netstack: don't allocate to get send buffer size

In a redis-benchmark PING_INLINE test, this reduces allocations by 7.7%.

PiperOrigin-RevId: 616263988

pkg/tcpip/stack/stack.go

@@ -495,6 +495,16 @@ func (s *Stack) TransportProtocolOption(transport tcpip.TransportProtocolNumber,
 	return transProtoState.proto.Option(option)
 }
 
+// SendBufSizeProto is a protocol that can return its send buffer size.
+type SendBufSizeProto interface {
+	SendBufferSize() tcpip.TCPSendBufferSizeRangeOption
+}
+
+// TCPSendBufferLimits returns the TCP send buffer size limit.
+func (s *Stack) TCPSendBufferLimits() tcpip.TCPSendBufferSizeRangeOption {
+	return s.transportProtocols[header.TCPProtocolNumber].proto.(SendBufSizeProto).SendBufferSize()
+}
+
 // SetTransportProtocolHandler sets the per-stack default handler for the given
 // protocol.
 //

pkg/tcpip/transport/tcp/endpoint.go

@@ -3228,12 +3228,10 @@ func (e *endpoint) SocketOptions() *tcpip.SocketOptions {
 }
 
 // GetTCPSendBufferLimits is used to get send buffer size limits for TCP.
-func GetTCPSendBufferLimits(s tcpip.StackHandler) tcpip.SendBufferSizeOption {
-	var ss tcpip.TCPSendBufferSizeRangeOption
-	if err := s.TransportProtocolOption(header.TCPProtocolNumber, &ss); err != nil {
-		panic(fmt.Sprintf("s.TransportProtocolOption(%d, %#v) = %s", header.TCPProtocolNumber, ss, err))
-	}
-
+func GetTCPSendBufferLimits(sh tcpip.StackHandler) tcpip.SendBufferSizeOption {
+	// This type assertion is safe because only the TCP stack calls this
+	// function.
+	ss := sh.(*stack.Stack).TCPSendBufferLimits()
 	return tcpip.SendBufferSizeOption{
 		Min:     ss.Min,
 		Default: ss.Default,

pkg/tcpip/transport/tcp/protocol.go

@@ -480,6 +480,13 @@ func (p *protocol) Option(option tcpip.GettableTransportProtocolOption) tcpip.Er
 	}
 }
 
+// SendBufferSize implements stack.SendBufSizeProto.
+func (p *protocol) SendBufferSize() tcpip.TCPSendBufferSizeRangeOption {
+	p.mu.RLock()
+	defer p.mu.RUnlock()
+	return p.sendBufferSize
+}
+
 // Close implements stack.TransportProtocol.Close.
 func (p *protocol) Close() {
 	p.dispatcher.close()