many: rename systemd.Kind to Backend for a bit more clarity (#10788)

add some doc comments
adjust/simplify tests using MockNewSystemd

overlord/snapstate/backend/mountunit_test.go

@@ -45,10 +45,6 @@ type ResultForAddMountUnitFile struct {
 type FakeSystemd struct {
 	systemd.Systemd
 
-	Kind  systemd.Kind
-	Mode  systemd.InstanceMode
-	Meter systemd.Reporter
-
 	AddMountUnitFileCalls  []ParamsForAddMountUnitFile
 	AddMountUnitFileResult ResultForAddMountUnitFile
 
@@ -103,8 +99,8 @@ func (s *mountunitSuite) TestAddMountUnit(c *C) {
 	expectedErr := errors.New("creation error")
 
 	var sysd *FakeSystemd
-	restore := systemd.MockNewSystemd(func(kind systemd.Kind, roodDir string, mode systemd.InstanceMode, meter systemd.Reporter) systemd.Systemd {
-		sysd = &FakeSystemd{Kind: kind, Mode: mode, Meter: meter}
+	restore := systemd.MockNewSystemd(func(be systemd.Backend, roodDir string, mode systemd.InstanceMode, meter systemd.Reporter) systemd.Systemd {
+		sysd = &FakeSystemd{}
 		sysd.AddMountUnitFileResult = ResultForAddMountUnitFile{"", expectedErr}
 		return sysd
 	})
@@ -138,8 +134,8 @@ func (s *mountunitSuite) TestRemoveMountUnit(c *C) {
 	expectedErr := errors.New("removal error")
 
 	var sysd *FakeSystemd
-	restore := systemd.MockNewSystemd(func(kind systemd.Kind, roodDir string, mode systemd.InstanceMode, meter systemd.Reporter) systemd.Systemd {
-		sysd = &FakeSystemd{Kind: kind, Mode: mode, Meter: meter}
+	restore := systemd.MockNewSystemd(func(be systemd.Backend, roodDir string, mode systemd.InstanceMode, meter systemd.Reporter) systemd.Systemd {
+		sysd = &FakeSystemd{}
 		sysd.RemoveMountUnitFileResult = expectedErr
 		return sysd
 	})
@@ -164,8 +160,8 @@ func (s *mountunitSuite) TestRemoveSnapMountUnitsFailOnList(c *C) {
 	expectedErr := errors.New("listing error")
 
 	var sysd *FakeSystemd
-	restore := systemd.MockNewSystemd(func(kind systemd.Kind, roodDir string, mode systemd.InstanceMode, meter systemd.Reporter) systemd.Systemd {
-		sysd = &FakeSystemd{Kind: kind, Mode: mode, Meter: meter}
+	restore := systemd.MockNewSystemd(func(be systemd.Backend, roodDir string, mode systemd.InstanceMode, meter systemd.Reporter) systemd.Systemd {
+		sysd = &FakeSystemd{}
 		sysd.ListMountUnitsResult = ResultForListMountUnits{nil, expectedErr}
 		return sysd
 	})
@@ -195,8 +191,8 @@ func (s *mountunitSuite) TestRemoveSnapMountUnitsFailOnRemoval(c *C) {
 	returnedMountPoints := []string{"/here", "/and/there"}
 
 	var sysd *FakeSystemd
-	restore := systemd.MockNewSystemd(func(kind systemd.Kind, roodDir string, mode systemd.InstanceMode, meter systemd.Reporter) systemd.Systemd {
-		sysd = &FakeSystemd{Kind: kind, Mode: mode, Meter: meter}
+	restore := systemd.MockNewSystemd(func(be systemd.Backend, roodDir string, mode systemd.InstanceMode, meter systemd.Reporter) systemd.Systemd {
+		sysd = &FakeSystemd{}
 		sysd.ListMountUnitsResult = ResultForListMountUnits{returnedMountPoints, nil}
 		sysd.RemoveMountUnitFileResult = expectedErr
 		return sysd
@@ -228,8 +224,8 @@ func (s *mountunitSuite) TestRemoveSnapMountUnitsHappy(c *C) {
 	returnedMountPoints := []string{"/here", "/and/there", "/here/too"}
 
 	var sysd *FakeSystemd
-	restore := systemd.MockNewSystemd(func(kind systemd.Kind, roodDir string, mode systemd.InstanceMode, meter systemd.Reporter) systemd.Systemd {
-		sysd = &FakeSystemd{Kind: kind, Mode: mode, Meter: meter}
+	restore := systemd.MockNewSystemd(func(be systemd.Backend, roodDir string, mode systemd.InstanceMode, meter systemd.Reporter) systemd.Systemd {
+		sysd = &FakeSystemd{}
 		sysd.ListMountUnitsResult = ResultForListMountUnits{returnedMountPoints, nil}
 		sysd.RemoveMountUnitFileResult = nil
 		return sysd

systemd/emulation.go

@@ -41,6 +41,10 @@ type emulation struct {
 
 var errNotImplemented = errors.New("not implemented in emulation mode")
 
+func (s *emulation) Backend() Backend {
+	return EmulationModeBackend
+}
+
 func (s *emulation) DaemonReload() error {
 	return errNotImplemented
 }

systemd/systemd.go

@@ -101,7 +101,7 @@ func (m *extMutex) Taken(errMsg string) {
 
 // MockNewSystemd can be used to replace the constructor of the
 // Systemd types with a function that returns a mock object.
-func MockNewSystemd(f func(kind Kind, rootDir string, mode InstanceMode, rep Reporter) Systemd) func() {
+func MockNewSystemd(f func(be Backend, rootDir string, mode InstanceMode, rep Reporter) Systemd) func() {
 	oldNewSystemd := newSystemd
 	newSystemd = f
 	return func() {
@@ -272,8 +272,22 @@ type MountUnitOptions struct {
 	Origin   string
 }
 
+// Backend identifies the implementation backend in use by a Systemd instance.
+type Backend int
+
+const (
+	// RunningSystemdBackend identifies the implementation backend
+	// talking to the running system systemd daemon.
+	RunningSystemdBackend Backend = iota
+	// EmulationModeBackend identifies the implementation backend
+	// emulating a subset of systemd against a filesystem.
+	EmulationModeBackend
+)
+
 // Systemd exposes a minimal interface to manage systemd via the systemctl command.
 type Systemd interface {
+	// Backend returns the underlying implementation backend.
+	Backend() Backend
 	// DaemonReload reloads systemd's configuration.
 	DaemonReload() error
 	// DaemonRexec reexecutes systemd's system manager, should be
@@ -385,26 +399,26 @@ type Reporter interface {
 	Notify(string)
 }
 
-func newSystemdReal(kind Kind, rootDir string, mode InstanceMode, rep Reporter) Systemd {
-	switch kind {
-	case FullImplementation:
+func newSystemdReal(be Backend, rootDir string, mode InstanceMode, rep Reporter) Systemd {
+	switch be {
+	case RunningSystemdBackend:
 		return &systemd{rootDir: rootDir, mode: mode, reporter: rep}
-	case EmulationMode:
+	case EmulationModeBackend:
 		return &emulation{rootDir: rootDir}
 	default:
-		panic(fmt.Sprintf("unsupported systemd kind %v", kind))
+		panic(fmt.Sprintf("unsupported systemd backend %v", be))
 	}
 }
 
 // New returns a Systemd that uses the default root directory and omits
 // --root argument when executing systemctl.
 func New(mode InstanceMode, rep Reporter) Systemd {
-	return newSystemd(FullImplementation, "", mode, rep)
+	return newSystemd(RunningSystemdBackend, "", mode, rep)
 }
 
 // NewUnderRoot returns a Systemd that operates on the given rootdir.
 func NewUnderRoot(rootDir string, mode InstanceMode, rep Reporter) Systemd {
-	return newSystemd(FullImplementation, rootDir, mode, rep)
+	return newSystemd(RunningSystemdBackend, rootDir, mode, rep)
 }
 
 // NewEmulationMode returns a Systemd that runs in emulation mode where
@@ -414,7 +428,7 @@ func NewEmulationMode(rootDir string) Systemd {
 	if rootDir == "" {
 		rootDir = dirs.GlobalRootDir
 	}
-	return newSystemd(EmulationMode, rootDir, SystemMode, nil)
+	return newSystemd(EmulationModeBackend, rootDir, SystemMode, nil)
 }
 
 // InstanceMode determines which instance of systemd to control.
@@ -435,13 +449,6 @@ const (
 	GlobalUserMode
 )
 
-type Kind int
-
-const (
-	FullImplementation Kind = iota
-	EmulationMode
-)
-
 type systemd struct {
 	rootDir  string
 	reporter Reporter
@@ -461,6 +468,10 @@ func (s *systemd) systemctl(args ...string) ([]byte, error) {
 	return systemctlCmd(args...)
 }
 
+func (s *systemd) Backend() Backend {
+	return RunningSystemdBackend
+}
+
 func (s *systemd) DaemonReload() error {
 	if s.mode == GlobalUserMode {
 		panic("cannot call daemon-reload with GlobalUserMode")

systemd/systemd_test.go

@@ -188,6 +188,10 @@ func (s *SystemdTestSuite) TestEnsureAtLeastFail(c *C) {
 	}
 }
 
+func (s *SystemdTestSuite) TestBackend(c *C) {
+	c.Check(New(SystemMode, s.rep).Backend(), Equals, RunningSystemdBackend)
+}
+
 func (s *SystemdTestSuite) TestDaemonReload(c *C) {
 	err := New(SystemMode, s.rep).DaemonReload()
 	c.Assert(err, IsNil)
@@ -1263,6 +1267,11 @@ func (s *SystemdTestSuite) TestStatusGlobalUserMode(c *C) {
 	c.Check(s.argses[2], DeepEquals, []string{"--user", "--global", "--root", rootDir, "is-enabled", "one"})
 }
 
+func (s *SystemdTestSuite) TestEmulationModeBackend(c *C) {
+	sysd := NewEmulationMode(dirs.GlobalRootDir)
+	c.Check(sysd.Backend(), Equals, EmulationModeBackend)
+}
+
 const unitTemplate = `
 [Unit]
 Description=Mount unit for foo, revision 42