unix: lock the thread for TestPrctlRetInt

The prctl system calls used in this test modify and examine the
properties of the current thread. For the test to work correctly (and
to avoid interfering with other tests), the thread must be locked for
those calls and should not be reused when the test completes.

Fixes golang/go#50607

Change-Id: I2b14e6c0440155f4287d97f0bb72099af291ba0d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/378555
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux_test.go

@@ -815,16 +815,35 @@ func TestEpoll(t *testing.T) {
 }
 
 func TestPrctlRetInt(t *testing.T) {
-	err := unix.Prctl(unix.PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)
-	if err != nil {
-		t.Skipf("Prctl: %v, skipping test", err)
+	skipc := make(chan bool, 1)
+	skip := func() {
+		skipc <- true
+		runtime.Goexit()
 	}
-	v, err := unix.PrctlRetInt(unix.PR_GET_NO_NEW_PRIVS, 0, 0, 0, 0)
-	if err != nil {
-		t.Fatalf("failed to perform prctl: %v", err)
-	}
-	if v != 1 {
-		t.Fatalf("unexpected return from prctl; got %v, expected %v", v, 1)
+
+	go func() {
+		// This test uses prctl to modify the calling thread, so run it on its own
+		// throwaway thread and do not unlock it when the goroutine exits.
+		runtime.LockOSThread()
+		defer close(skipc)
+
+		err := unix.Prctl(unix.PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)
+		if err != nil {
+			t.Logf("Prctl: %v, skipping test", err)
+			skip()
+		}
+
+		v, err := unix.PrctlRetInt(unix.PR_GET_NO_NEW_PRIVS, 0, 0, 0, 0)
+		if err != nil {
+			t.Errorf("failed to perform prctl: %v", err)
+		}
+		if v != 1 {
+			t.Errorf("unexpected return from prctl; got %v, expected %v", v, 1)
+		}
+	}()
+
+	if <-skipc {
+		t.SkipNow()
 	}
 }