add a function to set the UDP send buffer size

This function is the equivalent to the function used to set the UDP
receive buffer size. It's so similar that code generation is used to
make a copy of the setReceiveBuffer function.

sys_conn_buffers_write.go

@@ -0,0 +1,70 @@
+// Code generated by go generate. DO NOT EDIT.
+// Source: sys_conn_buffers.go
+
+package quic
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"syscall"
+
+	"github.com/quic-go/quic-go/internal/protocol"
+	"github.com/quic-go/quic-go/internal/utils"
+)
+
+func setSendBuffer(c net.PacketConn, logger utils.Logger) error {
+	conn, ok := c.(interface{ SetWriteBuffer(int) error })
+	if !ok {
+		return errors.New("connection doesn't allow setting of send buffer size. Not a *net.UDPConn?")
+	}
+
+	var syscallConn syscall.RawConn
+	if sc, ok := c.(interface {
+		SyscallConn() (syscall.RawConn, error)
+	}); ok {
+		var err error
+		syscallConn, err = sc.SyscallConn()
+		if err != nil {
+			syscallConn = nil
+		}
+	}
+	// The connection has a SetWriteBuffer method, but we couldn't obtain a syscall.RawConn.
+	// This shouldn't happen for a net.UDPConn, but is possible if the connection just implements the
+	// net.PacketConn interface and the SetWriteBuffer method.
+	// We have no way of checking if increasing the buffer size actually worked.
+	if syscallConn == nil {
+		return conn.SetWriteBuffer(protocol.DesiredSendBufferSize)
+	}
+
+	size, err := inspectWriteBuffer(syscallConn)
+	if err != nil {
+		return fmt.Errorf("failed to determine send buffer size: %w", err)
+	}
+	if size >= protocol.DesiredSendBufferSize {
+		logger.Debugf("Conn has send buffer of %d kiB (wanted: at least %d kiB)", size/1024, protocol.DesiredSendBufferSize/1024)
+		return nil
+	}
+	// Ignore the error. We check if we succeeded by querying the buffer size afterward.
+	_ = conn.SetWriteBuffer(protocol.DesiredSendBufferSize)
+	newSize, err := inspectWriteBuffer(syscallConn)
+	if newSize < protocol.DesiredSendBufferSize {
+		// Try again with RCVBUFFORCE on Linux
+		_ = forceSetSendBuffer(syscallConn, protocol.DesiredSendBufferSize)
+		newSize, err = inspectWriteBuffer(syscallConn)
+		if err != nil {
+			return fmt.Errorf("failed to determine send buffer size: %w", err)
+		}
+	}
+	if err != nil {
+		return fmt.Errorf("failed to determine send buffer size: %w", err)
+	}
+	if newSize == size {
+		return fmt.Errorf("failed to increase send buffer size (wanted: %d kiB, got %d kiB)", protocol.DesiredSendBufferSize/1024, newSize/1024)
+	}
+	if newSize < protocol.DesiredSendBufferSize {
+		return fmt.Errorf("failed to sufficiently increase send buffer size (was: %d kiB, wanted: %d kiB, got: %d kiB)", size/1024, protocol.DesiredSendBufferSize/1024, newSize/1024)
+	}
+	logger.Debugf("Increased send buffer size to %d kiB", newSize/1024)
+	return nil
+}