Read should return buffered data before errors.

Netstack should fully drain receive buffer on calls to read
when in an error state (such as RST received) before returning
an ECONNRESET.

PiperOrigin-RevId: 193734391

tcpip/transport/tcp/endpoint.go

@@ -376,10 +376,11 @@ func (e *endpoint) cleanup() {
 // Read reads data from the endpoint.
 func (e *endpoint) Read(*tcpip.FullAddress) (buffer.View, *tcpip.Error) {
 	e.mu.RLock()
-
 	// The endpoint can be read if it's connected, or if it's already closed
-	// but has some pending unread data.
-	if s := e.state; s != stateConnected && s != stateClosed {
+	// but has some pending unread data. Also note that a RST being received
+	// would cause the state to become stateError so we should allow the
+	// reads to proceed before returning a ECONNRESET.
+	if s := e.state; s != stateConnected && s != stateClosed && e.rcvBufUsed == 0 {
 		e.mu.RUnlock()
 		if s == stateError {
 			return buffer.View{}, e.hardError