Add tests/sharedstorage

Towards better test coverage of shared backing storage
mounts.

https://github.com/rfjakob/gocryptfs/issues/525

tests/sharedstorage/sharedstorage_test.go

@@ -0,0 +1,61 @@
+// test gocryptfs cipherdir mounted multiple times at the same time
+package sharedstorage
+
+import (
+	"testing"
+
+	"golang.org/x/sys/unix"
+
+	"github.com/rfjakob/gocryptfs/tests/test_helpers"
+)
+
+var flagSharestorage bool
+
+func TestMain(m *testing.M) {
+	flagSharestorage = false
+	m.Run()
+	flagSharestorage = true
+	m.Run()
+}
+
+// mountSharedstorage mounts `cipherdir` on `mnt` with or without the
+// `-sharedstorage` flag, depending on the global var `flagSharestorage`.
+func mountSharedstorage(t *testing.T, cipherdir string, mnt string) {
+	args := []string{"-extpass=echo test"}
+	if flagSharestorage {
+		args = append(args, "-sharedstorage")
+	}
+	test_helpers.MountOrFatal(t, cipherdir, mnt, args...)
+}
+
+func TestUnlink(t *testing.T) {
+	cipherdir := test_helpers.InitFS(t)
+	mnt1 := cipherdir + ".mnt1"
+	mnt2 := cipherdir + ".mnt2"
+	mountSharedstorage(t, cipherdir, mnt1)
+	defer test_helpers.UnmountPanic(mnt1)
+	mountSharedstorage(t, cipherdir, mnt2)
+	defer test_helpers.UnmountPanic(mnt2)
+
+	// Create dir via mnt1
+	if err := unix.Mkdir(mnt1+"/foo", 0700); err != nil {
+		t.Fatal(err)
+	}
+	// Replace dir with file via mnt2
+	if err := unix.Rmdir(mnt2 + "/foo"); err != nil {
+		t.Fatal(err)
+	}
+	if fd, err := unix.Creat(mnt2+"/foo", 0600); err != nil {
+		t.Fatal(err)
+	} else {
+		unix.Close(fd)
+	}
+	// Try to unlink via mnt1
+	if err := unix.Unlink(mnt1 + "/foo"); err != nil {
+		if flagSharestorage {
+			t.Fatal(err)
+		} else {
+			t.Logf("Unlink failed as expected: errno %d / %v", err, err)
+		}
+	}
+}