systemd: add a method to list mount units created by a snap

systemd/emulation.go

@@ -192,6 +192,10 @@ func (s *emulation) RemoveMountUnitFile(mountedDir string) error {
 	return nil
 }
 
+func (s *emulation) ListMountUnits(snapName, origin string) ([]string, error) {
+	return nil, errNotImplemented
+}
+
 func (s *emulation) Mask(service string) error {
 	_, err := systemctlCmd("--root", s.rootDir, "mask", service)
 	return err

systemd/systemd.go

@@ -280,6 +280,9 @@ type Systemd interface {
 	AddMountUnitFileWithOptions(unitOptions *MountUnitOptions) (string, error)
 	// RemoveMountUnitFile unmounts/stops/disables/removes a mount unit.
 	RemoveMountUnitFile(baseDir string) error
+	// ListMountUnits gets the list of targets of the mount units created by
+	// the `origin` module for the given snap
+	ListMountUnits(snapName, origin string) ([]string, error)
 	// Mask the given service.
 	Mask(service string) error
 	// Unmask the given service.
@@ -1275,6 +1278,101 @@ func (s *systemd) RemoveMountUnitFile(mountedDir string) error {
 	return nil
 }
 
+func workaroundSystemdQuoting(fragmentPath, where string) string {
+	// We know that the directory components of the fragment path do not need
+	// quoting and are therefore reliable. As for the file name, we workaround
+	// the wrong quoting of older systemd version by re-encoding the "Where"
+	// ourselves.
+	dir := filepath.Dir(fragmentPath)
+	baseName := EscapeUnitNamePath(where)
+	unitType := filepath.Ext(fragmentPath)
+	return filepath.Join(dir, baseName+unitType)
+}
+
+func extractOriginModule(systemdUnitPath string) (string, error) {
+	f, err := os.Open(systemdUnitPath)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	var originModule string
+	s := bufio.NewScanner(f)
+	prefix := snappyOriginModule + "="
+	for s.Scan() {
+		line := s.Text()
+		if strings.HasPrefix(line, prefix) {
+			originModule = line[len(prefix):]
+			break
+		}
+	}
+	return originModule, nil
+}
+
+func (s *systemd) ListMountUnits(snapName, origin string) ([]string, error) {
+	out, err := s.systemctl("show", "--property=Description,Where,FragmentPath", "*.mount")
+	if err != nil {
+		return nil, err
+	}
+
+	var mountPoints []string
+	if bytes.TrimSpace(out) == nil {
+		return mountPoints, nil
+	}
+	// Results are separated by a blank line, so we can split them like this:
+	units := bytes.Split(out, []byte("\n\n"))
+	for _, unitOutput := range units {
+		var where, description, fragmentPath string
+		lines := bytes.Split(bytes.Trim(unitOutput, "\n"), []byte("\n"))
+		for _, line := range lines {
+			splitVal := strings.SplitN(string(line), "=", 2)
+			if len(splitVal) != 2 {
+				return nil, fmt.Errorf("cannot parse systemctl output: %q", line)
+			}
+			switch splitVal[0] {
+			case "Description":
+				description = splitVal[1]
+			case "Where":
+				where = splitVal[1]
+			case "FragmentPath":
+				fragmentPath = splitVal[1]
+			default:
+				return nil, fmt.Errorf("unexpected property %q", splitVal[0])
+			}
+		}
+
+		ourDescription := fmt.Sprintf("Mount unit for %s", snapName)
+		if !strings.HasPrefix(description, ourDescription) {
+			continue
+		}
+
+		// Under Ubuntu 16.04, systemd improperly quotes the FragmentPath, so
+		// we must do some extra work here to get the correct path. This code
+		// can be removed once we stop supporting old distros
+		fragmentPath = workaroundSystemdQuoting(fragmentPath, where)
+
+		// only return units programmatically created by some snapd backend:
+		// the mount unit used to mount the snap's squashfs is generally
+		// uninteresting
+		originModule, err := extractOriginModule(fragmentPath)
+		if err != nil || originModule == "" {
+			continue
+		}
+
+		// If an `origin` was given, we must return only units created by it
+		if origin != "" && originModule != origin {
+			continue
+		}
+
+		if where == "" {
+			return nil, fmt.Errorf(`missing "Where" in mount unit %q`, fragmentPath)
+		}
+
+		mountPoints = append(mountPoints, where)
+	}
+	return mountPoints, nil
+}
+
 func (s *systemd) ReloadOrRestart(serviceName string) error {
 	if s.mode == GlobalUserMode {
 		panic("cannot call restart with GlobalUserMode")

systemd/systemd_test.go

@@ -1405,6 +1405,88 @@ func (s *SystemdTestSuite) TestUnmaskInEmulationMode(c *C) {
 		{"--root", "/path", "unmask", "foo"}})
 }
 
+func (s *SystemdTestSuite) TestListMountUnitsEmpty(c *C) {
+	s.outs = [][]byte{
+		[]byte("\n"),
+	}
+
+	sysd := New(SystemMode, nil)
+	units, err := sysd.ListMountUnits("some-snap", "")
+	c.Check(units, HasLen, 0)
+	c.Check(err, IsNil)
+}
+
+func (s *SystemdTestSuite) TestListMountUnitsMalformed(c *C) {
+	s.outs = [][]byte{
+		[]byte(`Description=Mount unit for some-snap, revision x1
+Where=/somewhere/here
+FragmentPath=/etc/systemd/system/somewhere-here.mount
+HereIsOneLineWithoutAnEqualSign
+`),
+	}
+
+	sysd := New(SystemMode, nil)
+	units, err := sysd.ListMountUnits("some-snap", "")
+	c.Check(units, HasLen, 0)
+	c.Check(err, ErrorMatches, "cannot parse systemctl output:.*")
+}
+
+func (s *SystemdTestSuite) TestListMountUnitsHappy(c *C) {
+	tmpDir, err := ioutil.TempDir("/tmp", "snapd-systemd-test-list-mounts-*")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(tmpDir)
+
+	var systemctlOutput string
+	createFakeUnit := func(fileName, snapName, where, origin string) error {
+		path := filepath.Join(tmpDir, fileName)
+		if len(systemctlOutput) > 0 {
+			systemctlOutput += "\n\n"
+		}
+		systemctlOutput += fmt.Sprintf(`Description=Mount unit for %s, revision x1
+Where=%s
+FragmentPath=%s
+`, snapName, where, path)
+		contents := fmt.Sprintf(`[Unit]
+Description=Mount unit for %s, revision x1
+
+[Mount]
+What=/does/not/matter
+Where=%s
+Type=doesntmatter
+Options=do,not,matter,either
+
+[Install]
+WantedBy=multi-user.target
+X-SnapdOrigin=%s
+`, snapName, where, origin)
+		return ioutil.WriteFile(path, []byte(contents), 0644)
+	}
+
+	// Prepare the unit files
+	err = createFakeUnit("somepath-somedir.mount", "some-snap", "/somepath/somedir", "module1")
+	c.Assert(err, IsNil)
+	err = createFakeUnit("somewhere-here.mount", "some-other-snap", "/somewhere/here", "module2")
+	c.Assert(err, IsNil)
+	err = createFakeUnit("somewhere-there.mount", "some-snap", "/somewhere/there", "module3")
+	c.Assert(err, IsNil)
+
+	s.outs = [][]byte{
+		[]byte(systemctlOutput),
+	}
+	sysd := New(SystemMode, nil)
+
+	// First, get all mount units for some-snap, without filter on the origin module
+	units, err := sysd.ListMountUnits("some-snap", "")
+	c.Check(units, DeepEquals, []string{"/somepath/somedir", "/somewhere/there"})
+	c.Check(err, IsNil)
+
+	// Now repeat the same, filtering on the origin module
+	s.i = 0 // this resets the systemctl output iterator back to the beginning
+	units, err = sysd.ListMountUnits("some-snap", "module3")
+	c.Check(units, DeepEquals, []string{"/somewhere/there"})
+	c.Check(err, IsNil)
+}
+
 func (s *SystemdTestSuite) TestMountHappy(c *C) {
 	sysd := New(SystemMode, nil)