udp: smoke-test that GSO actually engages, on real sockets in CI

Nothing previously asserted the offload path works end to end -- a silent fallback to per-packet sends would pass every test and only show up as a throughput regression. Send a batch through a real StdConn over loopback and assert (a) the run left as a single sendmmsg entry (GSO engaged, via a spy around the real syscall) and (b) the kernel carved the superpacket back into the exact original datagrams at the receiver. Fails rather than skips when the probe reports no GSO on a UDP_SEGMENT- capable kernel, so CI (make test, ubuntu-latest) guards engagement. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
2026-08-15 14:37:02 +02:00 · 2026-07-27 16:01:26 -05:00
parent 66cb98e13a
commit f9eb86df9d
1 changed files with 86 additions and 0 deletions
@@ -550,3 +550,89 @@ func TestWriteBatchEIODisablesGSOAndReplays(t *testing.T) {
 		t.Errorf("sendmmsg entry counts = %v, want %v", entryCounts, want)
 	}
 }
+
+// TestGSOEngagesOnLoopback is the offload smoke test: real sockets, real
+// UDP_SEGMENT cmsg, real kernel segmentation over loopback. It asserts
+// both that GSO *engaged* (the whole batch left in a single sendmmsg
+// entry -- a silent fallback to per-packet entries fails the test) and
+// that the kernel carved the superpacket back into the exact original
+// datagrams on the receive side. Runs in CI (make test on ubuntu-latest),
+// which is what guards against the offload path silently degrading.
+func TestGSOEngagesOnLoopback(t *testing.T) {
+	rx, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4(127, 0, 0, 1)})
+	if err != nil {
+		t.Fatalf("listen rx: %v", err)
+	}
+	defer rx.Close()
+	dst := rx.LocalAddr().(*net.UDPAddr).AddrPort()
+
+	uc, err := NewListener(testLogger(), netip.MustParseAddr("127.0.0.1"), 0, false, 8)
+	if err != nil {
+		t.Fatalf("NewListener: %v", err)
+	}
+	sc := uc.(*StdConn)
+	defer sc.Close()
+
+	if !sc.bw.gsoSupported {
+		var un unix.Utsname
+		_ = unix.Uname(&un)
+		release := string(un.Release[:])
+		if major, minor := parseRelease(release); major > 4 || (major == 4 && minor >= 18) {
+			t.Fatalf("kernel %q supports UDP_SEGMENT but the GSO probe failed", release)
+		}
+		t.Skipf("kernel %q predates UDP_SEGMENT (4.18)", release)
+	}
+
+	// Spy on the real syscall to count entries per sendmmsg without
+	// changing what hits the kernel.
+	var entryCounts []int
+	real := sc.bw.sendFn
+	sc.bw.sendFn = func(n int) (int, error) {
+		entryCounts = append(entryCounts, n)
+		return real(n)
+	}
+
+	const numPkts = 8
+	const pktLen = 1200
+	bufs := make([][]byte, numPkts)
+	addrs := make([]netip.AddrPort, numPkts)
+	for i := range bufs {
+		bufs[i] = make([]byte, pktLen)
+		for j := range bufs[i] {
+			bufs[i][j] = byte(i)
+		}
+		addrs[i] = dst
+	}
+
+	written, err := sc.WriteBatch(bufs, addrs, nil)
+	if err != nil {
+		t.Fatalf("WriteBatch: %v", err)
+	}
+	if written != numPkts {
+		t.Fatalf("written = %d, want %d", written, numPkts)
+	}
+	// GSO engaged means the run went out as ONE sendmmsg entry carrying a
+	// UDP_SEGMENT superpacket. Per-packet entries mean it silently fell
+	// back -- exactly the regression this test exists to catch.
+	if len(entryCounts) != 1 || entryCounts[0] != 1 {
+		t.Fatalf("sendmmsg entry counts = %v, want [1]: GSO did not engage", entryCounts)
+	}
+
+	// The kernel must deliver the original datagram boundaries and bytes.
+	_ = rx.SetReadDeadline(time.Now().Add(5 * time.Second))
+	got := make([]byte, pktLen+1)
+	for i := 0; i < numPkts; i++ {
+		n, _, err := rx.ReadFromUDP(got)
+		if err != nil {
+			t.Fatalf("rx read %d: %v", i, err)
+		}
+		if n != pktLen {
+			t.Fatalf("rx read %d: len=%d want %d (kernel segmented at wrong boundary)", i, n, pktLen)
+		}
+		for j := 0; j < n; j++ {
+			if got[j] != byte(i) {
+				t.Fatalf("rx read %d: byte %d = %#x, want %#x", i, j, got[j], byte(i))
+			}
+		}
+	}
+}