Remove pause/resume on session level buffer

Summary:
We currently pause all producers if the sum of the egress buffers of all transactions exceeds the write buffer limit.  This turns out to be deterimental to prioritization.

Now, we pass the underlying transport pause state or connection flow control state back to the handlers.  The previous diff in this stack introduces a per-stream buffer limit (64kb default).  To limit total session buffer size, limit the number of concurrent streams or lower the per-stream limit.

Reviewed By: lnicco

Differential Revision: D17097138

fbshipit-source-id: 9025c5be8b318963311c3aaad9ee9a03c0e2265e

quic/api/QuicSocket.h

@@ -299,6 +299,11 @@ class QuicSocket {
   virtual void
   setSendBuffer(StreamId id, size_t maxUnacked, size_t maxUnsent) = 0;
 
+  /**
+   * Return the amount of transport buffer space available for writing
+   */
+  virtual uint64_t getConnectionBufferAvailable() const = 0;
+
   /**
    * Get the flow control settings for the given stream (or connection flow
    * control by passing id=0).  Settings include send and receive window

quic/api/QuicTransportBase.cpp

@@ -539,7 +539,11 @@ void QuicTransportBase::setSendBuffer(
     size_t /*maxUnacked*/,
     size_t /*maxUnsent*/) {}
 
-uint64_t QuicTransportBase::bufferSpaceAvailable() {
+uint64_t QuicTransportBase::getConnectionBufferAvailable() const {
+  return bufferSpaceAvailable();
+}
+
+uint64_t QuicTransportBase::bufferSpaceAvailable() const {
   auto bytesBuffered = conn_->flowControlState.sumCurStreamBufferLen;
   auto totalBufferSpaceAvailable =
       conn_->transportSettings.totalBufferSpaceAvailable;

quic/api/QuicTransportBase.h

@@ -90,7 +90,9 @@ class QuicTransportBase : public QuicSocket {
 
   void setSendBuffer(StreamId id, size_t maxUnacked, size_t maxUnsent) override;
 
-  uint64_t bufferSpaceAvailable();
+  uint64_t getConnectionBufferAvailable() const override;
+
+  uint64_t bufferSpaceAvailable() const;
 
   folly::Expected<QuicSocket::FlowControlState, LocalErrorCode>
   getConnectionFlowControl() const override;

quic/api/test/MockQuicSocket.h

@@ -59,6 +59,7 @@ class MockQuicSocket : public QuicSocket {
   MOCK_CONST_METHOD0(getAppProtocol, folly::Optional<std::string>());
   MOCK_METHOD2(setReceiveWindow, void(StreamId, size_t));
   MOCK_METHOD3(setSendBuffer, void(StreamId, size_t, size_t));
+  MOCK_CONST_METHOD0(getConnectionBufferAvailable, uint64_t());
   MOCK_CONST_METHOD0(
       getConnectionFlowControl,
       folly::Expected<FlowControlState, LocalErrorCode>());