unix: don't overwrite unrelated file descriptors in TestDup

TestDup used a file descriptor without ensuring it was free,
leading to rare crashes in the runtime netpoller when the victim fd
was the polling descriptor.

Updates golang/go#29423

Change-Id: Idc8b6b47f7e966e045f57f2028e7b6b79e0fb3f3
Reviewed-on: https://go-review.googlesource.com/c/163638Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_unix_test.go

@@ -396,14 +396,24 @@ func TestDup(t *testing.T) {
 		t.Fatalf("Dup: %v", err)
 	}
 
-	err = unix.Dup2(newFd, newFd+1)
+	// Create and reserve a file descriptor.
+	// Dup2 automatically closes it before reusing it.
+	nullFile, err := os.Open("/dev/null")
+	if err != nil {
+		t.Fatal(err)
+	}
+	dupFd := int(file.Fd())
+	err = unix.Dup2(newFd, dupFd)
 	if err != nil {
 		t.Fatalf("Dup2: %v", err)
 	}
+	// Keep the dummy file open long enough to not be closed in
+	// its finalizer.
+	runtime.KeepAlive(nullFile)
 
 	b1 := []byte("Test123")
 	b2 := make([]byte, 7)
-	_, err = unix.Write(newFd+1, b1)
+	_, err = unix.Write(dupFd, b1)
 	if err != nil {
 		t.Fatalf("Write to dup2 fd failed: %v", err)
 	}