocketLite,
 
   virtual void cancelAllAppCallbacks(const QuicError& error) noexcept;
 
+  // Install a packet writer on the connection. Must be called before accept().
+  void setPacketWriter(std::unique_ptr<QuicPacketWriter> writer);
+
+  // Schedule a write loop iteration on the connection's EventBase. Safe to
+  // call from any thread; the write will execute on the EventBase thread.
+  void scheduleWrite();
+
   void scheduleTimeout(
       QuicTimerCallback* callback,
       std::chrono::milliseconds timeout);