caddy: Support SetReadBuffer and SyscallConn for QUIC (fix #3998)

Supersedes #3999

listeners.go

@@ -22,6 +22,7 @@ import (
 	"strings"
 	"sync"
 	"sync/atomic"
+	"syscall"
 	"time"
 )
 
@@ -230,6 +231,24 @@ func (fcpc *fakeClosePacketConn) Close() error {
 	return nil
 }
 
+// Supports QUIC implementation: https://github.com/caddyserver/caddy/issues/3998
+func (fcpc fakeClosePacketConn) SetReadBuffer(bytes int) error {
+	if conn, ok := fcpc.PacketConn.(interface{ SetReadBuffer(int) error }); ok {
+		return conn.SetReadBuffer(bytes)
+	}
+	return fmt.Errorf("SetReadBuffer() not implemented for %T", fcpc.PacketConn)
+}
+
+// Supports QUIC implementation: https://github.com/caddyserver/caddy/issues/3998
+func (fcpc fakeClosePacketConn) SyscallConn() (syscall.RawConn, error) {
+	if conn, ok := fcpc.PacketConn.(interface {
+		SyscallConn() (syscall.RawConn, error)
+	}); ok {
+		return conn.SyscallConn()
+	}
+	return nil, fmt.Errorf("SyscallConn() not implemented for %T", fcpc.PacketConn)
+}
+
 // ErrFakeClosed is the underlying error value returned by
 // fakeCloseListener.Accept() after Close() has been called,
 // indicating that it is pretending to be closed so that the
@@ -432,3 +451,11 @@ var (
 )
 
 const maxPortSpan = 65535
+
+// Interface guards (see https://github.com/caddyserver/caddy/issues/3998)
+var (
+	_ (interface{ SetReadBuffer(int) error }) = (*fakeClosePacketConn)(nil)
+	_ (interface {
+		SyscallConn() (syscall.RawConn, error)
+	}) = (*fakeClosePacketConn)(nil)
+)