Add TCP_NOTSENT_LOWAT setting

network/v2/sockopts_lowat.go

@@ -0,0 +1,20 @@
+//go:build linux || darwin
+
+package network
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// setNotSentLowat sets TCP_NOTSENT_LOWAT which limits the amount of
+// unsent data queued in the kernel write buffer. Once unsent data drops
+// below this threshold the socket becomes writable again, applying
+// back-pressure to the relay loop instead of piling up data in kernel
+// buffers. This reduces per-connection memory and bufferbloat.
+func setNotSentLowat(conn syscall.RawConn) {
+	conn.Control(func(fd uintptr) { //nolint: errcheck
+		unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_NOTSENT_LOWAT, tcpNotSentLowat) //nolint: errcheck
+	})
+}