Fix: set SO_REUSEADDR for UDP listeners on linux (#630)

common/sockopt/reuseaddr_linux.go

@@ -0,0 +1,19 @@
+package sockopt
+
+import (
+	"net"
+	"syscall"
+)
+
+func UDPReuseaddr(c *net.UDPConn) (err error) {
+	rc, err := c.SyscallConn()
+	if err != nil {
+		return
+	}
+
+	rc.Control(func(fd uintptr) {
+		err = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
+	})
+
+	return
+}

common/sockopt/reuseaddr_other.go

@@ -0,0 +1,11 @@
+// +build !linux
+
+package sockopt
+
+import (
+	"net"
+)
+
+func UDPReuseaddr(c *net.UDPConn) (err error) {
+	return
+}