Add integration tests for mounting.

In particular TestNonEmptyMountPoint, which is currently failing on
Linux due to the poor error handling we inherited from bazil.org/fuse.

mount_test.go

+package fuse_test
+
+import (
+	"io/ioutil"
+	"os"
+	"path"
+	"runtime"
+	"strings"
+	"testing"
+
+	"golang.org/x/net/context"
+
+	"github.com/jacobsa/fuse"
+	"github.com/jacobsa/fuse/fuseops"
+	"github.com/jacobsa/fuse/fuseutil"
+)
+
+////////////////////////////////////////////////////////////////////////
+// minimalFS
+////////////////////////////////////////////////////////////////////////
+
+// A minimal fuseutil.FileSystem that can successfully mount but do nothing
+// else.
+type minimalFS struct {
+	fuseutil.NotImplementedFileSystem
+}
+
+func (fs *minimalFS) StatFS(
+	ctx context.Context,
+	op *fuseops.StatFSOp) (err error) {
+	return
+}
+
+////////////////////////////////////////////////////////////////////////
+// Tests
+////////////////////////////////////////////////////////////////////////
+
+func TestSuccessfulMount(t *testing.T) {
+	ctx := context.Background()
+
+	// Set up a temporary directory.
+	dir, err := ioutil.TempDir("", "mount_test")
+	if err != nil {
+		t.Fatal("ioutil.TempDir: %v", err)
+	}
+
+	defer os.RemoveAll(dir)
+
+	// Mount.
+	fs := &minimalFS{}
+	mfs, err := fuse.Mount(
+		dir,
+		fuseutil.NewFileSystemServer(fs),
+		&fuse.MountConfig{})
+
+	if err != nil {
+		t.Fatalf("fuse.Mount: %v", err)
+	}
+
+	defer func() {
+		if err := mfs.Join(ctx); err != nil {
+			t.Errorf("Joining: %v", err)
+		}
+	}()
+
+	defer fuse.Unmount(mfs.Dir())
+}
+
+func TestNonEmptyMountPoint(t *testing.T) {
+	ctx := context.Background()
+
+	// osxfuse appears to be happy to mount over a non-empty mount point.
+	//
+	// We leave this test in for Linux, because it tickles the behavior of
+	// fusermount writing to stderr and exiting with an error code. We want to
+	// make sure that a descriptive error makes it back to the user.
+	if runtime.GOOS == "darwin" {
+		return
+	}
+
+	// Set up a temporary directory.
+	dir, err := ioutil.TempDir("", "mount_test")
+	if err != nil {
+		t.Fatal("ioutil.TempDir: %v", err)
+	}
+
+	defer os.RemoveAll(dir)
+
+	// Add a file within it.
+	err = ioutil.WriteFile(path.Join(dir, "foo"), []byte{}, 0600)
+	if err != nil {
+		t.Fatalf("ioutil.WriteFile: %v", err)
+	}
+
+	// Attempt to mount.
+	fs := &minimalFS{}
+	mfs, err := fuse.Mount(
+		dir,
+		fuseutil.NewFileSystemServer(fs),
+		&fuse.MountConfig{})
+
+	if err == nil {
+		fuse.Unmount(mfs.Dir())
+		mfs.Join(ctx)
+		t.Fatal("fuse.Mount returned nil")
+	}
+
+	const want = "not empty"
+	if got := err.Error(); !strings.Contains(got, want) {
+		t.Errorf("Unexpected error: %v", got)
+	}
+}
+
+func TestNonexistentMountPoint(t *testing.T) {
+	ctx := context.Background()
+
+	// Set up a temporary directory.
+	dir, err := ioutil.TempDir("", "mount_test")
+	if err != nil {
+		t.Fatal("ioutil.TempDir: %v", err)
+	}
+
+	defer os.RemoveAll(dir)
+
+	// Attempt to mount into a sub-directory that doesn't exist.
+	fs := &minimalFS{}
+	mfs, err := fuse.Mount(
+		path.Join(dir, "foo"),
+		fuseutil.NewFileSystemServer(fs),
+		&fuse.MountConfig{})
+
+	if err == nil {
+		fuse.Unmount(mfs.Dir())
+		mfs.Join(ctx)
+		t.Fatal("fuse.Mount returned nil")
+	}
+
+	const want = "no such file"
+	if got := err.Error(); !strings.Contains(got, want) {
+		t.Errorf("Unexpected error: %v", got)
+	}
+}