net: refactor readerAtEOF splice test

Refactor TestSplice/readerAtEOF to handle cases where we disable
splice on older kernels better.

If splice is disabled, net.splice and poll.Splice do not get to
observe EOF on the reader, because poll.Splice returns immediately
with EINVAL. The test fails unexpectedly, because the splice operation
is reported as not handled.

This change refactors the test to handle the aforementioned case
correctly, by not calling net.splice directly, but using a higher
level check.

Fixes #27355.

Change-Id: I0d5606b4775213f2dbbb84ef82ddfc3bab662a31
Reviewed-on: https://go-review.googlesource.com/132096
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/splice_test.go

@@ -122,6 +122,7 @@ func testSpliceBig(t *testing.T) {
 func testSpliceHonorsLimitedReader(t *testing.T) {
 	t.Run("stopsAfterN", testSpliceStopsAfterN)
 	t.Run("updatesN", testSpliceUpdatesN)
+	t.Run("readerAtLimit", testSpliceReaderAtLimit)
 }
 
 func testSpliceStopsAfterN(t *testing.T) {
@@ -208,7 +209,7 @@ func testSpliceUpdatesN(t *testing.T) {
 	}
 }
 
-func testSpliceReaderAtEOF(t *testing.T) {
+func testSpliceReaderAtLimit(t *testing.T) {
 	clientUp, serverUp, err := spliceTestSocketPair("tcp")
 	if err != nil {
 		t.Fatal(err)
@@ -222,21 +223,63 @@ func testSpliceReaderAtEOF(t *testing.T) {
 	defer clientDown.Close()
 	defer serverDown.Close()
 
-	serverUp.Close()
-	_, err, handled := splice(serverDown.(*TCPConn).fd, serverUp)
-	if !handled {
-		t.Errorf("closed connection: got err = %v, handled = %t, want handled = true", err, handled)
-	}
 	lr := &io.LimitedReader{
 		N: 0,
 		R: serverUp,
 	}
-	_, err, handled = splice(serverDown.(*TCPConn).fd, lr)
+	_, err, handled := splice(serverDown.(*TCPConn).fd, lr)
 	if !handled {
 		t.Errorf("exhausted LimitedReader: got err = %v, handled = %t, want handled = true", err, handled)
 	}
 }
 
+func testSpliceReaderAtEOF(t *testing.T) {
+	clientUp, serverUp, err := spliceTestSocketPair("tcp")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer clientUp.Close()
+	clientDown, serverDown, err := spliceTestSocketPair("tcp")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer clientDown.Close()
+
+	serverUp.Close()
+
+	// We'd like to call net.splice here and check the handled return
+	// value, but we disable splice on old Linux kernels.
+	//
+	// In that case, poll.Splice and net.splice return a non-nil error
+	// and handled == false. We'd ideally like to see handled == true
+	// because the source reader is at EOF, but if we're running on an old
+	// kernel, and splice is disabled, we won't see EOF from net.splice,
+	// because we won't touch the reader at all.
+	//
+	// Trying to untangle the errors from net.splice and match them
+	// against the errors created by the poll package would be brittle,
+	// so this is a higher level test.
+	//
+	// The following ReadFrom should return immediately, regardless of
+	// whether splice is disabled or not. The other side should then
+	// get a goodbye signal. Test for the goodbye signal.
+	msg := "bye"
+	go func() {
+		serverDown.(*TCPConn).ReadFrom(serverUp)
+		io.WriteString(serverDown, msg)
+		serverDown.Close()
+	}()
+
+	buf := make([]byte, 3)
+	_, err = io.ReadFull(clientDown, buf)
+	if err != nil {
+		t.Errorf("clientDown: %v", err)
+	}
+	if string(buf) != msg {
+		t.Errorf("clientDown got %q, want %q", buf, msg)
+	}
+}
+
 func testSpliceIssue25985(t *testing.T) {
 	front, err := newLocalListener("tcp")
 	if err != nil {