refactor common test code into package nftest

Converting more test functions to use it (and then splitting out test
functions into their own files) is left for a follow-up commit.

internal/nftest/nftest.go

@@ -0,0 +1,133 @@
+// Package nftest contains utility functions for nftables testing.
+package nftest
+
+import (
+	"bytes"
+	"fmt"
+	"log"
+	"strings"
+	"testing"
+
+	"github.com/google/nftables"
+	"github.com/mdlayher/netlink"
+)
+
+// Recorder provides an nftables connection that does not send to the Linux
+// kernel but instead records netlink messages into the recorder. The recorded
+// requests can later be obtained using Requests and compared using Diff.
+type Recorder struct {
+	requests []netlink.Message
+}
+
+// Conn opens an nftables connection that records netlink messages into the
+// Recorder.
+func (r *Recorder) Conn() (*nftables.Conn, error) {
+	return nftables.New(nftables.WithTestDial(
+		func(req []netlink.Message) ([]netlink.Message, error) {
+			r.requests = append(r.requests, req...)
+
+			// TODO: generate and return acknowledgements
+			for _, msg := range req {
+				log.Printf("msg: %+v", msg)
+			}
+			return req, nil
+		}))
+}
+
+// Requests returns the recorded netlink messages (typically nftables requests).
+func (r *Recorder) Requests() []netlink.Message {
+	return r.requests
+}
+
+// NewRecorder returns a ready-to-use Recorder.
+func NewRecorder() *Recorder {
+	return &Recorder{}
+}
+
+// Diff returns the first difference between the specified netlink messages and
+// the expected netlink message payloads.
+func Diff(got []netlink.Message, want [][]byte) string {
+	for idx, msg := range got {
+		b, err := msg.MarshalBinary()
+		if err != nil {
+			return fmt.Sprintf("msg.MarshalBinary: %v", err)
+		}
+		if len(b) < 16 {
+			continue
+		}
+		b = b[16:]
+		if len(want) == 0 {
+			return fmt.Sprintf("no want entry for message %d: %x", idx, b)
+		}
+		if got, want := b, want[0]; !bytes.Equal(got, want) {
+			return fmt.Sprintf("message %d: %s", idx, linediff(nfdump(got), nfdump(want)))
+		}
+		want = want[1:]
+	}
+	return ""
+}
+
+// MatchRulesetBytes is a test helper that ensures the fillRuleset modifications
+// correspond to the provided want netlink message payloads
+func MatchRulesetBytes(t *testing.T, fillRuleset func(c *nftables.Conn), want [][]byte) {
+	t.Helper()
+
+	rec := NewRecorder()
+
+	c, err := rec.Conn()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	c.FlushRuleset()
+
+	fillRuleset(c)
+
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+
+	if diff := Diff(rec.Requests(), want); diff != "" {
+		t.Errorf("unexpected netlink messages: diff: %s", diff)
+	}
+}
+
+// nfdump returns a hexdump of 4 bytes per line (like nft --debug=all), allowing
+// users to make sense of large byte literals more easily.
+func nfdump(b []byte) string {
+	var buf bytes.Buffer
+	i := 0
+	for ; i < len(b); i += 4 {
+		// TODO: show printable characters as ASCII
+		fmt.Fprintf(&buf, "%02x %02x %02x %02x\n",
+			b[i],
+			b[i+1],
+			b[i+2],
+			b[i+3])
+	}
+	for ; i < len(b); i++ {
+		fmt.Fprintf(&buf, "%02x ", b[i])
+	}
+	return buf.String()
+}
+
+// linediff returns a side-by-side diff of two nfdump() return values, flagging
+// lines which are not equal with an exclamation point prefix.
+func linediff(a, b string) string {
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "got -- want\n")
+	linesA := strings.Split(a, "\n")
+	linesB := strings.Split(b, "\n")
+	for idx, lineA := range linesA {
+		if idx >= len(linesB) {
+			break
+		}
+		lineB := linesB[idx]
+		prefix := "! "
+		if lineA == lineB {
+			prefix = "  "
+		}
+		fmt.Fprintf(&buf, "%s%s -- %s\n", prefix, lineA, lineB)
+	}
+	return buf.String()
+}

nftables_test.go

@@ -30,6 +30,7 @@ import (
 	"github.com/google/nftables"
 	"github.com/google/nftables/binaryutil"
 	"github.com/google/nftables/expr"
+	"github.com/google/nftables/internal/nftest"
 	"github.com/mdlayher/netlink"
 	"github.com/vishvananda/netns"
 	"golang.org/x/sys/unix"
@@ -5057,34 +5058,8 @@ func TestNotrack(t *testing.T) {
 		[]byte("\x00\x00\x00\x0a"),
 	}
 
-	c, err := nftables.New(nftables.WithTestDial(
-		func(req []netlink.Message) ([]netlink.Message, error) {
-			for idx, msg := range req {
-				b, err := msg.MarshalBinary()
-				if err != nil {
-					t.Fatal(err)
-				}
-				if len(b) < 16 {
-					continue
-				}
-				b = b[16:]
-				if len(want) == 0 {
-					t.Errorf("no want entry for message %d: %x", idx, b)
-					continue
-				}
-				if got, want := b, want[0]; !bytes.Equal(got, want) {
-					t.Errorf("message %d: %s", idx, linediff(nfdump(got), nfdump(want)))
-				}
-				want = want[1:]
-			}
-			return req, nil
-		}))
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	c.FlushRuleset()
-
+	nftest.MatchRulesetBytes(t,
+		func(c *nftables.Conn) {
 			filter := c.AddTable(&nftables.Table{
 				Family: nftables.TableFamilyIPv4,
 				Name:   "filter",
@@ -5106,10 +5081,8 @@ func TestNotrack(t *testing.T) {
 					&expr.Notrack{},
 				},
 			})
-
-	if err := c.Flush(); err != nil {
-		t.Fatal(err)
-	}
+		},
+		want)
 }
 
 func TestQuota(t *testing.T) {