internal/netlink: generalise implementation from container

This is useful for uevent implementation.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/netlink/netlink_test.go

@@ -0,0 +1,36 @@
+package netlink
+
+import (
+	"os"
+	"syscall"
+	"testing"
+)
+
+func init() { nlPidOnce.Do(func() {}); nlPid = 1 }
+
+type payloadTestCase struct {
+	name string
+	f    func(c *conn)
+	want []byte
+}
+
+// checkPayload runs multiple payloadTestCase against a stub conn and checks
+// the outgoing message written to its buffer page.
+func checkPayload(t *testing.T, testCases []payloadTestCase) {
+	t.Helper()
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			c := conn{
+				pos: syscall.NLMSG_HDRLEN,
+				buf: make([]byte, os.Getpagesize()),
+			}
+			tc.f(&c)
+			if got := c.pending(); string(got) != string(tc.want) {
+				t.Errorf("pending: %#v, want %#v", got, tc.want)
+			}
+		})
+	}
+}