Link refactor (#941)

* Link refactoring

* More refactoring

* More tweaking

* Cleaner shutdowns, UNIX socket support, more tweaks

* Actorise links, remove mutex

* SOCKS support

src/core/link_tcp_linux.go

@@ -0,0 +1,46 @@
+//go:build linux
+// +build linux
+
+package core
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// WARNING: This context is used both by net.Dialer and net.Listen in tcp.go
+
+func (t *linkTCP) tcpContext(network, address string, c syscall.RawConn) error {
+	var control error
+	var bbr error
+
+	control = c.Control(func(fd uintptr) {
+		bbr = unix.SetsockoptString(int(fd), unix.IPPROTO_TCP, unix.TCP_CONGESTION, "bbr")
+	})
+
+	// Log any errors
+	if bbr != nil {
+		t.links.core.log.Debugln("Failed to set tcp_congestion_control to bbr for socket, SetsockoptString error:", bbr)
+	}
+	if control != nil {
+		t.links.core.log.Debugln("Failed to set tcp_congestion_control to bbr for socket, Control error:", control)
+	}
+
+	// Return nil because errors here are not considered fatal for the connection, it just means congestion control is suboptimal
+	return nil
+}
+
+func (t *linkTCP) getControl(sintf string) func(string, string, syscall.RawConn) error {
+	return func(network, address string, c syscall.RawConn) error {
+		var err error
+		btd := func(fd uintptr) {
+			err = unix.BindToDevice(int(fd), sintf)
+		}
+		_ = c.Control(btd)
+		if err != nil {
+			t.links.core.log.Debugln("Failed to set SO_BINDTODEVICE:", sintf)
+		}
+		return t.tcpContext(network, address, c)
+	}
+}