unix: use same TestSelect on all Unices

Also change t.Error messages to use "got foo, expected bar" notation.

Change-Id: Ia0b452f96b08cbb69a0b64cc2a467b5cf94e72da
Reviewed-on: https://go-review.googlesource.com/c/sys/+/207858
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_unix_test.go

@@ -512,6 +512,57 @@ func TestPoll(t *testing.T) {
 	}
 }
 
+func TestSelect(t *testing.T) {
+	n, err := unix.Select(0, nil, nil, nil, &unix.Timeval{Sec: 0, Usec: 0})
+	if err != nil {
+		t.Fatalf("Select: %v", err)
+	}
+	if n != 0 {
+		t.Fatalf("Select: got %v ready file descriptors, expected 0", n)
+	}
+
+	dur := 250 * time.Millisecond
+	tv := unix.NsecToTimeval(int64(dur))
+	start := time.Now()
+	n, err = unix.Select(0, nil, nil, nil, &tv)
+	took := time.Since(start)
+	if err != nil {
+		t.Fatalf("Select: %v", err)
+	}
+	if n != 0 {
+		t.Fatalf("Select: got %v ready file descriptors, expected 0", n)
+	}
+
+	// On some BSDs the actual timeout might also be slightly less than the requested.
+	// Add an acceptable margin to avoid flaky tests.
+	if took < dur*2/3 {
+		t.Errorf("Select: got %v timeout, expected at least %v", took, dur)
+	}
+
+	rr, ww, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer rr.Close()
+	defer ww.Close()
+
+	if _, err := ww.Write([]byte("HELLO GOPHER")); err != nil {
+		t.Fatal(err)
+	}
+
+	rFdSet := &unix.FdSet{}
+	fd := int(rr.Fd())
+	rFdSet.Set(fd)
+
+	n, err = unix.Select(fd+1, rFdSet, nil, nil, nil)
+	if err != nil {
+		t.Fatalf("Select: %v", err)
+	}
+	if n != 1 {
+		t.Fatalf("Select: got %v ready file descriptors, expected 1", n)
+	}
+}
+
 func TestGetwd(t *testing.T) {
 	fd, err := os.Open(".")
 	if err != nil {