Fix the domain/tenant mismatch that failed the second live migration

The second production attempt failed during recovery-mode apply, with the
mail server already stopped and the store already at schema v6:

    create Account restore-13: invalidForeignKey | Object id: Domain#d

v0.16 requires a tenant-scoped Account to sit on a Domain owned by that
same tenant, for its primary domain and for every alias. v0.15 imposed no
such rule, and migrate_v016.py carries the two facts over independently:
_build_domains sets a domain's memberTenantId only for domains declared as
their own `domain` principal with a `tenant`, while _build_user sets the
account's from the account's own record. A domain that exists only inside
an email address is inferred, gets no tenant, and every tenant-scoped
account using it is then rejected.

Established by reproduction rather than inference: a synthetic v0.15
principal dump, run through the unpatched upstream converter and applied to
a real 0.16.14 in recovery mode, reproduces the error character for
character - the `#d` is the server's own object id for the offending
domain, not a plan client-id. The same harness establishes which directions
are constrained: a tenant-scoped account on a tenant-less domain or on
another tenant's domain is rejected; a global account on a tenant-owned
domain is accepted.

  - applyplan.ReconcileDomainTenants repairs the plan between convert and
    apply. Where a tenant-less domain is used only by accounts of one
    tenant, the domain adopts that tenant - the sole assignment that both
    applies and keeps every account. Where accounts genuinely disagree it
    changes nothing and reports why, because forcing such a plan through
    would mean dropping mailboxes.
  - stalwartapi.FetchTenantLayout maps tenant membership over the 0.15 REST
    API and predicts the outcome with the same rule the server enforces, so
    preflight either warns about the domains that will adopt a tenant or
    fails - while the service is still running.
  - The plan is parsed generically rather than through the typed Operation.
    A real export.json mixes shapes: `create` maps a client-id to an object,
    `update` carries a flat one. The typed form failed on the first `update`
    line, found by running against actual converter output. Numbers decode
    as json.Number so a 10 GiB quota is not rewritten as 1.073741824e+10.

Corrects the record: the previous commit claimed the converter emits every
Account with `tenantId: null` and made preflight refuse every multi-tenant
install on that basis. The field is memberTenantId, the converter does
populate it, and the export had been inspected for a key no version of the
script ever writes. The refusal is now narrowed to what v0.16 genuinely
cannot represent.

The same fix has been prepared for migrate_v016.py upstream. The tool
downloads that script rather than vendoring it, so the repair stays here
until a released version carries it, and is a no-op on a consistent plan.

internal/applyplan/tenants.go

@@ -0,0 +1,386 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package applyplan
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+)
+
+// This file repairs one specific defect in Stalwart's own migrate_v016.py,
+// which this tool downloads rather than vendors and therefore cannot fix at
+// the source.
+//
+// In v0.15 a domain's tenant and a principal's tenant were independent
+// facts. A tenant-scoped user could sit on a domain owned by no tenant at
+// all - which is the normal outcome whenever the domain was never declared
+// as its own `domain` principal, because migrate_v016.py then infers the
+// domain from an email address, and inferred domains carry no tenant.
+//
+// v0.16 rejects that arrangement. A tenant-scoped Account may only
+// reference a Domain owned by the same tenant, as a primary domain or as an
+// alias, and the server answers:
+//
+//	invalidForeignKey |   Object id:   Domain#d
+//
+// on the Domain reference otherwise. This was confirmed against a real
+// 0.16.14 in recovery mode, not inferred: the reverse direction (a global,
+// tenant-less account on a tenant-owned domain) applies without complaint,
+// and only the tenant-scoped-account-to-tenant-less-domain direction fails.
+//
+// That error cost a production migration a full restore, because it
+// surfaced during apply - after the old service was already stopped, and
+// after the store had been irreversibly upgraded to schema v6. Hence both
+// halves of the fix: repair the plan here, and refuse the genuinely
+// unrepresentable cases in preflight, before anything is stopped.
+//
+// The same fix has been prepared for migrate_v016.py upstream; this stays
+// until a released migrate_v016.py carries it, and is written to be a no-op
+// against a plan that is already consistent.
+
+// TenantAdoption records one domain that took on a tenant it did not
+// previously declare, so the operator is told rather than silently given a
+// different ownership model than they had.
+type TenantAdoption struct {
+	Domain  string // domain name, e.g. "example.com"
+	Tenant  string // tenant name, or the raw reference if the name is unknown
+	Because string // the principal whose membership forced it
+}
+
+// TenantConflict is a domain whose users disagree about which tenant owns
+// it. v0.16 cannot represent this, so it is reported rather than repaired.
+type TenantConflict struct {
+	Domain  string
+	Detail  string
+	Tenants []string
+}
+
+// TenantReconcileResult is what a reconciliation changed and what it could
+// not change.
+type TenantReconcileResult struct {
+	Adoptions []TenantAdoption
+	Conflicts []TenantConflict
+}
+
+// OK reports whether the plan is now internally consistent.
+func (r TenantReconcileResult) OK() bool { return len(r.Conflicts) == 0 }
+
+// String renders a short operator-facing summary.
+func (r TenantReconcileResult) String() string {
+	if len(r.Adoptions) == 0 && len(r.Conflicts) == 0 {
+		return "no domain/tenant mismatches"
+	}
+	var b strings.Builder
+	for _, a := range r.Adoptions {
+		fmt.Fprintf(&b, "domain %s adopted tenant %s (required by %s)\n", a.Domain, a.Tenant, a.Because)
+	}
+	for _, c := range r.Conflicts {
+		fmt.Fprintf(&b, "domain %s: %s\n", c.Domain, c.Detail)
+	}
+	return strings.TrimRight(b.String(), "\n")
+}
+
+// PlanOp is one line of an apply plan, kept in the generic form it was
+// read in.
+//
+// It is deliberately not the typed Operation above. A real export.json
+// mixes shapes: `create` operations map a client-id to an object, while
+// `update` operations - SystemSettings, BlobStore, SearchStore - carry a
+// flat object with no client-id layer at all. Parsing a whole plan into the
+// typed form fails on the first `update` line, and a reconciliation that
+// cannot read the plan it is meant to repair is worse than none.
+type PlanOp map[string]any
+
+// ReadPlan parses an apply plan - migrate_v016.py's export.json, or one
+// this tool generated.
+//
+// Numbers are decoded as json.Number so they survive the round-trip
+// unchanged. Decoding into float64 would rewrite a quota of 10737418240 as
+// 1.073741824e+10, which is a different document than the one the
+// conversion produced.
+func ReadPlan(path string) ([]PlanOp, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("applyplan: open %s: %w", path, err)
+	}
+	defer f.Close()
+
+	var ops []PlanOp
+	sc := bufio.NewScanner(f)
+	// Plans carry certificates and DKIM keys inline, so lines run well past
+	// bufio.Scanner's default 64KiB limit.
+	sc.Buffer(make([]byte, 0, 64<<10), 8<<20)
+	for line := 1; sc.Scan(); line++ {
+		raw := strings.TrimSpace(sc.Text())
+		if raw == "" {
+			continue
+		}
+		dec := json.NewDecoder(strings.NewReader(raw))
+		dec.UseNumber()
+		var op PlanOp
+		if err := dec.Decode(&op); err != nil {
+			return nil, fmt.Errorf("applyplan: %s line %d: %w", path, line, err)
+		}
+		ops = append(ops, op)
+	}
+	if err := sc.Err(); err != nil {
+		return nil, fmt.Errorf("applyplan: read %s: %w", path, err)
+	}
+	return ops, nil
+}
+
+// WritePlan writes plan operations atomically, so a failure part-way
+// through can never leave a truncated plan that would apply half a
+// migration.
+func WritePlan(path string, ops []PlanOp) error {
+	dir := filepath.Dir(path)
+	tmp, err := os.CreateTemp(dir, filepath.Base(path)+".tmp*")
+	if err != nil {
+		return fmt.Errorf("applyplan: create temp file in %s: %w", dir, err)
+	}
+	tmpName := tmp.Name()
+	defer os.Remove(tmpName)
+
+	enc := json.NewEncoder(tmp)
+	for _, op := range ops {
+		if err := enc.Encode(op); err != nil {
+			tmp.Close()
+			return fmt.Errorf("applyplan: write %s: %w", tmpName, err)
+		}
+	}
+	if err := tmp.Sync(); err != nil {
+		tmp.Close()
+		return fmt.Errorf("applyplan: sync %s: %w", tmpName, err)
+	}
+	if err := tmp.Close(); err != nil {
+		return fmt.Errorf("applyplan: close %s: %w", tmpName, err)
+	}
+	if err := os.Chmod(tmpName, 0o640); err != nil {
+		return fmt.Errorf("applyplan: chmod %s: %w", tmpName, err)
+	}
+	if err := os.Rename(tmpName, path); err != nil {
+		return fmt.Errorf("applyplan: replace %s: %w", path, err)
+	}
+	return nil
+}
+
+// objectName is the operation's target type, e.g. "Domain".
+func (op PlanOp) objectName() string {
+	s, _ := op["object"].(string)
+	return s
+}
+
+// createBodies returns the client-id-keyed objects a `create` operation
+// carries. Operations whose value is a flat object - every `update` - yield
+// nothing, which is what keeps them untouched.
+func (op PlanOp) createBodies() map[string]map[string]any {
+	value, ok := op["value"].(map[string]any)
+	if !ok {
+		return nil
+	}
+	out := map[string]map[string]any{}
+	for cid, body := range value {
+		if b, ok := body.(map[string]any); ok {
+			out[cid] = b
+		}
+	}
+	if len(out) == 0 {
+		return nil
+	}
+	return out
+}
+
+// refCID strips the leading "#" from a plan back-reference. Values that are
+// not back-references (an already-resolved server id, say) are returned
+// unchanged, which is what makes comparing two references safe.
+func refCID(v any) string {
+	s, ok := v.(string)
+	if !ok || s == "" {
+		return ""
+	}
+	return strings.TrimPrefix(s, "#")
+}
+
+// domainRefs returns every domain reference an account or mailing list
+// makes: its primary domain, plus one per alias. Aliases matter as much as
+// the primary - a tenant-scoped account with an alias on a tenant-less
+// domain is rejected exactly like one whose primary domain mismatches.
+func domainRefs(body map[string]any) []string {
+	var refs []string
+	if c := refCID(body["domainId"]); c != "" {
+		refs = append(refs, c)
+	}
+	aliases, ok := body["aliases"].(map[string]any)
+	if !ok {
+		return refs
+	}
+	for _, a := range aliases {
+		alias, ok := a.(map[string]any)
+		if !ok {
+			continue
+		}
+		if c := refCID(alias["domainId"]); c != "" {
+			refs = append(refs, c)
+		}
+	}
+	return refs
+}
+
+func stringField(body map[string]any, key string) string {
+	s, _ := body[key].(string)
+	return s
+}
+
+// ReconcileDomainTenants makes every domain's tenant agree with the
+// accounts and mailing lists that use it, editing ops in place.
+//
+// Where a domain has no tenant but every tenant-scoped principal using it
+// agrees on one, the domain adopts that tenant: it is the only assignment
+// that lets the plan apply while keeping every account, and it is safe in
+// the other direction because a global account on a tenant-owned domain is
+// accepted.
+//
+// Where the principals disagree, nothing is changed and the disagreement is
+// reported. Forcing such a plan through would mean dropping accounts, and
+// dropping accounts silently is how mailboxes get lost.
+func ReconcileDomainTenants(ops []PlanOp) TenantReconcileResult {
+	var result TenantReconcileResult
+
+	domains := map[string]map[string]any{} // cid -> domain body
+	tenantNames := map[string]string{}     // cid -> tenant name
+	for _, op := range ops {
+		bodies := op.createBodies()
+		switch op.objectName() {
+		case "Domain":
+			for cid, body := range bodies {
+				domains[cid] = body
+			}
+		case "Tenant":
+			for cid, body := range bodies {
+				tenantNames[cid] = stringField(body, "name")
+			}
+		}
+	}
+	if len(domains) == 0 || len(tenantNames) == 0 {
+		return result // single-tenant install: nothing can mismatch
+	}
+
+	tenantName := func(ref string) string {
+		if n := tenantNames[refCID(ref)]; n != "" {
+			return n
+		}
+		return refCID(ref)
+	}
+
+	// domain cid -> tenant ref -> an example principal requiring it.
+	required := map[string]map[string]string{}
+	for _, op := range ops {
+		object := op.objectName()
+		if object != "Account" && object != "MailingList" {
+			continue
+		}
+		for _, body := range op.createBodies() {
+			tRef, _ := body["memberTenantId"].(string)
+			if tRef == "" {
+				continue // a global principal constrains nothing
+			}
+			label := fmt.Sprintf("%s %q", object, stringField(body, "name"))
+			for _, dCID := range domainRefs(body) {
+				if required[dCID] == nil {
+					required[dCID] = map[string]string{}
+				}
+				if _, seen := required[dCID][tRef]; !seen {
+					required[dCID][tRef] = label
+				}
+			}
+		}
+	}
+
+	for _, dCID := range sortedKeys(required) {
+		wanted := required[dCID]
+		dom, ok := domains[dCID]
+		if !ok {
+			continue // reference to something this plan does not create
+		}
+		dName := stringField(dom, "name")
+		if dName == "" {
+			dName = dCID
+		}
+
+		if len(wanted) > 1 {
+			var names, parts []string
+			for _, tRef := range sortedKeys(wanted) {
+				names = append(names, tenantName(tRef))
+				parts = append(parts, fmt.Sprintf("%s (e.g. %s)", tenantName(tRef), wanted[tRef]))
+			}
+			result.Conflicts = append(result.Conflicts, TenantConflict{
+				Domain:  dName,
+				Tenants: names,
+				Detail: fmt.Sprintf("used by principals from more than one tenant: %s. "+
+					"v0.16 requires every tenant-scoped account to sit on a domain owned by its own "+
+					"tenant, and a domain can belong to at most one tenant", strings.Join(parts, ", ")),
+			})
+			continue
+		}
+
+		tRef := sortedKeys(wanted)[0]
+		because := wanted[tRef]
+		current, _ := dom["memberTenantId"].(string)
+		if refCID(current) == refCID(tRef) {
+			continue // already consistent
+		}
+		if current != "" {
+			result.Conflicts = append(result.Conflicts, TenantConflict{
+				Domain:  dName,
+				Tenants: []string{tenantName(current), tenantName(tRef)},
+				Detail: fmt.Sprintf("belongs to tenant %s, but %s belongs to tenant %s and uses it. "+
+					"v0.16 rejects an account whose tenant differs from its domain's",
+					tenantName(current), because, tenantName(tRef)),
+			})
+			continue
+		}
+
+		dom["memberTenantId"] = tRef
+		result.Adoptions = append(result.Adoptions, TenantAdoption{
+			Domain: dName, Tenant: tenantName(tRef), Because: because,
+		})
+	}
+
+	return result
+}
+
+// ReconcileDomainTenantsFile reads a plan, reconciles it, and rewrites it
+// only if something actually changed and nothing conflicted.
+func ReconcileDomainTenantsFile(path string) (TenantReconcileResult, error) {
+	ops, err := ReadPlan(path)
+	if err != nil {
+		return TenantReconcileResult{}, err
+	}
+	result := ReconcileDomainTenants(ops)
+	if !result.OK() {
+		return result, fmt.Errorf("applyplan: %s cannot be made consistent with v0.16's "+
+			"tenant rules:\n%s", path, result.String())
+	}
+	if len(result.Adoptions) == 0 {
+		return result, nil
+	}
+	if err := WritePlan(path, ops); err != nil {
+		return result, err
+	}
+	return result, nil
+}
+
+func sortedKeys[V any](m map[string]V) []string {
+	out := make([]string, 0, len(m))
+	for k := range m {
+		out = append(out, k)
+	}
+	sort.Strings(out)
+	return out
+}

internal/applyplan/tenants_test.go

@@ -0,0 +1,329 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package applyplan
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// The four cases below are the ones checked against a real Stalwart 0.16.14
+// in recovery mode, via `stalwart-cli apply`, before this code was written:
+//
+//	tenant account  -> tenant-less domain : invalidForeignKey  (repaired here)
+//	tenant account  -> other tenant's dom : invalidForeignKey  (unrepresentable)
+//	global account  -> tenant-owned domain: accepted           (left alone)
+//	tenant account  -> own tenant's domain: accepted           (left alone)
+func plan(ops ...PlanOp) []PlanOp { return ops }
+
+func tenantOp(cid, name string) PlanOp {
+	return PlanOp{"@type": "create", "object": "Tenant",
+		"value": map[string]any{cid: map[string]any{"name": name}}}
+}
+
+func domainOp(cid, name string, tenantRef string) PlanOp {
+	body := map[string]any{"name": name}
+	if tenantRef != "" {
+		body["memberTenantId"] = tenantRef
+	}
+	return PlanOp{"@type": "create", "object": "Domain",
+		"value": map[string]any{cid: body}}
+}
+
+func accountOp(cid, name, domainRef, tenantRef string, aliasDomainRefs ...string) PlanOp {
+	body := map[string]any{"@type": "User", "name": name, "domainId": domainRef}
+	if tenantRef != "" {
+		body["memberTenantId"] = tenantRef
+	}
+	if len(aliasDomainRefs) > 0 {
+		aliases := map[string]any{}
+		for i, r := range aliasDomainRefs {
+			aliases[string(rune('0'+i))] = map[string]any{"name": name, "domainId": r}
+		}
+		body["aliases"] = aliases
+	}
+	return PlanOp{"@type": "create", "object": "Account",
+		"value": map[string]any{cid: body}}
+}
+
+func domainTenant(t *testing.T, ops []PlanOp, cid string) string {
+	t.Helper()
+	for _, op := range ops {
+		if op.objectName() != "Domain" {
+			continue
+		}
+		if body, ok := op.createBodies()[cid]; ok {
+			s, _ := body["memberTenantId"].(string)
+			return s
+		}
+	}
+	t.Fatalf("no domain %q in plan", cid)
+	return ""
+}
+
+func TestReconcileAdoptsTenantForInferredDomain(t *testing.T) {
+	ops := plan(
+		tenantOp("t0", "acme"),
+		domainOp("d0", "acme-corp.test", "#t0"),
+		domainOp("d1", "inferred.test", ""), // never declared in v0.15
+		accountOp("a0", "bob", "#d0", "#t0", "#d1"),
+	)
+	res := ReconcileDomainTenants(ops)
+	if !res.OK() {
+		t.Fatalf("unexpected conflicts: %s", res.String())
+	}
+	if got := domainTenant(t, ops, "d1"); got != "#t0" {
+		t.Errorf("inferred domain tenant = %q, want %q", got, "#t0")
+	}
+	if len(res.Adoptions) != 1 || res.Adoptions[0].Domain != "inferred.test" {
+		t.Errorf("adoptions = %+v, want one for inferred.test", res.Adoptions)
+	}
+	if res.Adoptions[0].Tenant != "acme" {
+		t.Errorf("adoption tenant = %q, want the tenant name %q", res.Adoptions[0].Tenant, "acme")
+	}
+}
+
+func TestReconcileLeavesGlobalAccountOnTenantDomainAlone(t *testing.T) {
+	// Confirmed accepted by 0.16.14: a global account may live on a
+	// tenant-owned domain. Nothing should be invented for it.
+	ops := plan(
+		tenantOp("t0", "acme"),
+		domainOp("d0", "acme-corp.test", "#t0"),
+		accountOp("a0", "admin", "#d0", ""),
+	)
+	res := ReconcileDomainTenants(ops)
+	if !res.OK() || len(res.Adoptions) != 0 {
+		t.Fatalf("expected no changes, got %s", res.String())
+	}
+}
+
+func TestReconcileLeavesTenantlessDomainWithOnlyGlobalUsers(t *testing.T) {
+	ops := plan(
+		tenantOp("t0", "acme"),
+		domainOp("d0", "global.test", ""),
+		accountOp("a0", "carol", "#d0", ""),
+	)
+	res := ReconcileDomainTenants(ops)
+	if got := domainTenant(t, ops, "d0"); got != "" {
+		t.Errorf("global domain gained tenant %q; it has no tenant-scoped users", got)
+	}
+	if len(res.Adoptions) != 0 {
+		t.Errorf("adoptions = %+v, want none", res.Adoptions)
+	}
+}
+
+func TestReconcileReportsTwoTenantsSharingADomain(t *testing.T) {
+	ops := plan(
+		tenantOp("t0", "alpha"),
+		tenantOp("t1", "beta"),
+		domainOp("d0", "shared.test", ""),
+		accountOp("a0", "u1", "#d0", "#t0"),
+		accountOp("a1", "u2", "#d0", "#t1"),
+	)
+	res := ReconcileDomainTenants(ops)
+	if res.OK() {
+		t.Fatal("expected a conflict for a domain shared by two tenants")
+	}
+	if got := domainTenant(t, ops, "d0"); got != "" {
+		t.Errorf("conflicting domain was modified to %q; it must be left untouched", got)
+	}
+	c := res.Conflicts[0]
+	if c.Domain != "shared.test" || len(c.Tenants) != 2 {
+		t.Errorf("conflict = %+v, want shared.test across two tenants", c)
+	}
+	for _, want := range []string{"alpha", "beta"} {
+		if !strings.Contains(c.Detail, want) {
+			t.Errorf("conflict detail %q does not name tenant %q", c.Detail, want)
+		}
+	}
+}
+
+func TestReconcileReportsAccountOnAnotherTenantsDomain(t *testing.T) {
+	ops := plan(
+		tenantOp("t0", "alpha"),
+		tenantOp("t1", "beta"),
+		domainOp("d0", "owned.test", "#t0"),
+		accountOp("a0", "u2", "#d0", "#t1"),
+	)
+	res := ReconcileDomainTenants(ops)
+	if res.OK() {
+		t.Fatal("expected a conflict when an account's tenant differs from its domain's")
+	}
+	if got := domainTenant(t, ops, "d0"); got != "#t0" {
+		t.Errorf("domain ownership changed to %q; declared ownership must not be overwritten", got)
+	}
+}
+
+func TestReconcileIsNoOpWithoutTenants(t *testing.T) {
+	ops := plan(
+		domainOp("d0", "example.test", ""),
+		accountOp("a0", "alice", "#d0", ""),
+	)
+	res := ReconcileDomainTenants(ops)
+	if !res.OK() || len(res.Adoptions) != 0 {
+		t.Fatalf("single-tenant plan should be untouched, got %s", res.String())
+	}
+	if res.String() != "no domain/tenant mismatches" {
+		t.Errorf("summary = %q", res.String())
+	}
+}
+
+func TestReconcileFileRoundTripsAndRewritesOnlyWhenChanged(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "export.json")
+
+	ops := plan(
+		tenantOp("t0", "acme"),
+		domainOp("d0", "acme-corp.test", "#t0"),
+		domainOp("d1", "inferred.test", ""),
+		accountOp("a0", "bob", "#d0", "#t0", "#d1"),
+	)
+	if err := WritePlan(path, ops); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	res, err := ReconcileDomainTenantsFile(path)
+	if err != nil {
+		t.Fatalf("reconcile file: %v", err)
+	}
+	if len(res.Adoptions) != 1 {
+		t.Fatalf("adoptions = %+v", res.Adoptions)
+	}
+
+	reread, err := ReadPlan(path)
+	if err != nil {
+		t.Fatalf("reread: %v", err)
+	}
+	if got := domainTenant(t, reread, "d1"); got != "#t0" {
+		t.Errorf("rewritten plan has tenant %q, want %q", got, "#t0")
+	}
+
+	// Second pass must be a no-op: the plan is already consistent.
+	before, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res2, err := ReconcileDomainTenantsFile(path)
+	if err != nil {
+		t.Fatalf("second reconcile: %v", err)
+	}
+	if len(res2.Adoptions) != 0 {
+		t.Errorf("second pass adopted %+v, want none", res2.Adoptions)
+	}
+	after, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(before) != string(after) {
+		t.Error("second pass rewrote an already-consistent plan")
+	}
+}
+
+func TestReconcileFileRefusesConflictingPlan(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "export.json")
+	ops := plan(
+		tenantOp("t0", "alpha"),
+		tenantOp("t1", "beta"),
+		domainOp("d0", "shared.test", ""),
+		accountOp("a0", "u1", "#d0", "#t0"),
+		accountOp("a1", "u2", "#d0", "#t1"),
+	)
+	if err := WritePlan(path, ops); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	before, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := ReconcileDomainTenantsFile(path); err == nil {
+		t.Fatal("expected an error for an unrepresentable plan")
+	}
+	after, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(before) != string(after) {
+		t.Error("a plan that could not be repaired was rewritten anyway")
+	}
+}
+
+func TestReadPlanHandlesLongLines(t *testing.T) {
+	// Plans embed certificates and DKIM private keys inline, which blow
+	// past bufio.Scanner's default 64KiB line limit.
+	dir := t.TempDir()
+	path := filepath.Join(dir, "big.json")
+	huge := strings.Repeat("x", 200<<10)
+	ops := plan(PlanOp{"@type": "update", "object": "Certificate",
+		"value": map[string]any{"c0": map[string]any{"cert": huge}}})
+	if err := WritePlan(path, ops); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	got, err := ReadPlan(path)
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if len(got) != 1 || got[0].createBodies()["c0"]["cert"] != huge {
+		t.Error("long line did not round-trip")
+	}
+}
+
+// A real export.json mixes `create` operations (client-id keyed) with
+// `update` operations carrying a flat object. An earlier version of this
+// code modelled every operation the first way and failed to parse a real
+// plan at the first `update` line - found by running it against actual
+// migrate_v016.py output rather than hand-built fixtures.
+func TestReconcileHandlesRealExportShapes(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "export.json")
+	raw := strings.Join([]string{
+		`{"@type":"create","object":"Tenant","value":{"create-0":{"name":"acme","quotas":{}}}}`,
+		`{"@type":"create","object":"Domain","value":{"create-1":{"name":"acme-corp.test","memberTenantId":"#create-0"},"create-2":{"name":"inferred.test"}}}`,
+		`{"@type":"create","object":"Account","value":{"restore-5":{"@type":"User","name":"bob","domainId":"#create-1","memberTenantId":"#create-0","aliases":{"0":{"name":"bob","domainId":"#create-2"}},"quotas":{"maxDiskQuota":10737418240}}}}`,
+		`{"@type":"update","object":"SystemSettings","value":{"defaultDomainId":"#create-1","defaultHostname":"mail.acme-corp.test"}}`,
+		`{"@type":"update","object":"BlobStore","value":{"@type":"Default"}}`,
+		"",
+	}, "\n")
+	if err := os.WriteFile(path, []byte(raw), 0o640); err != nil {
+		t.Fatal(err)
+	}
+
+	res, err := ReconcileDomainTenantsFile(path)
+	if err != nil {
+		t.Fatalf("reconcile: %v", err)
+	}
+	if len(res.Adoptions) != 1 || res.Adoptions[0].Domain != "inferred.test" {
+		t.Fatalf("adoptions = %+v", res.Adoptions)
+	}
+
+	out, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// A large quota must survive as an integer: decoding through float64
+	// would rewrite it as 1.073741824e+10 and hand the server a different
+	// document than the conversion produced.
+	if !strings.Contains(string(out), "10737418240") {
+		t.Errorf("quota was reformatted; plan now reads:\n%s", out)
+	}
+	// The update operations must come back untouched.
+	for _, want := range []string{`"defaultHostname":"mail.acme-corp.test"`, `"object":"BlobStore"`} {
+		if !strings.Contains(strings.ReplaceAll(string(out), ", ", ","), want) {
+			t.Errorf("rewritten plan lost %s:\n%s", want, out)
+		}
+	}
+
+	reread, err := ReadPlan(path)
+	if err != nil {
+		t.Fatalf("reread: %v", err)
+	}
+	if len(reread) != 5 {
+		t.Errorf("operation count = %d, want 5", len(reread))
+	}
+	if got := domainTenant(t, reread, "create-2"); got != "#create-0" {
+		t.Errorf("inferred domain tenant = %q, want %q", got, "#create-0")
+	}
+}

internal/preflight/checks.go

@@ -328,24 +328,46 @@ func (c *Checker) Run(ctx context.Context, store *checkpoint.Store, rs *checkpoi
 				BaseURL: c.opts.AdminURL, Username: c.opts.AdminUser,
 				Password: c.opts.AdminPassword, HTTPClient: c.opts.HTTPClient,
 			}
-			tenants, err := client.TenantNames(ctx)
+			layout, err := client.FetchTenantLayout(ctx)
 			if err != nil {
 				return CheckResult{
 					Status: StatusWarn,
-					Detail: fmt.Sprintf("couldn't determine whether this instance is multi-tenant: %v - if it is, the migration will fail after the service is stopped", err),
+					Detail: fmt.Sprintf("couldn't map this instance's tenants: %v - if it is multi-tenant, "+
+						"a domain/tenant mismatch would only surface during the conversion", err),
 				}, ""
 			}
-			if len(tenants) > 0 {
+			if len(layout.Tenants) == 0 {
+				return CheckResult{Status: StatusOK, Detail: "single-tenant: no tenant principals, so no account can mismatch its domain"}, ""
+			}
+
+			plan := layout.Analyze()
+			if len(plan.Problems) > 0 {
+				details := make([]string, 0, len(plan.Problems))
+				for _, p := range plan.Problems {
+					details = append(details, fmt.Sprintf("%s: %s", p.Domain, p.Detail))
+				}
 				return CheckResult{
 					Status: StatusFail,
-					Detail: fmt.Sprintf("this instance has %d tenant(s) (%s), and Stalwart's migrate_v016.py does not carry tenant "+
-						"membership onto accounts: it creates the Tenant and Domains, then every Account with a null tenantId, and the "+
-						"apply is rejected with invalidForeignKey - during recovery-mode migration, with the service already stopped. "+
-						"Migrate a multi-tenant install by hand, or wait for a converter that handles it",
-						len(tenants), strings.Join(tenants, ", ")),
+					Detail: fmt.Sprintf("this instance has %d tenant(s) (%s) in an arrangement v0.16 cannot represent - %s. "+
+						"Resolve this in v0.15 first: give each tenant its own domains, or move the accounts into one tenant",
+						len(layout.Tenants), strings.Join(layout.Tenants, ", "), strings.Join(details, "; ")),
 				}, ""
 			}
-			return CheckResult{Status: StatusOK, Detail: "single-tenant: no tenant principals, so the conversion's null tenantId is harmless"}, ""
+			if len(plan.Adoptions) > 0 {
+				return CheckResult{
+					Status: StatusWarn,
+					Detail: fmt.Sprintf("this instance has %d tenant(s) (%s); %d domain(s) (%s) have no tenant of their own but are "+
+						"used only by accounts of a single tenant. v0.16 requires them to match, so the conversion will assign each "+
+						"domain to that tenant - the accounts migrate intact, but those domains become tenant-owned",
+						len(layout.Tenants), strings.Join(layout.Tenants, ", "),
+						len(plan.Adoptions), strings.Join(plan.Adoptions, ", ")),
+				}, ""
+			}
+			return CheckResult{
+				Status: StatusOK,
+				Detail: fmt.Sprintf("%d tenant(s) (%s), and every account already sits on a domain of its own tenant",
+					len(layout.Tenants), strings.Join(layout.Tenants, ", ")),
+			}, ""
 		}); err != nil {
 			return report, err
 		}

internal/stalwartapi/principal.go

@@ -209,12 +209,15 @@ func accountKey(p restPrincipal) string {
 
 // TenantNames returns the tenant principals on a v0.15.x instance.
 //
-// Multi-tenancy has to be detected before a migration starts, because
-// Stalwart's own converter does not survive it: it emits the Tenant and the
-// Domains correctly and then every Account with `tenantId: null`, so the
-// account references a tenant-owned domain while belonging to no tenant and
-// the apply is rejected with `invalidForeignKey`. Observed on a real
-// migration, at the point where the mail server was already stopped.
+// Multi-tenancy has to be established before a migration starts. v0.16
+// requires a tenant-scoped account to sit on a domain owned by that same
+// tenant; v0.15 did not, and Stalwart's converter carries the two facts
+// over independently, so an install that is valid today can convert into a
+// plan the new server rejects with `invalidForeignKey` on the Domain
+// reference - during the recovery-mode apply, with the mail server already
+// stopped and the store already at schema v6. See FetchTenantLayout, which
+// builds on this to predict that outcome, and applyplan.ReconcileDomainTenants,
+// which repairs the plan.
 func (c *Client) TenantNames(ctx context.Context) ([]string, error) {
 	tenants, err := c.restPrincipals(ctx, "tenant")
 	if err != nil {

internal/stalwartapi/tenantlayout.go

@@ -0,0 +1,291 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package stalwartapi
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"sort"
+	"strings"
+)
+
+// TenantLayout is who-belongs-to-which-tenant on a v0.15.x instance, in
+// enough detail to predict whether the v0.16 conversion will produce a plan
+// the new server accepts.
+//
+// v0.16 requires a tenant-scoped account to sit on a domain owned by that
+// same tenant, for its primary domain and for every alias. v0.15 imposed no
+// such rule, so an install can be perfectly valid today and unconvertible
+// tomorrow. Establishing that here - while the server is still running and
+// nothing has been stopped - is the whole point: the alternative is finding
+// out during the recovery-mode apply, which is the one moment in the run
+// with no way forward and no way back.
+type TenantLayout struct {
+	// Tenants is every tenant principal's name.
+	Tenants []string
+	// DomainTenant maps a declared domain name to its tenant name. Domains
+	// that exist only inside an email address are absent, which mirrors the
+	// converter: it infers those domains and gives them no tenant.
+	DomainTenant map[string]string
+	// Principals is every account, group and mailing list, with the tenant
+	// it belongs to and the domains it touches.
+	Principals []PrincipalTenancy
+}
+
+// PrincipalTenancy is one account's tenant and the domains it references.
+type PrincipalTenancy struct {
+	Name    string
+	Type    string
+	Tenant  string   // "" for a global principal
+	Domains []string // primary plus alias domains, lowercased
+}
+
+// flexString reads a field that a v0.15 instance may return either as a
+// plain string or wrapped - migrate_v016.py's own pv_string tolerates the
+// same shapes, and a preflight that only understood one of them would
+// silently see every account as global.
+func flexString(raw json.RawMessage) string {
+	if len(raw) == 0 {
+		return ""
+	}
+	var s string
+	if err := json.Unmarshal(raw, &s); err == nil {
+		return s
+	}
+	var obj map[string]json.RawMessage
+	if err := json.Unmarshal(raw, &obj); err == nil {
+		for _, key := range []string{"string", "name", "id"} {
+			if v, ok := obj[key]; ok {
+				var inner string
+				if json.Unmarshal(v, &inner) == nil && inner != "" {
+					return inner
+				}
+			}
+		}
+		return ""
+	}
+	var list []json.RawMessage
+	if err := json.Unmarshal(raw, &list); err == nil && len(list) > 0 {
+		return flexString(list[0])
+	}
+	return ""
+}
+
+// detailedPrincipal is the per-principal view, which carries the tenant the
+// paginated list does not reliably include.
+type detailedPrincipal struct {
+	Type   json.RawMessage `json:"type"`
+	Name   json.RawMessage `json:"name"`
+	Tenant json.RawMessage `json:"tenant"`
+	Emails []string        `json:"emails"`
+}
+
+// principalDetail fetches one principal by name. The response may or may
+// not be wrapped in a "data" envelope depending on the point release, so
+// both are accepted.
+func (c *Client) principalDetail(ctx context.Context, name string) (detailedPrincipal, error) {
+	var out detailedPrincipal
+	endpoint := strings.TrimRight(c.BaseURL, "/") + "/api/principal/" + url.PathEscape(name)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, endpoint, nil)
+	if err != nil {
+		return out, err
+	}
+	req.SetBasicAuth(c.Username, c.Password)
+	resp, err := c.httpClient().Do(req)
+	if err != nil {
+		return out, fmt.Errorf("stalwartapi: fetch principal %q: %w", name, err)
+	}
+	body, readErr := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return out, fmt.Errorf("stalwartapi: GET %s returned %s", endpoint, resp.Status)
+	}
+	if readErr != nil {
+		return out, fmt.Errorf("stalwartapi: read principal %q: %w", name, readErr)
+	}
+
+	var envelope struct {
+		Data json.RawMessage `json:"data"`
+	}
+	payload := body
+	if json.Unmarshal(body, &envelope) == nil && len(envelope.Data) > 0 {
+		payload = envelope.Data
+	}
+	if err := json.Unmarshal(payload, &out); err != nil {
+		return out, fmt.Errorf("stalwartapi: parse principal %q: %w", name, err)
+	}
+	return out, nil
+}
+
+// domainsOf returns every domain a principal touches: the domain in its
+// name, if it is an address, plus one per email.
+func domainsOf(name string, emails []string) []string {
+	seen := map[string]bool{}
+	var out []string
+	add := func(addr string) {
+		at := strings.LastIndex(addr, "@")
+		if at < 0 || at == len(addr)-1 {
+			return
+		}
+		d := strings.ToLower(strings.TrimSpace(addr[at+1:]))
+		if d == "" || seen[d] {
+			return
+		}
+		seen[d] = true
+		out = append(out, d)
+	}
+	add(name)
+	for _, e := range emails {
+		add(e)
+	}
+	sort.Strings(out)
+	return out
+}
+
+// FetchTenantLayout builds a TenantLayout from a v0.15.x instance.
+//
+// It returns an empty layout, and no error, on a single-tenant install:
+// there is nothing that can mismatch, and that is the common case.
+func (c *Client) FetchTenantLayout(ctx context.Context) (*TenantLayout, error) {
+	tenants, err := c.TenantNames(ctx)
+	if err != nil {
+		return nil, err
+	}
+	layout := &TenantLayout{Tenants: tenants, DomainTenant: map[string]string{}}
+	if len(tenants) == 0 {
+		return layout, nil
+	}
+
+	domains, err := c.restPrincipals(ctx, "domain")
+	if err != nil {
+		return nil, err
+	}
+	for _, d := range domains {
+		if d.Name == "" {
+			continue
+		}
+		detail, err := c.principalDetail(ctx, d.Name)
+		if err != nil {
+			return nil, err
+		}
+		layout.DomainTenant[strings.ToLower(d.Name)] = flexString(detail.Tenant)
+	}
+
+	for _, pType := range []string{"individual", "group", "list"} {
+		principals, err := c.restPrincipals(ctx, pType)
+		if err != nil {
+			return nil, err
+		}
+		for _, p := range principals {
+			if p.Name == "" {
+				continue
+			}
+			detail, err := c.principalDetail(ctx, p.Name)
+			if err != nil {
+				return nil, err
+			}
+			emails := p.Emails
+			if len(detail.Emails) > 0 {
+				emails = detail.Emails
+			}
+			layout.Principals = append(layout.Principals, PrincipalTenancy{
+				Name:    p.Name,
+				Type:    pType,
+				Tenant:  flexString(detail.Tenant),
+				Domains: domainsOf(p.Name, emails),
+			})
+		}
+	}
+	return layout, nil
+}
+
+// TenancyProblem is one domain whose users cannot all be represented in
+// v0.16.
+type TenancyProblem struct {
+	Domain string
+	Detail string
+}
+
+// TenancyPlan is what the conversion will have to do to this layout.
+type TenancyPlan struct {
+	// Adoptions are domains with no tenant of their own that will be given
+	// one, because the only tenant-scoped accounts using them agree.
+	Adoptions []string
+	// Problems are the domains v0.16 cannot represent at all.
+	Problems []TenancyProblem
+}
+
+// Analyze predicts whether this layout converts cleanly, applying exactly
+// the rule the server enforces and the same repair applyplan performs.
+func (l *TenantLayout) Analyze() TenancyPlan {
+	var plan TenancyPlan
+	if len(l.Tenants) == 0 {
+		return plan
+	}
+
+	// domain -> tenant -> an example principal requiring it
+	required := map[string]map[string]string{}
+	for _, p := range l.Principals {
+		if p.Tenant == "" {
+			continue // a global principal constrains nothing
+		}
+		for _, d := range p.Domains {
+			if required[d] == nil {
+				required[d] = map[string]string{}
+			}
+			if _, ok := required[d][p.Tenant]; !ok {
+				required[d][p.Tenant] = p.Name
+			}
+		}
+	}
+
+	domains := make([]string, 0, len(required))
+	for d := range required {
+		domains = append(domains, d)
+	}
+	sort.Strings(domains)
+
+	for _, d := range domains {
+		wanted := required[d]
+		names := make([]string, 0, len(wanted))
+		for t := range wanted {
+			names = append(names, t)
+		}
+		sort.Strings(names)
+
+		if len(names) > 1 {
+			var parts []string
+			for _, t := range names {
+				parts = append(parts, fmt.Sprintf("%s (e.g. %s)", t, wanted[t]))
+			}
+			plan.Problems = append(plan.Problems, TenancyProblem{
+				Domain: d,
+				Detail: fmt.Sprintf("used by accounts from more than one tenant: %s - v0.16 allows a domain "+
+					"to belong to at most one tenant, and requires each tenant-scoped account to sit on its "+
+					"own tenant's domain", strings.Join(parts, ", ")),
+			})
+			continue
+		}
+
+		want := names[0]
+		switch declared, isDeclared := l.DomainTenant[d]; {
+		case !isDeclared || declared == "":
+			// Either inferred from an address, or declared with no tenant.
+			// Either way the conversion gives it no tenant and the account
+			// is rejected - this is the case that broke production.
+			plan.Adoptions = append(plan.Adoptions, d)
+		case declared != want:
+			plan.Problems = append(plan.Problems, TenancyProblem{
+				Domain: d,
+				Detail: fmt.Sprintf("belongs to tenant %s, but %s (tenant %s) uses it - v0.16 rejects an "+
+					"account whose tenant differs from its domain's", declared, wanted[want], want),
+			})
+		}
+	}
+	return plan
+}

internal/stalwartapi/tenantlayout_test.go

@@ -0,0 +1,138 @@
+// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package stalwartapi
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+func TestFlexStringAcceptsEveryShapeAV015InstanceReturns(t *testing.T) {
+	cases := map[string]string{
+		`"acme"`:              "acme",
+		`{"string":"acme"}`:   "acme",
+		`{"name":"acme"}`:     "acme",
+		`["acme","other"]`:    "acme",
+		`null`:                "",
+		`{}`:                  "",
+		`[]`:                  "",
+		`{"other":"ignored"}`: "",
+	}
+	for raw, want := range cases {
+		if got := flexString(json.RawMessage(raw)); got != want {
+			t.Errorf("flexString(%s) = %q, want %q", raw, got, want)
+		}
+	}
+	if got := flexString(nil); got != "" {
+		t.Errorf("flexString(nil) = %q", got)
+	}
+}
+
+func TestDomainsOfCollectsNameAndAliasDomains(t *testing.T) {
+	got := domainsOf("[email protected]", []string{"[email protected]", "[email protected]", "malformed", "trailing@"})
+	want := []string{"alias.net", "example.com"}
+	if strings.Join(got, ",") != strings.Join(want, ",") {
+		t.Errorf("domainsOf = %v, want %v", got, want)
+	}
+}
+
+func TestAnalyzeIsEmptyForSingleTenant(t *testing.T) {
+	l := &TenantLayout{DomainTenant: map[string]string{}}
+	plan := l.Analyze()
+	if len(plan.Adoptions) != 0 || len(plan.Problems) != 0 {
+		t.Errorf("single-tenant layout produced %+v", plan)
+	}
+}
+
+func TestAnalyzeFlagsUndeclaredDomainForAdoption(t *testing.T) {
+	// The production failure: a tenant account with an address on a domain
+	// that was never declared, so the conversion gives it no tenant.
+	l := &TenantLayout{
+		Tenants:      []string{"acme"},
+		DomainTenant: map[string]string{"acme-corp.test": "acme"},
+		Principals: []PrincipalTenancy{
+			{Name: "bob", Tenant: "acme", Domains: []string{"acme-corp.test", "inferred.test"}},
+		},
+	}
+	plan := l.Analyze()
+	if len(plan.Problems) != 0 {
+		t.Fatalf("unexpected problems: %+v", plan.Problems)
+	}
+	if len(plan.Adoptions) != 1 || plan.Adoptions[0] != "inferred.test" {
+		t.Errorf("adoptions = %v, want [inferred.test]", plan.Adoptions)
+	}
+}
+
+func TestAnalyzeIgnoresGlobalAccountsOnTenantDomains(t *testing.T) {
+	// Verified against 0.16.14: this direction applies cleanly, so it must
+	// not be reported as anything.
+	l := &TenantLayout{
+		Tenants:      []string{"acme"},
+		DomainTenant: map[string]string{"acme-corp.test": "acme"},
+		Principals: []PrincipalTenancy{
+			{Name: "admin", Tenant: "", Domains: []string{"acme-corp.test"}},
+		},
+	}
+	plan := l.Analyze()
+	if len(plan.Adoptions) != 0 || len(plan.Problems) != 0 {
+		t.Errorf("global account on a tenant domain produced %+v", plan)
+	}
+}
+
+func TestAnalyzeReportsDomainSharedByTwoTenants(t *testing.T) {
+	l := &TenantLayout{
+		Tenants:      []string{"alpha", "beta"},
+		DomainTenant: map[string]string{},
+		Principals: []PrincipalTenancy{
+			{Name: "u1", Tenant: "alpha", Domains: []string{"shared.test"}},
+			{Name: "u2", Tenant: "beta", Domains: []string{"shared.test"}},
+		},
+	}
+	plan := l.Analyze()
+	if len(plan.Problems) != 1 {
+		t.Fatalf("problems = %+v, want one", plan.Problems)
+	}
+	if len(plan.Adoptions) != 0 {
+		t.Errorf("a conflicting domain was also queued for adoption: %v", plan.Adoptions)
+	}
+	for _, want := range []string{"alpha", "beta", "u1", "u2"} {
+		if !strings.Contains(plan.Problems[0].Detail, want) {
+			t.Errorf("detail %q omits %q", plan.Problems[0].Detail, want)
+		}
+	}
+}
+
+func TestAnalyzeReportsAccountOnAnotherTenantsDomain(t *testing.T) {
+	l := &TenantLayout{
+		Tenants:      []string{"alpha", "beta"},
+		DomainTenant: map[string]string{"owned.test": "alpha"},
+		Principals: []PrincipalTenancy{
+			{Name: "u2", Tenant: "beta", Domains: []string{"owned.test"}},
+		},
+	}
+	plan := l.Analyze()
+	if len(plan.Problems) != 1 || plan.Problems[0].Domain != "owned.test" {
+		t.Fatalf("problems = %+v", plan.Problems)
+	}
+}
+
+func TestAnalyzeAcceptsAConsistentMultiTenantLayout(t *testing.T) {
+	l := &TenantLayout{
+		Tenants: []string{"alpha", "beta"},
+		DomainTenant: map[string]string{
+			"alpha.test": "alpha",
+			"beta.test":  "beta",
+		},
+		Principals: []PrincipalTenancy{
+			{Name: "u1", Tenant: "alpha", Domains: []string{"alpha.test"}},
+			{Name: "u2", Tenant: "beta", Domains: []string{"beta.test"}},
+			{Name: "admin", Tenant: "", Domains: []string{"alpha.test"}},
+		},
+	}
+	plan := l.Analyze()
+	if len(plan.Adoptions) != 0 || len(plan.Problems) != 0 {
+		t.Errorf("a already-consistent multi-tenant layout produced %+v", plan)
+	}
+}