feat(run): Add the --dry option to the run command

internals/cli/cmd_run.go

@@ -17,6 +17,7 @@ package cli
 import (
 	"errors"
 	"fmt"
+	"maps"
 	"os"
 	"os/signal"
 	"strconv"
@@ -62,22 +63,29 @@ var sharedRunEnterArgsHelp = map[string]string{
 	"--args":        "Provide additional arguments to a service",
 	"--identities":  "Seed identities from file (like update-identities --replace)",
 }
+var runArgsHelp = map[string]string{
+	"--dry": "Start {{.DisplayName}} without side-effects",
+}
 
 type cmdRun struct {
 	client *client.Client
 
 	socketPath string
 	pebbleDir  string
 
+	DryRun bool `long:"dry"`
 	sharedRunEnterOpts
 }
 
 func init() {
+	argsHelp := runArgsHelp
+	maps.Copy(argsHelp, sharedRunEnterArgsHelp)
+
 	AddCommand(&CmdInfo{
 		Name:        "run",
 		Summary:     cmdRunSummary,
 		Description: cmdRunDescription,
-		ArgsHelp:    sharedRunEnterArgsHelp,
+		ArgsHelp:    argsHelp,
 		New: func(opts *CmdOptions) flags.Commander {
 			return &cmdRun{
 				client:     opts.Client,
@@ -185,6 +193,7 @@ func runDaemon(rcmd *cmdRun, ch chan os.Signal, ready chan<- func()) error {
 	dopts := daemon.Options{
 		Dir:        rcmd.pebbleDir,
 		SocketPath: rcmd.socketPath,
+		DryRun:     rcmd.DryRun,
 	}
 	if rcmd.Verbose {
 		dopts.ServiceOutput = os.Stdout
@@ -195,6 +204,16 @@ func runDaemon(rcmd *cmdRun, ch chan os.Signal, ready chan<- func()) error {
 	if err != nil {
 		return err
 	}
+
+	if rcmd.DryRun {
+		// Validation has been performed by initializing the daemon above, so we
+		// don't need the rest of the code to execute for dry-run.
+		// If a hook for manager-specific dry run logic is added in the future, it
+		// should be run immediately above this block.o
+		logger.Noticef("No error encountered: dry-run successful.")
+		return nil
+	}
+
 	if err := d.Init(); err != nil {
 		return err
 	}

internals/daemon/daemon.go

@@ -84,6 +84,9 @@ type Options struct {
 	// OverlordExtension is an optional interface used to extend the capabilities
 	// of the Overlord.
 	OverlordExtension overlord.Extension
+
+	// If true, no state will be written to file.
+	DryRun bool
 }
 
 // A Daemon listens for requests and routes them to the right command
@@ -852,6 +855,7 @@ func New(opts *Options) (*Daemon, error) {
 		RestartHandler: d,
 		ServiceOutput:  opts.ServiceOutput,
 		Extension:      opts.OverlordExtension,
+		DryRun:         opts.DryRun,
 	}
 
 	ovld, err := overlord.New(&ovldOptions)

internals/overlord/backend.go

@@ -25,9 +25,15 @@ type overlordStateBackend struct {
 	path           string
 	ensureBefore   func(d time.Duration)
 	requestRestart func(t restart.RestartType)
+
+	// If this is true, the backend will not actually save anything to file.
+	DryRun bool
 }
 
 func (osb *overlordStateBackend) Checkpoint(data []byte) error {
+	if osb.DryRun {
+		return nil
+	}
 	return osutil.AtomicWriteFile(osb.path, data, 0600, 0)
 }
 

internals/overlord/overlord.go

@@ -76,6 +76,9 @@ type Options struct {
 	ServiceOutput io.Writer
 	// Extension allows extending the overlord with externally defined features.
 	Extension Extension
+	// DryRun must be true if state in storage is not meant to be altered.
+	// Otherwise, the Overlord will operate normally.
+	DryRun bool
 }
 
 // Overlord is the central manager of the system, keeping track
@@ -106,6 +109,9 @@ type Overlord struct {
 	logMgr     *logstate.LogManager
 
 	extension Extension
+
+	// If true, no state will be written to file.
+	DryRun bool
 }
 
 // New creates an Overlord with all its state managers.
@@ -116,6 +122,7 @@ func New(opts *Options) (*Overlord, error) {
 		loopTomb:  new(tomb.Tomb),
 		inited:    true,
 		extension: opts.Extension,
+		DryRun:    opts.DryRun,
 	}
 
 	if !filepath.IsAbs(o.pebbleDir) {
@@ -133,6 +140,7 @@ func New(opts *Options) (*Overlord, error) {
 	backend := &overlordStateBackend{
 		path:         statePath,
 		ensureBefore: o.ensureBefore,
+		DryRun:       opts.DryRun,
 	}
 	s, restartMgr, err := loadState(statePath, opts.RestartHandler, backend)
 	if err != nil {