[release-branch.go1.7] syscall: use MNT_NOWAIT in TestGetfsstat

Fixes test failure when VMWare's shared folder filesystem is present.

MNT_NOWAIT is what the mount(8) command does.

Fixes #16937

Change-Id: Id436185f544b7069db46c8716d6a0bf580b31da0
Reviewed-on: https://go-review.googlesource.com/28550
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-on: https://go-review.googlesource.com/28650
Run-TryBot: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/syscall_bsd_test.go

@@ -7,28 +7,45 @@
 package syscall_test
 
 import (
+	"os/exec"
 	"syscall"
 	"testing"
 )
 
 const MNT_WAIT = 1
+const MNT_NOWAIT = 2
 
 func TestGetfsstat(t *testing.T) {
-	n, err := syscall.Getfsstat(nil, MNT_WAIT)
+	const flags = MNT_NOWAIT // see Issue 16937
+	n, err := syscall.Getfsstat(nil, flags)
+	t.Logf("Getfsstat(nil, %d) = (%v, %v)", flags, n, err)
 	if err != nil {
 		t.Fatal(err)
 	}
 
 	data := make([]syscall.Statfs_t, n)
-	n, err = syscall.Getfsstat(data, MNT_WAIT)
+	n2, err := syscall.Getfsstat(data, flags)
+	t.Logf("Getfsstat([]syscall.Statfs_t, %d) = (%v, %v)", flags, n2, err)
 	if err != nil {
 		t.Fatal(err)
 	}
-
-	empty := syscall.Statfs_t{}
-	for _, stat := range data {
-		if stat == empty {
-			t.Fatal("an empty Statfs_t struct was returned")
+	if n != n2 {
+		t.Errorf("Getfsstat(nil) = %d, but subsequent Getfsstat(slice) = %d", n, n2)
+	}
+	for i, stat := range data {
+		if stat == (syscall.Statfs_t{}) {
+			t.Errorf("index %v is an empty Statfs_t struct", i)
+		}
+	}
+	if t.Failed() {
+		for i, stat := range data[:n2] {
+			t.Logf("data[%v] = %+v", i, stat)
+		}
+		mount, err := exec.Command("mount").CombinedOutput()
+		if err != nil {
+			t.Logf("mount: %v\n%s", err, mount)
+		} else {
+			t.Logf("mount: %s", mount)
 		}
 	}
 }