feat(cli): add --dry option to pebble run

internals/cli/cmd_run.go

@@ -48,6 +48,7 @@ type sharedRunEnterOpts struct {
 	Hold       bool       `long:"hold"`
 	HTTP       string     `long:"http"`
 	Verbose    bool       `short:"v" long:"verbose"`
+	Dry        bool       `long:"dry"`
 	Args       [][]string `long:"args" terminator:";"`
 }
 
@@ -56,6 +57,7 @@ var sharedRunEnterArgsHelp = map[string]string{
 	"--hold":        "Do not start default services automatically",
 	"--http":        `Start HTTP API listening on this address (e.g., ":4000")`,
 	"--verbose":     "Log all output from services to stdout",
+	"--dry":         "Attempt to run without actually running",
 	"--args":        `Provide additional arguments to a service`,
 }
 
@@ -162,6 +164,9 @@ func runDaemon(rcmd *cmdRun, ch chan os.Signal, ready chan<- func()) error {
 	if err != nil {
 		return err
 	}
+	if rcmd.Dry {
+		return nil
+	}
 	if err := d.Init(); err != nil {
 		return err
 	}

internals/daemon/daemon_test.go

@@ -97,8 +97,13 @@ func (h *fakeHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 }
 
 type fakeManager struct {
-	id          string
-	ensureCalls int
+	id                         string
+	ensureCalls, dryStartCalls int
+}
+
+func (m *fakeManager) DryStart() error {
+	m.dryStartCalls++
+	return nil
 }
 
 func (m *fakeManager) Ensure() error {
@@ -111,7 +116,7 @@ type fakeExtension struct {
 }
 
 func (f *fakeExtension) ExtraManagers(o *overlord.Overlord) ([]overlord.StateManager, error) {
-	f.mgr = fakeManager{id: "expected", ensureCalls: 0}
+	f.mgr = fakeManager{id: "expected", ensureCalls: 0, dryStartCalls: 0}
 	result := []overlord.StateManager{&f.mgr}
 	return result, nil
 }

internals/overlord/cmdstate/manager.go

@@ -37,6 +37,11 @@ func NewManager(runner *state.TaskRunner) *CommandManager {
 	return manager
 }
 
+// DryStart is part of the overlord.StateManager interface.
+func (m *CommandManager) DryStart() error {
+	return nil
+}
+
 // Ensure is part of the overlord.StateManager interface.
 func (m *CommandManager) Ensure() error {
 	return nil

internals/overlord/logstate/manager.go

@@ -109,6 +109,11 @@ func (m *LogManager) ServiceStarted(service *plan.Service, buffer *servicelog.Ri
 	}
 }
 
+// DryStart implements overlord.StateManager.
+func (m *LogManager) DryStart() error {
+	return nil
+}
+
 // Ensure implements overlord.StateManager.
 func (m *LogManager) Ensure() error {
 	return nil

internals/overlord/overlord.go

@@ -16,6 +16,7 @@
 package overlord
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -94,7 +95,6 @@ type Overlord struct {
 
 // New creates an Overlord with all its state managers.
 func New(opts *Options) (*Overlord, error) {
-
 	o := &Overlord{
 		pebbleDir: opts.PebbleDir,
 		loopTomb:  new(tomb.Tomb),
@@ -175,6 +175,10 @@ func New(opts *Options) (*Overlord, error) {
 	// before it.
 	o.stateEng.AddManager(o.runner)
 
+	// Dry start all managers.
+	if err := o.stateEng.DryStart(); err != nil {
+		return nil, err
+	}
 	return o, nil
 }
 
@@ -358,17 +362,16 @@ func (o *Overlord) settle(timeout time.Duration, beforeCleanups func()) error {
 	var errs []error
 	for !done {
 		if timeout > 0 && time.Since(t0) > timeout {
-			err := fmt.Errorf("Settle is not converging")
 			if len(errs) != 0 {
-				return &ensureError{append(errs, err)}
+				return newMultiError("settle is not converging", errs)
 			}
-			return err
+			return errors.New("settle is not converging")
 		}
 		next := o.ensureTimerReset()
 		err := o.stateEng.Ensure()
 		switch ee := err.(type) {
 		case nil:
-		case *ensureError:
+		case *multiError:
 			errs = append(errs, ee.errs...)
 		default:
 			errs = append(errs, err)
@@ -395,7 +398,7 @@ func (o *Overlord) settle(timeout time.Duration, beforeCleanups func()) error {
 		}
 	}
 	if len(errs) != 0 {
-		return &ensureError{errs}
+		return newMultiError("state ensure errors", errs)
 	}
 	return nil
 }
@@ -474,6 +477,7 @@ func FakeWithState(handleRestart func(restart.RestartType)) *Overlord {
 	s := state.New(fakeBackend{o: o})
 	o.stateEng = NewStateEngine(s)
 	o.runner = state.NewTaskRunner(s)
+	o.stateEng.DryStart()
 	return o
 }
 

internals/overlord/overlord_test.go

@@ -171,6 +171,10 @@ type witnessManager struct {
 	ensureCallback func(s *state.State) error
 }
 
+func (wm *witnessManager) DryStart() error {
+	return nil
+}
+
 func (wm *witnessManager) Ensure() error {
 	if wm.expectedEnsure--; wm.expectedEnsure == 0 {
 		close(wm.ensureCalled)
@@ -562,6 +566,10 @@ func newSampleManager(s *state.State, runner *state.TaskRunner) *sampleManager {
 	return sm
 }
 
+func (sm *sampleManager) DryStart() error {
+	return nil
+}
+
 func (sm *sampleManager) Ensure() error {
 	if sm.ensureCallback != nil {
 		sm.ensureCallback()
@@ -621,7 +629,7 @@ func (ovs *overlordSuite) TestSettleNotConverging(c *C) {
 	err := o.Settle(250 * time.Millisecond)
 	s.Lock()
 
-	c.Check(err, ErrorMatches, `Settle is not converging`)
+	c.Check(err, ErrorMatches, `settle is not converging`)
 
 }
 

internals/overlord/servstate/manager.go

@@ -244,6 +244,12 @@ func (m *ServiceManager) acquirePlan() (release func(), err error) {
 	return release, nil
 }
 
+// DryStart implements StateManager.DryStart.
+func (m *ServiceManager) DryStart() error {
+	_, err := m.Plan()
+	return err
+}
+
 // Ensure implements StateManager.Ensure.
 func (m *ServiceManager) Ensure() error {
 	return nil

internals/overlord/state/taskrunner.go

@@ -331,6 +331,10 @@ func (r *TaskRunner) tryUndo(t *Task) {
 	}
 }
 
+func (r *TaskRunner) DryStart() error {
+	return nil
+}
+
 // Ensure starts new goroutines for all known tasks with no pending
 // dependencies.
 // Note that Ensure will lock the state.

internals/overlord/stateengine.go

@@ -15,7 +15,10 @@
 package overlord
 
 import (
+	"bytes"
+	"errors"
 	"fmt"
+	"strings"
 	"sync"
 
 	"github.com/canonical/pebble/internals/logger"
@@ -25,6 +28,10 @@ import (
 // StateManager is implemented by types responsible for observing
 // the system and manipulating it to reflect the desired state.
 type StateManager interface {
+	// DryStart prepares the manager to run its activities without
+	// incurring unwanted side effects.
+	DryStart() error
+
 	// Ensure forces a complete evaluation of the current state.
 	// See StateEngine.Ensure for more details.
 	Ensure() error
@@ -53,8 +60,9 @@ type StateStopper interface {
 // cope with Ensure calls in any order, coordinating among themselves
 // solely via the state.
 type StateEngine struct {
-	state   *state.State
-	stopped bool
+	state      *state.State
+	dryStarted bool
+	stopped    bool
 	// managers in use
 	mgrLock  sync.Mutex
 	managers []StateManager
@@ -72,12 +80,68 @@ func (se *StateEngine) State() *state.State {
 	return se.state
 }
 
-type ensureError struct {
-	errs []error
+// multiError collects multiple errors that affected an operation.
+type multiError struct {
+	header string
+	errs   []error
+}
+
+// newMultiError returns a new multiError struct initialized with
+// the given format string that explains what operation potentially
+// went wrong. multiError can be nested and will render correctly
+// in these cases.
+func newMultiError(header string, errs []error) error {
+	return &multiError{header: header, errs: errs}
 }
 
-func (e *ensureError) Error() string {
-	return fmt.Sprintf("state ensure errors: %v", e.errs)
+// Error formats the error string.
+func (me *multiError) Error() string {
+	return me.nestedError(0)
+}
+
+// helper to ensure formating of nested multiErrors works.
+func (me *multiError) nestedError(level int) string {
+	indent := strings.Repeat(" ", level)
+	buf := bytes.NewBufferString(fmt.Sprintf("%s:\n", me.header))
+	if level > 8 {
+		return "circular or too deep error nesting (max 8)?!"
+	}
+	for i, err := range me.errs {
+		switch v := err.(type) {
+		case *multiError:
+			fmt.Fprintf(buf, "%s- %v", indent, v.nestedError(level+1))
+		default:
+			fmt.Fprintf(buf, "%s- %v", indent, err)
+		}
+		if i < len(me.errs)-1 {
+			fmt.Fprintf(buf, "\n")
+		}
+	}
+	return buf.String()
+}
+
+var errStateEngineStopped = errors.New("state engine already stopped")
+
+// DryStart ensures all managers are ready to run their activities
+// before the first call to Ensure() is made.
+func (se *StateEngine) DryStart() error {
+	se.mgrLock.Lock()
+	defer se.mgrLock.Unlock()
+	if se.stopped {
+		return errStateEngineStopped
+	}
+	var errs []error
+	for _, m := range se.managers {
+		err := m.DryStart()
+		if err != nil {
+			errs = append(errs, err)
+		}
+	}
+	if len(errs) != 0 {
+		return newMultiError("dry-start failed", errs)
+	}
+	se.dryStarted = true
+	return nil
 }
 
 // Ensure asks every manager to ensure that they are doing the necessary
@@ -91,8 +155,11 @@ func (e *ensureError) Error() string {
 func (se *StateEngine) Ensure() error {
 	se.mgrLock.Lock()
 	defer se.mgrLock.Unlock()
+	if !se.dryStarted {
+		return errors.New("state engine did not dry-start")
+	}
 	if se.stopped {
-		return fmt.Errorf("state engine already stopped")
+		return errStateEngineStopped
 	}
 	var errs []error
 	for _, m := range se.managers {
@@ -103,7 +170,7 @@ func (se *StateEngine) Ensure() error {
 		}
 	}
 	if len(errs) != 0 {
-		return &ensureError{errs}
+		return newMultiError("state ensure errors", errs)
 	}
 	return nil
 }