unix: add TestEpoll on linux

This tests the epoll functionality on linux and should hopefully help
catch GOARCHes where additional padding of EpollEvent is needed (see
e.g.  CL 189877).

Change-Id: Icd19746a60a63016a5d46535a2cc557ca7a0d474
Reviewed-on: https://go-review.googlesource.com/c/sys/+/205398
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux_test.go

@@ -659,3 +659,40 @@ func openMountByID(mountID int) (f *os.File, err error) {
 	}
 	return nil, errors.New("mountID not found")
 }
+
+func TestEpoll(t *testing.T) {
+	efd, err := unix.EpollCreate1(unix.EPOLL_CLOEXEC)
+	if err != nil {
+		t.Fatalf("EpollCreate1: %v", err)
+	}
+	defer unix.Close(efd)
+
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer r.Close()
+	defer w.Close()
+
+	fd := int(r.Fd())
+	ev := unix.EpollEvent{Events: unix.EPOLLIN, Fd: int32(fd)}
+
+	err = unix.EpollCtl(efd, unix.EPOLL_CTL_ADD, fd, &ev)
+	if err != nil {
+		t.Fatalf("EpollCtl: %v", err)
+	}
+
+	if _, err := w.Write([]byte("HELLO GOPHER")); err != nil {
+		t.Fatal(err)
+	}
+
+	events := make([]unix.EpollEvent, 128)
+	n, err := unix.EpollWait(efd, events, 1)
+	if err != nil {
+		t.Fatalf("EpollWait: %v", err)
+	}
+
+	if n != 1 {
+		t.Logf("EpollWait: wrong number of events: got %v, expected 1", n)
+	}
+}