Merge pull request #8 from LINUXexpert-org/docker-cutover

Cut over a container, or refuse to for a reason

ARCHITECTURE.md

@@ -290,9 +290,28 @@ against an already-migrated store.
 - The original unit is preserved and recorded as the `service-unit`
   artifact *before* the rewrite, so an operator restoring by hand isn't
   reconstructing a unit file from memory.
-- Docker deployments are refused: cutting over a container means pulling a
-  new image and recreating the container, not swapping a binary and
-  rewriting a unit.
+- Docker deployments recreate rather than rewrite, because a container
+  cannot be edited in place the way a unit file can. That makes silent loss
+  the default failure: a container rebuilt without its capabilities, its
+  custom network or its device mappings starts cleanly and is quietly not
+  the server it was. So the same rule the unit rewrite follows applies
+  here - a definition this only partly understands is one it must not
+  rebuild - and cutover refuses a container using anything outside the set
+  it carries across, naming what it found. The list is conservative and
+  deliberately not exhaustive; docker's HostConfig has far more fields than
+  it checks, and one it does not know about is a reason not to be
+  recreating that container at all.
+- The old container is renamed rather than removed, and the old image is
+  never pruned. Together they are the container's manual restore path, the
+  nearest equivalent to the preserved binary of §4.2: one command starts
+  the previous container again. The `docker inspect` of the container as it
+  was is preserved as the `container-definition` artifact before anything
+  is replaced, for the same reason the unit file is.
+- **Status: the container path is implemented and not yet reachable from
+  the CLI** - `run` does not pass container options, so a container is
+  still refused there. Wiring it up, and lifting preflight's refusal for
+  the containers it can now handle, is the remaining work in
+  [#3](https://github.com/LINUXexpert-org/stalwart-migrator/issues/3).
 - Quota recalculation is the one step allowed to fail without failing the
   phase. Stale counters are an accounting problem; a failed cutover is one
   an operator has to respond to by restoring a machine that is otherwise
@@ -815,9 +834,16 @@ happens to need them. `preflight.DeploymentKind` is a type alias for
   §4.5 lists exactly which two details are inferred. A smoke test against a
   real 0.16 instance would settle both, and would let this step be promoted
   from "warns on failure" to a hard check.
-- **Cutover doesn't handle Docker.** It refuses container deployments
-  outright, since cutting one over means pulling an image and recreating
-  the container rather than swapping a binary and rewriting a unit.
+- **Docker is implemented but not yet wired to the CLI.** Preflight
+  inspects a container and reports what stands in the way; stage pulls and
+  verifies an image; the recovery cycle runs in a throwaway container
+  against the live data; and cutover recreates the container, refusing one
+  whose definition it would not carry across intact. What is missing is
+  `run` passing those options and preflight lifting its refusal for the
+  containers now handled. Compose stays refused deliberately: recreating a
+  compose-managed container out from under compose leaves the container and
+  the compose file disagreeing, and the next `compose up` reverts the
+  migration.
 - **Cutover ignores systemd drop-ins.** It rewrites only the main unit
   file, so an `ExecStart` or `Environment` override in
   `/etc/systemd/system/stalwart.service.d/*.conf` is invisible to it -

internal/cutover/container.go

@@ -0,0 +1,187 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package cutover
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+
+	"github.com/LINUXexpert-org/stalwart-migrator/internal/checkpoint"
+	"github.com/LINUXexpert-org/stalwart-migrator/internal/preflight"
+)
+
+// ArtifactContainerDefinition is the preserved `docker inspect` of the
+// container as it was before cutover replaced it - the container's
+// equivalent of ArtifactServiceUnit, and for the same reason. Recovery is
+// out of scope (ARCHITECTURE.md §4.8), so what this tool owes an operator
+// putting a machine back by hand is the definition they would otherwise be
+// reconstructing from memory.
+const ArtifactContainerDefinition = "container-definition"
+
+// ContainerOptions configures cutting over a container deployment.
+type ContainerOptions struct {
+	// ContainerName is the live container, which must already be stopped.
+	ContainerName string
+
+	// StagedImage is the image ID stage verified - an ID rather than a tag
+	// deliberately, so what runs is what was checked.
+	StagedImage string
+
+	// PreserveDir is where the inspected definition is written.
+	PreserveDir string
+
+	DockerBinary string
+}
+
+func (o ContainerOptions) docker() string {
+	if o.DockerBinary == "" {
+		return "docker"
+	}
+	return o.DockerBinary
+}
+
+// runContainerCutover replaces the container with one running the staged
+// image, carrying across the parts of its definition this tool understands
+// and refusing outright when it finds parts it does not.
+//
+// The old container is renamed rather than removed, and the old image is
+// never pruned. Together they are the manual restore path: an operator can
+// start the previous container again with one command, which is as close to
+// the preserved-binary guarantee (§4.2) as a container gets.
+func runContainerCutover(ctx context.Context, rs *checkpoint.RunState, step stepFunc, opts ContainerOptions) error {
+	if opts.ContainerName == "" {
+		return fmt.Errorf("cutover: no container name")
+	}
+	if opts.StagedImage == "" {
+		return fmt.Errorf("cutover: no staged image - stage the target image before cutting over to it")
+	}
+
+	var facts preflight.ContainerFacts
+
+	if err := step("preserve-container-definition", func() (checkpoint.StepOutcome, error) {
+		raw, err := dockerOut(ctx, opts.docker(), "inspect", opts.ContainerName)
+		if err != nil {
+			return checkpoint.StepOutcome{}, fmt.Errorf("inspect %s: %w (%s)", opts.ContainerName, err, raw)
+		}
+		if opts.PreserveDir == "" {
+			return checkpoint.StepOutcome{}, fmt.Errorf("no directory to preserve the container definition in")
+		}
+		if err := os.MkdirAll(opts.PreserveDir, 0o750); err != nil {
+			return checkpoint.StepOutcome{}, err
+		}
+		dest := filepath.Join(opts.PreserveDir, opts.ContainerName+".inspect.json")
+		if err := os.WriteFile(dest, []byte(raw), 0o640); err != nil {
+			return checkpoint.StepOutcome{}, err
+		}
+		sum, size, err := hashFile(dest)
+		if err != nil {
+			return checkpoint.StepOutcome{}, err
+		}
+		// Recorded before anything is replaced, so a crash between
+		// preserving and recreating still leaves the original findable.
+		rs.RecordArtifact(ArtifactContainerDefinition, checkpoint.Artifact{Path: dest, SHA256: sum, SizeBytes: size})
+
+		facts, err = preflight.InspectContainer(ctx, opts.ContainerName)
+		if err != nil {
+			return checkpoint.StepOutcome{}, err
+		}
+		return checkpoint.StepOutcome{Detail: "preserved the container definition at " + dest}, nil
+	}); err != nil {
+		return err
+	}
+
+	if err := step("container-is-recreatable", func() (checkpoint.StepOutcome, error) {
+		if len(facts.Unsupported) > 0 {
+			return checkpoint.StepOutcome{}, fmt.Errorf(
+				"this container uses configuration cutting over would not carry across: %s. Recreating it without those would "+
+					"start cleanly and quietly not be the server it was, so this tool will not do it. Migrate this one by hand: "+
+					"the definition is preserved as %s, and the staged image is %s",
+				strings.Join(facts.Unsupported, "; "), rs.Artifacts[ArtifactContainerDefinition].Path, opts.StagedImage)
+		}
+		return checkpoint.StepOutcome{Detail: "the container's definition is entirely within what a recreate carries across"}, nil
+	}); err != nil {
+		return err
+	}
+
+	retired := opts.ContainerName + "-premigration"
+	if rs.SourceVersion != "" {
+		retired += "-" + rs.SourceVersion
+	}
+
+	if err := step("retire-old-container", func() (checkpoint.StepOutcome, error) {
+		// Renamed, not removed. The old container plus the old image - which
+		// nothing here prunes - is what an operator restores by hand.
+		if out, err := dockerOut(ctx, opts.docker(), "rename", opts.ContainerName, retired); err != nil {
+			return checkpoint.StepOutcome{}, fmt.Errorf("rename %s: %w (%s)", opts.ContainerName, err, out)
+		}
+		return checkpoint.StepOutcome{Detail: fmt.Sprintf("kept the previous container as %s, still on image %s", retired, facts.Image)}, nil
+	}); err != nil {
+		return err
+	}
+
+	return step("create-container", func() (checkpoint.StepOutcome, error) {
+		args := []string{"run", "-d", "--name", opts.ContainerName}
+		if facts.RestartPolicy != "" && facts.RestartPolicy != "no" {
+			args = append(args, "--restart", facts.RestartPolicy)
+		}
+		for _, e := range facts.Env {
+			// Recovery-mode variables must never survive into a normal
+			// start: leaving STALWART_RECOVERY_MODE set would recovery-boot
+			// on every restart, the same footgun §4.5 strips from a unit.
+			if strings.HasPrefix(e, "STALWART_RECOVERY_") {
+				continue
+			}
+			args = append(args, "-e", e)
+		}
+		for _, m := range facts.Mounts {
+			src := m.Name
+			if src == "" {
+				src = m.Source
+			}
+			spec := src + ":" + m.Destination
+			if !m.RW {
+				spec += ":ro"
+			}
+			args = append(args, "-v", spec)
+		}
+		for port, bindings := range facts.Ports {
+			for _, b := range bindings {
+				spec := b.HostPort + ":" + strings.SplitN(port, "/", 2)[0]
+				if b.HostIP != "" {
+					spec = b.HostIP + ":" + spec
+				}
+				args = append(args, "-p", spec)
+			}
+		}
+		for k, v := range facts.Labels {
+			args = append(args, "--label", k+"="+v)
+		}
+		args = append(args, opts.StagedImage)
+
+		if out, err := dockerOut(ctx, opts.docker(), args...); err != nil {
+			return checkpoint.StepOutcome{}, fmt.Errorf(
+				"create %s from %s: %w (%s). The previous container is still here as %s",
+				opts.ContainerName, opts.StagedImage, err, out, retired)
+		}
+		return checkpoint.StepOutcome{Detail: fmt.Sprintf("recreated %s on image %s", opts.ContainerName, opts.StagedImage)}, nil
+	})
+}
+
+// stepFunc is cutover.Run's checkpointed step runner, passed in so the
+// container path reports through the same Report the systemd path does.
+type stepFunc func(name string, fn func() (checkpoint.StepOutcome, error)) error
+
+func dockerOut(ctx context.Context, name string, args ...string) (string, error) {
+	cmd := exec.CommandContext(ctx, name, args...)
+	var out bytes.Buffer
+	cmd.Stdout = &out
+	cmd.Stderr = &out
+	err := cmd.Run()
+	return strings.TrimSpace(out.String()), err
+}

internal/cutover/container_test.go

@@ -0,0 +1,254 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package cutover
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/LINUXexpert-org/stalwart-migrator/internal/checkpoint"
+)
+
+// inspectJSON builds a `docker inspect` document. extra is merged into
+// HostConfig so a test can add the configuration a recreate would drop.
+func inspectJSON(t *testing.T, extraHost map[string]any, networks map[string]any) string {
+	t.Helper()
+	host := map[string]any{
+		"PortBindings":  map[string]any{"143/tcp": []map[string]string{{"HostIp": "0.0.0.0", "HostPort": "143"}}},
+		"RestartPolicy": map[string]any{"Name": "unless-stopped"},
+		"NetworkMode":   "bridge",
+		"LogConfig":     map[string]any{"Type": "json-file"},
+	}
+	for k, v := range extraHost {
+		host[k] = v
+	}
+	if networks == nil {
+		networks = map[string]any{"bridge": map[string]any{}}
+	}
+	doc := []map[string]any{{
+		"Name":  "/stalwart",
+		"Image": "sha256:old",
+		"Config": map[string]any{
+			"Image": "stalwartlabs/stalwart:v0.15.5",
+			"Env":   []string{"TZ=UTC", "STALWART_RECOVERY_MODE=1"},
+		},
+		"State":           map[string]any{"Running": false},
+		"Mounts":          []map[string]any{{"Type": "volume", "Name": "stalwart-data", "Destination": "/opt/stalwart", "RW": true}},
+		"HostConfig":      host,
+		"NetworkSettings": map[string]any{"Networks": networks},
+	}}
+	b, err := json.Marshal(doc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return string(b)
+}
+
+// fakeDockerCutover installs a docker that records arguments and serves the
+// given inspect document.
+func fakeDockerCutover(t *testing.T, doc string) (log string) {
+	t.Helper()
+	dir := t.TempDir()
+	log = filepath.Join(dir, "args.log")
+	inspectFile := filepath.Join(dir, "inspect.json")
+	if err := os.WriteFile(inspectFile, []byte(doc), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	script := fmt.Sprintf(`#!/bin/sh
+echo "$@" >> %q
+case "$1" in
+  inspect) cat %q ;;
+  rename) exit 0 ;;
+  run) echo newcontainerid ;;
+esac
+`, log, inspectFile)
+	if err := os.WriteFile(filepath.Join(dir, "docker"), []byte(script), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", dir+string(os.PathListSeparator)+os.Getenv("PATH"))
+	return log
+}
+
+func runContainerFor(t *testing.T, doc string) (*checkpoint.RunState, Report, error) {
+	t.Helper()
+	fakeDockerCutover(t, doc)
+	store := checkpoint.NewStore(t.TempDir())
+	rs, err := store.Create("0.15.5", "0.16.14")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var report Report
+	step := func(name string, fn func() (checkpoint.StepOutcome, error)) error {
+		outcome, err := store.RunStep(rs, checkpoint.PhaseCutover, name, fn)
+		if err != nil {
+			report.Results = append(report.Results, CheckResult{Name: name, Status: StatusFail, Detail: err.Error()})
+			return err
+		}
+		report.Results = append(report.Results, CheckResult{Name: name, Status: StatusOK, Detail: outcome.Detail})
+		return nil
+	}
+	err = runContainerCutover(context.Background(), rs, step, ContainerOptions{
+		ContainerName: "stalwart", StagedImage: "sha256:new", PreserveDir: t.TempDir(),
+	})
+	return rs, report, err
+}
+
+func TestContainerCutoverPreservesTheDefinitionFirst(t *testing.T) {
+	rs, _, err := runContainerFor(t, inspectJSON(t, nil, nil))
+	if err != nil {
+		t.Fatalf("runContainerCutover: %v", err)
+	}
+	art, ok := rs.Artifacts[ArtifactContainerDefinition]
+	if !ok {
+		t.Fatal("the container definition was not recorded as an artifact")
+	}
+	if art.SHA256 == "" || art.SizeBytes == 0 {
+		t.Errorf("artifact recorded without a checksum or size: %+v", art)
+	}
+	body, err := os.ReadFile(art.Path)
+	if err != nil {
+		t.Fatalf("preserved definition unreadable: %v", err)
+	}
+	if !strings.Contains(string(body), "stalwart-data") {
+		t.Error("preserved definition does not contain the container's mounts")
+	}
+}
+
+// The old container is kept, not removed: with the old image unpruned it is
+// the manual restore path (ARCHITECTURE.md §4.8).
+func TestContainerCutoverRetiresRatherThanRemoves(t *testing.T) {
+	log := fakeDockerCutover(t, inspectJSON(t, nil, nil))
+	store := checkpoint.NewStore(t.TempDir())
+	rs, _ := store.Create("0.15.5", "0.16.14")
+	step := func(name string, fn func() (checkpoint.StepOutcome, error)) error {
+		_, err := store.RunStep(rs, checkpoint.PhaseCutover, name, fn)
+		return err
+	}
+	if err := runContainerCutover(context.Background(), rs, step, ContainerOptions{
+		ContainerName: "stalwart", StagedImage: "sha256:new", PreserveDir: t.TempDir(),
+	}); err != nil {
+		t.Fatal(err)
+	}
+	args := readLog(t, log)
+	if !strings.Contains(args, "rename stalwart stalwart-premigration-0.15.5") {
+		t.Errorf("old container was not retired by rename:\n%s", args)
+	}
+	if strings.Contains(args, "rm stalwart") || strings.Contains(args, "image rm") || strings.Contains(args, "prune") {
+		t.Errorf("cutover removed something it should have kept:\n%s", args)
+	}
+}
+
+func TestContainerCutoverRecreatesWithTheCarriedSettings(t *testing.T) {
+	log := fakeDockerCutover(t, inspectJSON(t, nil, nil))
+	store := checkpoint.NewStore(t.TempDir())
+	rs, _ := store.Create("0.15.5", "0.16.14")
+	step := func(name string, fn func() (checkpoint.StepOutcome, error)) error {
+		_, err := store.RunStep(rs, checkpoint.PhaseCutover, name, fn)
+		return err
+	}
+	if err := runContainerCutover(context.Background(), rs, step, ContainerOptions{
+		ContainerName: "stalwart", StagedImage: "sha256:new", PreserveDir: t.TempDir(),
+	}); err != nil {
+		t.Fatal(err)
+	}
+	args := readLog(t, log)
+	for _, want := range []string{
+		"run -d --name stalwart",
+		"--restart unless-stopped",
+		"-e TZ=UTC",
+		"-v stalwart-data:/opt/stalwart",
+		"-p 0.0.0.0:143:143",
+		"sha256:new",
+	} {
+		if !strings.Contains(args, want) {
+			t.Errorf("recreate missing %q\ngot: %s", want, args)
+		}
+	}
+	// Leaving recovery mode set would recovery-boot on every restart - the
+	// footgun §4.5 strips from a unit.
+	if strings.Contains(args, "STALWART_RECOVERY_MODE") {
+		t.Errorf("recovery-mode env survived into the recreated container:\n%s", args)
+	}
+}
+
+// The hazard this design exists for: a container carrying configuration a
+// recreate would drop must be refused, not quietly rebuilt without it.
+func TestContainerCutoverRefusesConfigurationItWouldDrop(t *testing.T) {
+	for _, tc := range []struct {
+		name  string
+		host  map[string]any
+		nets  map[string]any
+		wants string
+	}{
+		{"capabilities", map[string]any{"CapAdd": []string{"NET_ADMIN"}}, nil, "capabilities"},
+		{"devices", map[string]any{"Devices": []any{map[string]any{}}}, nil, "device mappings"},
+		{"sysctls", map[string]any{"Sysctls": map[string]string{"net.core.somaxconn": "1024"}}, nil, "sysctls"},
+		{"privileged", map[string]any{"Privileged": true}, nil, "privileged"},
+		{"log driver", map[string]any{"LogConfig": map[string]any{"Type": "syslog"}}, nil, "log driver"},
+		{"user network", nil, map[string]any{"mailnet": map[string]any{}}, "user-defined network"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			_, _, err := runContainerFor(t, inspectJSON(t, tc.host, tc.nets))
+			if err == nil {
+				t.Fatalf("expected a refusal for a container with %s", tc.name)
+			}
+			if !strings.Contains(err.Error(), tc.wants) {
+				t.Errorf("refusal should name %q, got: %v", tc.wants, err)
+			}
+			if !strings.Contains(err.Error(), "by hand") {
+				t.Errorf("refusal should tell the operator what to do instead, got: %v", err)
+			}
+		})
+	}
+}
+
+// A refusal must happen before anything is touched.
+func TestContainerCutoverRefusesBeforeRetiringAnything(t *testing.T) {
+	log := fakeDockerCutover(t, inspectJSON(t, map[string]any{"Privileged": true}, nil))
+	store := checkpoint.NewStore(t.TempDir())
+	rs, _ := store.Create("0.15.5", "0.16.14")
+	step := func(name string, fn func() (checkpoint.StepOutcome, error)) error {
+		_, err := store.RunStep(rs, checkpoint.PhaseCutover, name, fn)
+		return err
+	}
+	if err := runContainerCutover(context.Background(), rs, step, ContainerOptions{
+		ContainerName: "stalwart", StagedImage: "sha256:new", PreserveDir: t.TempDir(),
+	}); err == nil {
+		t.Fatal("expected a refusal")
+	}
+	if args := readLog(t, log); strings.Contains(args, "rename") || strings.Contains(args, "run -d") {
+		t.Errorf("refusal came after the container was already changed:\n%s", args)
+	}
+}
+
+func TestContainerCutoverNeedsAStagedImage(t *testing.T) {
+	store := checkpoint.NewStore(t.TempDir())
+	rs, _ := store.Create("0.15.5", "0.16.14")
+	step := func(name string, fn func() (checkpoint.StepOutcome, error)) error {
+		_, err := store.RunStep(rs, checkpoint.PhaseCutover, name, fn)
+		return err
+	}
+	if err := runContainerCutover(context.Background(), rs, step, ContainerOptions{
+		ContainerName: "stalwart", PreserveDir: t.TempDir(),
+	}); err == nil {
+		t.Fatal("expected a refusal with no staged image")
+	}
+}
+
+func readLog(t *testing.T, path string) string {
+	t.Helper()
+	b, err := os.ReadFile(path)
+	if os.IsNotExist(err) {
+		return ""
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	return string(b)
+}

internal/cutover/cutover.go

@@ -43,6 +43,12 @@ type Options struct {
 	// moved the old binary aside, so this path is normally empty by now.
 	BinaryPath string
 
+	// Container, when set, cuts over a container deployment instead of a
+	// binary and a unit file. Opt-in: a Docker deployment without it is
+	// still refused, so a caller that has not been taught to stage an image
+	// cannot reach a half-built container path by accident.
+	Container *ContainerOptions
+
 	// ServiceUnitPath is the systemd unit to rewrite. ConfigPath, if set,
 	// becomes the unit's --config argument.
 	ServiceUnitPath string
@@ -147,9 +153,19 @@ func BuildPlan(rs *checkpoint.RunState, opts Options) (Plan, error) {
 		kind = service.Kind(rs.Topology.DeploymentKind)
 	}
 	if kind == service.Docker {
-		return p, fmt.Errorf(
-			"cutover: this run's deployment is a Docker container, where cutting over means pulling a new image and recreating the " +
-				"container rather than swapping a binary and rewriting a unit. This tool doesn't automate that - do it by hand")
+		if opts.Container == nil {
+			return p, fmt.Errorf(
+				"cutover: this run's deployment is a Docker container, where cutting over means pulling a new image and recreating the " +
+					"container rather than swapping a binary and rewriting a unit. No container options were given, so there is nothing " +
+					"to recreate it from - stage the target image and pass them, or do it by hand")
+		}
+		if opts.Container.StagedImage == "" {
+			return p, fmt.Errorf("cutover: no staged image to cut over to - stage the target image first")
+		}
+		// The binary checks below are about a file and a unit, neither of
+		// which a container has.
+		p.Target = "docker container " + opts.Container.ContainerName
+		return p, nil
 	}
 	deployment := opts.Deployment
 	deployment.Kind = kind
@@ -229,103 +245,118 @@ func Run(ctx context.Context, store *checkpoint.Store, rs *checkpoint.RunState,
 		return nil
 	}
 
-	if err := step("verify-staged-binary", func() (checkpoint.StepOutcome, error) {
-		got, err := preflight.DetectVersion(ctx, plan.StagedBinaryPath)
-		if err != nil {
-			return checkpoint.StepOutcome{}, fmt.Errorf("couldn't read the staged binary's version: %w", err)
+	// A container is replaced rather than edited, so none of the binary
+	// path applies: there is no file to install and no unit to rewrite.
+	// What follows either branch - confirming it answers, and the quota
+	// recalculation - is the same question whatever started it.
+	if opts.Container != nil {
+		if err := runContainerCutover(ctx, rs, step, *opts.Container); err != nil {
+			return report, err
 		}
-		if rs.TargetVersion != "" && got != rs.TargetVersion {
-			return checkpoint.StepOutcome{}, fmt.Errorf(
-				"staged binary %s reports version %s, but this run targets %s - installing it would migrate to a version nobody planned for",
-				plan.StagedBinaryPath, got, rs.TargetVersion)
+		if err := step("wait-running", func() (checkpoint.StepOutcome, error) {
+			if err := service.WaitFor(ctx, controller, true, startTimeoutOr(opts)); err != nil {
+				return checkpoint.StepOutcome{}, err
+			}
+			return checkpoint.StepOutcome{Detail: fmt.Sprintf("%s is running the migrated instance", controller.Target())}, nil
+		}); err != nil {
+			return report, err
+		}
+	} else {
+		if err := step("verify-staged-binary", func() (checkpoint.StepOutcome, error) {
+			got, err := preflight.DetectVersion(ctx, plan.StagedBinaryPath)
+			if err != nil {
+				return checkpoint.StepOutcome{}, fmt.Errorf("couldn't read the staged binary's version: %w", err)
+			}
+			if rs.TargetVersion != "" && got != rs.TargetVersion {
+				return checkpoint.StepOutcome{}, fmt.Errorf(
+					"staged binary %s reports version %s, but this run targets %s - installing it would migrate to a version nobody planned for",
+					plan.StagedBinaryPath, got, rs.TargetVersion)
+			}
+			return checkpoint.StepOutcome{Detail: fmt.Sprintf("staged binary %s reports %s, matching this run's target", plan.StagedBinaryPath, got), Extra: got}, nil
+		}); err != nil {
+			return report, err
 		}
-		return checkpoint.StepOutcome{Detail: fmt.Sprintf("staged binary %s reports %s, matching this run's target", plan.StagedBinaryPath, got), Extra: got}, nil
-	}); err != nil {
-		return report, err
-	}
 
-	if err := step("install-binary", func() (checkpoint.StepOutcome, error) {
-		sum, size, err := installBinary(plan.StagedBinaryPath, plan.BinaryPath)
-		if err != nil {
-			return checkpoint.StepOutcome{}, err
+		if err := step("install-binary", func() (checkpoint.StepOutcome, error) {
+			sum, size, err := installBinary(plan.StagedBinaryPath, plan.BinaryPath)
+			if err != nil {
+				return checkpoint.StepOutcome{}, err
+			}
+			rs.RecordArtifact(ArtifactNewBinary, checkpoint.Artifact{Path: plan.BinaryPath, SHA256: sum, SizeBytes: size})
+			return checkpoint.StepOutcome{Detail: fmt.Sprintf("installed %s as %s (%d bytes)", plan.StagedBinaryPath, plan.BinaryPath, size)}, nil
+		}); err != nil {
+			return report, err
 		}
-		rs.RecordArtifact(ArtifactNewBinary, checkpoint.Artifact{Path: plan.BinaryPath, SHA256: sum, SizeBytes: size})
-		return checkpoint.StepOutcome{Detail: fmt.Sprintf("installed %s as %s (%d bytes)", plan.StagedBinaryPath, plan.BinaryPath, size)}, nil
-	}); err != nil {
-		return report, err
-	}
 
-	if err := step("install-config", func() (checkpoint.StepOutcome, error) {
-		if plan.ConfigSource == "" {
+		if err := step("install-config", func() (checkpoint.StepOutcome, error) {
+			if plan.ConfigSource == "" {
+				return checkpoint.StepOutcome{
+					Verdict: string(StatusSkipped),
+					Detail:  "no converted config to install - the unit is repointed at whatever is already at ConfigPath",
+				}, nil
+			}
+			owner, err := installConfig(plan.ConfigSource, plan.ConfigPath, opts.ConfigOwnerReference)
+			if err != nil {
+				return checkpoint.StepOutcome{}, err
+			}
+			return checkpoint.StepOutcome{Detail: fmt.Sprintf("installed %s as %s (%s)", plan.ConfigSource, plan.ConfigPath, owner)}, nil
+		}); err != nil {
+			return report, err
+		}
+
+		if err := step("update-service-definition", func() (checkpoint.StepOutcome, error) {
+			preserved, err := preserveUnit(plan.ServiceUnitPath, rs.RunID)
+			if err != nil {
+				return checkpoint.StepOutcome{}, err
+			}
+			sum, size, err := hashFile(preserved)
+			if err != nil {
+				return checkpoint.StepOutcome{}, err
+			}
+			// Recorded before the rewrite, so a crash between preserving and
+			// rewriting still leaves the original findable.
+			rs.RecordArtifact(ArtifactServiceUnit, checkpoint.Artifact{Path: preserved, SHA256: sum, SizeBytes: size})
+
+			original, err := os.ReadFile(preserved)
+			if err != nil {
+				return checkpoint.StepOutcome{}, fmt.Errorf("cutover: read preserved unit %s: %w", preserved, err)
+			}
+			rewritten, err := RewriteUnit(string(original), plan.BinaryPath, plan.ConfigPath)
+			if err != nil {
+				return checkpoint.StepOutcome{}, err
+			}
+			if err := writeFileAtomic(plan.ServiceUnitPath, []byte(rewritten), 0o644); err != nil {
+				return checkpoint.StepOutcome{}, err
+			}
 			return checkpoint.StepOutcome{
-				Verdict: string(StatusSkipped),
-				Detail:  "no converted config to install - the unit is repointed at whatever is already at ConfigPath",
+				Detail: fmt.Sprintf("pointed %s at %s; the original is preserved at %s", plan.ServiceUnitPath, plan.BinaryPath, preserved),
+				Extra:  preserved,
 			}, nil
+		}); err != nil {
+			return report, err
 		}
-		owner, err := installConfig(plan.ConfigSource, plan.ConfigPath, opts.ConfigOwnerReference)
-		if err != nil {
-			return checkpoint.StepOutcome{}, err
-		}
-		return checkpoint.StepOutcome{Detail: fmt.Sprintf("installed %s as %s (%s)", plan.ConfigSource, plan.ConfigPath, owner)}, nil
-	}); err != nil {
-		return report, err
-	}
 
-	if err := step("update-service-definition", func() (checkpoint.StepOutcome, error) {
-		preserved, err := preserveUnit(plan.ServiceUnitPath, rs.RunID)
-		if err != nil {
-			return checkpoint.StepOutcome{}, err
+		if err := step("reload-service-definition", func() (checkpoint.StepOutcome, error) {
+			if err := controller.ReloadConfig(ctx); err != nil {
+				return checkpoint.StepOutcome{}, err
+			}
+			return checkpoint.StepOutcome{Detail: "service manager re-read the updated definition"}, nil
+		}); err != nil {
+			return report, err
 		}
-		sum, size, err := hashFile(preserved)
-		if err != nil {
-			return checkpoint.StepOutcome{}, err
-		}
-		// Recorded before the rewrite, so a crash between preserving and
-		// rewriting still leaves the original findable.
-		rs.RecordArtifact(ArtifactServiceUnit, checkpoint.Artifact{Path: preserved, SHA256: sum, SizeBytes: size})
 
-		original, err := os.ReadFile(preserved)
-		if err != nil {
-			return checkpoint.StepOutcome{}, fmt.Errorf("cutover: read preserved unit %s: %w", preserved, err)
+		startTimeout := startTimeoutOr(opts)
+		if err := step("start-service", func() (checkpoint.StepOutcome, error) {
+			if err := controller.Start(ctx); err != nil {
+				return checkpoint.StepOutcome{}, err
+			}
+			if err := service.WaitFor(ctx, controller, true, startTimeout); err != nil {
+				return checkpoint.StepOutcome{}, err
+			}
+			return checkpoint.StepOutcome{Detail: fmt.Sprintf("%s is running the migrated instance", controller.Target())}, nil
+		}); err != nil {
+			return report, err
 		}
-		rewritten, err := RewriteUnit(string(original), plan.BinaryPath, plan.ConfigPath)
-		if err != nil {
-			return checkpoint.StepOutcome{}, err
-		}
-		if err := writeFileAtomic(plan.ServiceUnitPath, []byte(rewritten), 0o644); err != nil {
-			return checkpoint.StepOutcome{}, err
-		}
-		return checkpoint.StepOutcome{
-			Detail: fmt.Sprintf("pointed %s at %s; the original is preserved at %s", plan.ServiceUnitPath, plan.BinaryPath, preserved),
-			Extra:  preserved,
-		}, nil
-	}); err != nil {
-		return report, err
-	}
-
-	if err := step("reload-service-definition", func() (checkpoint.StepOutcome, error) {
-		if err := controller.ReloadConfig(ctx); err != nil {
-			return checkpoint.StepOutcome{}, err
-		}
-		return checkpoint.StepOutcome{Detail: "service manager re-read the updated definition"}, nil
-	}); err != nil {
-		return report, err
-	}
-
-	startTimeout := opts.StartTimeout
-	if startTimeout <= 0 {
-		startTimeout = 60 * time.Second
-	}
-	if err := step("start-service", func() (checkpoint.StepOutcome, error) {
-		if err := controller.Start(ctx); err != nil {
-			return checkpoint.StepOutcome{}, err
-		}
-		if err := service.WaitFor(ctx, controller, true, startTimeout); err != nil {
-			return checkpoint.StepOutcome{}, err
-		}
-		return checkpoint.StepOutcome{Detail: fmt.Sprintf("%s is running the migrated instance", controller.Target())}, nil
-	}); err != nil {
-		return report, err
 	}
 
 	healthTimeout := opts.HealthTimeout
@@ -587,3 +618,12 @@ func installConfig(src, dst, reference string) (ownership string, err error) {
 	}
 	return fmt.Sprintf("mode %v, ownership unchanged (no reference file to copy it from)", perm), nil
 }
+
+// startTimeoutOr is the wait for the service to come up, defaulted. Shared
+// because both branches wait for the same thing.
+func startTimeoutOr(opts Options) time.Duration {
+	if opts.StartTimeout > 0 {
+		return opts.StartTimeout
+	}
+	return 60 * time.Second
+}

internal/preflight/container.go

@@ -45,6 +45,23 @@ type ContainerFacts struct {
 	Labels  map[string]string
 	Mounts  []Mount
 	Running bool
+
+	// The rest is what cutover would have to carry across when it recreates
+	// the container. Recreating is the container equivalent of rewriting a
+	// unit file, except that a unit can be edited in place and a container
+	// cannot - so anything not carried here is silently dropped, which is
+	// the failure UnsupportedForRecreate exists to prevent.
+	Env           []string
+	Ports         map[string][]PortBinding
+	RestartPolicy string
+	NetworkMode   string
+	Unsupported   []string // populated by UnsupportedForRecreate
+}
+
+// PortBinding is one published port.
+type PortBinding struct {
+	HostIP   string `json:"HostIp"`
+	HostPort string `json:"HostPort"`
 }
 
 // ComposeProject returns the compose project managing this container, or
@@ -95,11 +112,37 @@ type inspectOutput struct {
 	Config struct {
 		Image  string            `json:"Image"`
 		Labels map[string]string `json:"Labels"`
+		Env    []string          `json:"Env"`
+		User   string            `json:"User"`
 	} `json:"Config"`
 	State struct {
 		Running bool `json:"Running"`
 	} `json:"State"`
-	Mounts []Mount `json:"Mounts"`
+	Mounts     []Mount `json:"Mounts"`
+	HostConfig struct {
+		PortBindings  map[string][]PortBinding `json:"PortBindings"`
+		RestartPolicy struct {
+			Name string `json:"Name"`
+		} `json:"RestartPolicy"`
+		NetworkMode string            `json:"NetworkMode"`
+		CapAdd      []string          `json:"CapAdd"`
+		CapDrop     []string          `json:"CapDrop"`
+		Devices     []any             `json:"Devices"`
+		Sysctls     map[string]string `json:"Sysctls"`
+		Ulimits     []any             `json:"Ulimits"`
+		Privileged  bool              `json:"Privileged"`
+		ExtraHosts  []string          `json:"ExtraHosts"`
+		DNS         []string          `json:"Dns"`
+		GroupAdd    []string          `json:"GroupAdd"`
+		SecurityOpt []string          `json:"SecurityOpt"`
+		Tmpfs       map[string]string `json:"Tmpfs"`
+		LogConfig   struct {
+			Type string `json:"Type"`
+		} `json:"LogConfig"`
+	} `json:"HostConfig"`
+	NetworkSettings struct {
+		Networks map[string]any `json:"Networks"`
+	} `json:"NetworkSettings"`
 }
 
 // InspectContainer reads the facts about containerName. An error here is
@@ -123,14 +166,70 @@ func InspectContainer(ctx context.Context, containerName string) (ContainerFacts
 		return ContainerFacts{}, fmt.Errorf("preflight: docker inspect %s returned no container", containerName)
 	}
 	c := got[0]
-	return ContainerFacts{
-		Name:    strings.TrimPrefix(c.Name, "/"),
-		Image:   c.Config.Image,
-		ImageID: c.Image,
-		Labels:  c.Config.Labels,
-		Mounts:  c.Mounts,
-		Running: c.State.Running,
-	}, nil
+	f := ContainerFacts{
+		Name:          strings.TrimPrefix(c.Name, "/"),
+		Image:         c.Config.Image,
+		ImageID:       c.Image,
+		Labels:        c.Config.Labels,
+		Mounts:        c.Mounts,
+		Running:       c.State.Running,
+		Env:           c.Config.Env,
+		Ports:         c.HostConfig.PortBindings,
+		RestartPolicy: c.HostConfig.RestartPolicy.Name,
+		NetworkMode:   c.HostConfig.NetworkMode,
+	}
+	f.Unsupported = unsupportedForRecreate(c)
+	return f, nil
+}
+
+// unsupportedForRecreate names every piece of this container's
+// configuration that recreating it would not carry across.
+//
+// Cutover recreates rather than edits, because a container cannot be edited
+// in place the way a unit file can. That makes silent loss the default
+// failure: a container recreated without its capabilities, its custom
+// network or its device mappings starts cleanly and is quietly not the
+// server it was. §4.5 already refuses to edit a unit line it only partly
+// understands; this is the same rule, applied where the whole definition
+// has to be rebuilt.
+//
+// The list is deliberately conservative and deliberately not exhaustive -
+// docker's HostConfig has far more fields than these. It names the ones a
+// mail server plausibly uses, and anything it does not know about is a
+// reason this tool should not be recreating that container at all.
+func unsupportedForRecreate(c inspectOutput) []string {
+	var out []string
+	add := func(cond bool, what string) {
+		if cond {
+			out = append(out, what)
+		}
+	}
+	h := c.HostConfig
+	add(len(h.CapAdd) > 0, "added capabilities (--cap-add)")
+	add(len(h.CapDrop) > 0, "dropped capabilities (--cap-drop)")
+	add(len(h.Devices) > 0, "device mappings (--device)")
+	add(len(h.Sysctls) > 0, "sysctls (--sysctl)")
+	add(len(h.Ulimits) > 0, "ulimits (--ulimit)")
+	add(h.Privileged, "privileged mode (--privileged)")
+	add(len(h.ExtraHosts) > 0, "extra hosts (--add-host)")
+	add(len(h.DNS) > 0, "custom DNS (--dns)")
+	add(len(h.GroupAdd) > 0, "supplementary groups (--group-add)")
+	add(len(h.SecurityOpt) > 0, "security options (--security-opt)")
+	add(len(h.Tmpfs) > 0, "tmpfs mounts (--tmpfs)")
+	add(h.LogConfig.Type != "" && h.LogConfig.Type != "json-file", "a non-default log driver (--log-driver "+h.LogConfig.Type+")")
+	add(c.Config.User != "", "a container user (--user "+c.Config.User+")")
+
+	// A user-defined network is a name in NetworkSettings.Networks that is
+	// not one of docker's built-ins. Recreating without it puts the server
+	// somewhere nothing else can reach it.
+	for name := range c.NetworkSettings.Networks {
+		switch name {
+		case "bridge", "host", "none":
+		default:
+			out = append(out, "a user-defined network ("+name+")")
+		}
+	}
+	return out
 }
 
 // runContainerChecks adds the checks that only apply to a container. They