unix: add ParseOneSocketControlMessage to parse control messages without allocating

Fixes golang/go#54714.

Change-Id: If711272937078b6c696756823aa4dfcec358b719
Reviewed-on: https://go-review.googlesource.com/c/sys/+/425917
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>

unix/sockcmsg_unix.go

@@ -52,6 +52,20 @@ func ParseSocketControlMessage(b []byte) ([]SocketControlMessage, error) {
 	return msgs, nil
 }
 
+// ParseOneSocketControlMessage parses a single socket control message from b, returning the message header,
+// message data (a slice of b), and the remainder of b after that single message.
+// When there are no remaining messages, len(remainder) == 0.
+func ParseOneSocketControlMessage(b []byte) (hdr Cmsghdr, data []byte, remainder []byte, err error) {
+	h, dbuf, err := socketControlMessageHeaderAndData(b)
+	if err != nil {
+		return Cmsghdr{}, nil, nil, err
+	}
+	if i := cmsgAlignOf(int(h.Len)); i < len(b) {
+		remainder = b[i:]
+	}
+	return *h, dbuf, remainder, nil
+}
+
 func socketControlMessageHeaderAndData(b []byte) (*Cmsghdr, []byte, error) {
 	h := (*Cmsghdr)(unsafe.Pointer(&b[0]))
 	if h.Len < SizeofCmsghdr || uint64(h.Len) > uint64(len(b)) {

unix/syscall_unix_test.go

@@ -322,7 +322,7 @@ func passFDChild() {
 	}
 }
 
-// TestUnixRightsRoundtrip tests that UnixRights, ParseSocketControlMessage,
+// TestUnixRightsRoundtrip tests that UnixRights, ParseSocketControlMessage, ParseOneSocketControlMessage,
 // and ParseUnixRights are able to successfully round-trip lists of file descriptors.
 func TestUnixRightsRoundtrip(t *testing.T) {
 	testCases := [...][][]int{
@@ -350,6 +350,23 @@ func TestUnixRightsRoundtrip(t *testing.T) {
 		if len(scms) != len(testCase) {
 			t.Fatalf("expected %v SocketControlMessage; got scms = %#v", len(testCase), scms)
 		}
+
+		var c int
+		for len(b) > 0 {
+			hdr, data, remainder, err := unix.ParseOneSocketControlMessage(b)
+			if err != nil {
+				t.Fatalf("ParseOneSocketControlMessage: %v", err)
+			}
+			if scms[c].Header != hdr || !bytes.Equal(scms[c].Data, data) {
+				t.Fatal("expected SocketControlMessage header and data to match")
+			}
+			b = remainder
+			c++
+		}
+		if c != len(scms) {
+			t.Fatalf("expected %d SocketControlMessages; got %d", len(scms), c)
+		}
+
 		for i, scm := range scms {
 			gotFds, err := unix.ParseUnixRights(&scm)
 			if err != nil {