Walk the control buffer by hand instead of allocating a message list per frame

ts.go

@@ -80,21 +80,31 @@ func enableRxTimestamps(fd int) error {
 // means the mac did not produce a stamp.
 const scmTimestampingLen = 3 * int(unsafe.Sizeof(unix.Timespec{}))
 
+// A control message's payload starts at its header rounded up to a pointer, and
+// the next starts at its length rounded up the same way.
+const (
+	cmsgAlignMask = unix.SizeofPtr - 1
+	cmsgDataOff   = (unix.SizeofCmsghdr + cmsgAlignMask) &^ cmsgAlignMask
+)
+
+// Walked by hand rather than through ParseSocketControlMessage, which allocates
+// per call on a path that runs for every frame received.
 func hwTimestamp(oob []byte) (int64, bool) {
-	msgs, err := unix.ParseSocketControlMessage(oob)
-	if err != nil {
-		return 0, false
-	}
-	for _, m := range msgs {
-		if m.Header.Level != unix.SOL_SOCKET || m.Header.Type != unix.SCM_TIMESTAMPING {
-			continue
-		}
-		if len(m.Data) < scmTimestampingLen {
+	for off := 0; off+unix.SizeofCmsghdr <= len(oob); {
+		h := (*unix.Cmsghdr)(unsafe.Pointer(&oob[off]))
+		n := int(h.Len)
+		if n < unix.SizeofCmsghdr || off+n > len(oob) {
 			return 0, false
 		}
-		ts := (*[3]unix.Timespec)(unsafe.Pointer(&m.Data[0]))
-		ns := ts[2].Nano()
-		return ns, ns != 0
+		if h.Level == unix.SOL_SOCKET && h.Type == unix.SCM_TIMESTAMPING {
+			if cmsgDataOff+scmTimestampingLen > n {
+				return 0, false
+			}
+			ts := (*[3]unix.Timespec)(unsafe.Pointer(&oob[off+cmsgDataOff]))
+			ns := ts[2].Nano()
+			return ns, ns != 0
+		}
+		off += (n + cmsgAlignMask) &^ cmsgAlignMask
 	}
 	return 0, false
 }

ts_test.go

@@ -0,0 +1,99 @@
+package main
+
+import (
+	"testing"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+func putCmsg(buf []byte, off int, level, typ int32, dataLen int) int {
+	h := (*unix.Cmsghdr)(unsafe.Pointer(&buf[off]))
+	h.SetLen(unix.CmsgLen(dataLen))
+	h.Level = level
+	h.Type = typ
+	return off + unix.CmsgSpace(dataLen)
+}
+
+func putStamp(buf []byte, off int, raw unix.Timespec) int {
+	next := putCmsg(buf, off, unix.SOL_SOCKET, unix.SCM_TIMESTAMPING, scmTimestampingLen)
+	ts := (*[3]unix.Timespec)(unsafe.Pointer(&buf[off+cmsgDataOff]))
+	ts[2] = raw
+	return next
+}
+
+func TestHwTimestampReadsRawHardwareClock(t *testing.T) {
+	buf := make([]byte, cmsgLen)
+	n := putStamp(buf, 0, unix.Timespec{Sec: 5, Nsec: 7})
+
+	got, ok := hwTimestamp(buf[:n])
+	if !ok || got != 5e9+7 {
+		t.Errorf("hwTimestamp = %d, %v; want 5000000007, true", got, ok)
+	}
+}
+
+func TestHwTimestampSkipsOtherMessages(t *testing.T) {
+	buf := make([]byte, cmsgLen)
+	off := putCmsg(buf, 0, unix.SOL_SOCKET, unix.SCM_RIGHTS, 4)
+	n := putStamp(buf, off, unix.Timespec{Sec: 1, Nsec: 2})
+
+	got, ok := hwTimestamp(buf[:n])
+	if !ok || got != 1e9+2 {
+		t.Errorf("hwTimestamp = %d, %v; want 1000000002, true", got, ok)
+	}
+}
+
+func TestHwTimestampRejectsZeroStamp(t *testing.T) {
+	buf := make([]byte, cmsgLen)
+	n := putStamp(buf, 0, unix.Timespec{})
+
+	if got, ok := hwTimestamp(buf[:n]); ok {
+		t.Errorf("hwTimestamp = %d, %v; want a zero stamp refused", got, ok)
+	}
+}
+
+func TestHwTimestampWithoutAStamp(t *testing.T) {
+	buf := make([]byte, cmsgLen)
+	n := putCmsg(buf, 0, unix.SOL_SOCKET, unix.SCM_RIGHTS, 4)
+
+	if _, ok := hwTimestamp(buf[:n]); ok {
+		t.Error("a buffer carrying no timestamp yielded one")
+	}
+	if _, ok := hwTimestamp(nil); ok {
+		t.Error("an empty buffer yielded a timestamp")
+	}
+}
+
+func TestHwTimestampRefusesBadLengths(t *testing.T) {
+	buf := make([]byte, cmsgLen)
+	n := putStamp(buf, 0, unix.Timespec{Sec: 1})
+	if _, ok := hwTimestamp(buf[:n-1]); ok {
+		t.Error("a message running past the buffer yielded a timestamp")
+	}
+
+	short := make([]byte, cmsgLen)
+	putCmsg(short, 0, unix.SOL_SOCKET, unix.SCM_TIMESTAMPING, scmTimestampingLen)
+	(*unix.Cmsghdr)(unsafe.Pointer(&short[0])).SetLen(unix.SizeofCmsghdr - 1)
+	if _, ok := hwTimestamp(short); ok {
+		t.Error("a message shorter than its own header yielded a timestamp")
+	}
+
+	trunc := make([]byte, cmsgLen)
+	tn := putCmsg(trunc, 0, unix.SOL_SOCKET, unix.SCM_TIMESTAMPING, scmTimestampingLen/3)
+	if _, ok := hwTimestamp(trunc[:tn]); ok {
+		t.Error("a truncated stamp yielded a timestamp")
+	}
+}
+
+func BenchmarkHwTimestamp(b *testing.B) {
+	buf := make([]byte, cmsgLen)
+	n := putStamp(buf, 0, unix.Timespec{Sec: 5, Nsec: 7})
+	oob := buf[:n]
+
+	b.ReportAllocs()
+	for b.Loop() {
+		if _, ok := hwTimestamp(oob); !ok {
+			b.Fatal("no timestamp")
+		}
+	}
+}