fix!: order privilege events by sequence number and merge snapshots by position (#333)

## Why
The privilege cache could keep a grant authz-service had revoked:
- **Unordered keys:** each routing key has its own transient queue, so a late `Privilege.Added`/`User.Added` resurrected a revoked grant.
- **Startup gap:** services fetched `/authz` before binding their queues, so revocations published in between were lost until restart.

Design: ADR-0015 (docs PR, Proposed).

## What
- `Process` orders events by authz-service's global `sequenceNo` per (email, company). All four events come from the Company aggregate, so seq order equals commit order. An event only overrides older facts, and `User.Removed` stamps every privilege.
- Events without a sequence number fail closed: additions are dropped, and removals hold until the next snapshot. Negative or huge sequence numbers are dropped.
- `Fetch` checks the status and retries 503 (60×1 s, 30 s HTTP timeout). It reads `X-Authz-Sequence`, merges the snapshot as facts at that position, and raises a floor; snapshots older than the floor are ignored. A missing header merges at 0 with a warning (rollout window only).
- `CompaniesByUser` returns `[]`, and unknown privileges create no state. CLAUDE.md is rewritten.

**BREAKING:** `Process` without `SequenceNo` no longer grants, so service tests must set it. Services must call `Fetch()` after `conn.Start`.

## Verification
- `go test -race`: 98.3% coverage, including table-driven reorderings, snapshot-merge cases and a revocation-during-Fetch race test.
- 26 mutants on the ordering, merge and retry checks: all killed (each compiled and produced `--- FAIL`).
- prek passes.

**Expert review:** two rounds. Round 1: Security, Go Backend, Event Sourcing and Database experts reviewed both diffs. Round 2: Security and Event Sourcing re-reviewed the fixes. A final Event Sourcing review covered the committed-events wrapper. Fixed from the reviews: older snapshots merged after newer ones (floor check); a lagging read view serving snapshots that miss revocations (503 plus catch-up); a reset's TRUNCATE emptying a REPEATABLE READ snapshot (LOCK TABLE privileges, verified on PostgreSQL); seq-0 removals undone by older additions (pending stamp); late-committing events skipped by read view backfills (CommittedEventStore with LOCK TABLE events IN SHARE MODE, verified on PostgreSQL 18); an unbounded lock wait (lock_timeout plus retries); catch-up firing on ordinary lag (5 s stall, 10 s cooldown, 10 min deadline); plus smaller items (unknown privileges, invalid seqs, `require` in a goroutine, wrapped errors, `[]` not nil). **Deliberately deferred (tracked in Ambix):** stored-but-unpublished revocations (user decision: authz-service outbox); the readview library's commit-order gap for other services (upstream); removing the missing-header fallback and alerting on /authz 503s; moving Fetch after conn.Start in the 13 consumers (separate bump PRs).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01DVGsVQ8AMFR4NZoxyCoEqS
Reviewed-on: https://gitea.unbound.se/shiny/authz_client/pulls/333

client_test.go

@@ -3,11 +3,14 @@ package client
 import (
 	"context"
 	"fmt"
+	"math"
 	"net/http"
 	"net/http/httptest"
 	"sort"
 	"sync"
+	"sync/atomic"
 	"testing"
+	"time"
 
 	goamqp "codeberg.org/messaging/go-messaging-amqp"
 	spec "codeberg.org/messaging/messaging"
@@ -27,9 +30,10 @@ func TestPrivilegeHandler_Process_PrivilegeRemoved(t *testing.T) {
 	handler := New(WithBaseURL("base"))
 
 	err := handler.Process(&PrivilegeAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
-		Privilege: PrivilegeAdmin,
+		SequenceNo: 1,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
+		Privilege:  PrivilegeAdmin,
 	})
 	assert.NoError(t, err)
 
@@ -40,9 +44,10 @@ func TestPrivilegeHandler_Process_PrivilegeRemoved(t *testing.T) {
 	assert.Equal(t, []string{"abc-123"}, companies)
 
 	err = handler.Process(&PrivilegeRemoved{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
-		Privilege: PrivilegeAdmin,
+		SequenceNo: 2,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
+		Privilege:  PrivilegeAdmin,
 	})
 	assert.NoError(t, err)
 
@@ -57,14 +62,16 @@ func TestPrivilegeHandler_Process_UserAdded_And_UserRemoved(t *testing.T) {
 	handler := New(WithBaseURL("base"))
 
 	err := handler.Process(&UserAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
+		SequenceNo: 1,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
 	})
 	assert.NoError(t, err)
 
 	err = handler.Process(&UserAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-456",
+		SequenceNo: 2,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-456",
 	})
 	assert.NoError(t, err)
 
@@ -75,14 +82,16 @@ func TestPrivilegeHandler_Process_UserAdded_And_UserRemoved(t *testing.T) {
 	assert.Equal(t, []string{"abc-123", "abc-456"}, companies)
 
 	err = handler.Process(&UserRemoved{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
+		SequenceNo: 3,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
 	})
 	assert.NoError(t, err)
 
 	err = handler.Process(&UserRemoved{
-		Email:     "jim@example.org",
-		CompanyID: "abc-456",
+		SequenceNo: 4,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-456",
 	})
 	assert.NoError(t, err)
 
@@ -96,8 +105,8 @@ func TestPrivilegeHandler_Process_UserAdded_Keeps_Existing_Privileges(t *testing
 	// Each routing key has its own transient queue, so Privilege.Added can be
 	// processed before the User.Added published before it. The privilege must
 	// survive either order: nothing re-reads it until the next Fetch at start.
-	userAdded := &UserAdded{Email: "jim@example.org", CompanyID: "abc-123"}
-	privilegeAdded := &PrivilegeAdded{Email: "jim@example.org", CompanyID: "abc-123", Privilege: PrivilegeAdmin}
+	userAdded := &UserAdded{SequenceNo: 1, Email: "jim@example.org", CompanyID: "abc-123"}
+	privilegeAdded := &PrivilegeAdded{SequenceNo: 2, Email: "jim@example.org", CompanyID: "abc-123", Privilege: PrivilegeAdmin}
 	for name, order := range map[string][]any{
 		"user added first":      {userAdded, privilegeAdded},
 		"privilege added first": {privilegeAdded, userAdded},
@@ -118,14 +127,15 @@ func TestPrivilegeHandler_Process_UserAdded_Keeps_Existing_Privileges(t *testing
 func TestPrivilegeHandler_Process_UserAdded_After_UserRemoved_Starts_Empty(t *testing.T) {
 	handler := New(WithBaseURL("base"))
 
-	assert.NoError(t, handler.Process(&UserAdded{Email: "jim@example.org", CompanyID: "abc-123"}))
+	assert.NoError(t, handler.Process(&UserAdded{SequenceNo: 1, Email: "jim@example.org", CompanyID: "abc-123"}))
 	assert.NoError(t, handler.Process(&PrivilegeAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
-		Privilege: PrivilegeAdmin,
+		SequenceNo: 2,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
+		Privilege:  PrivilegeAdmin,
 	}))
-	assert.NoError(t, handler.Process(&UserRemoved{Email: "jim@example.org", CompanyID: "abc-123"}))
-	assert.NoError(t, handler.Process(&UserAdded{Email: "jim@example.org", CompanyID: "abc-123"}))
+	assert.NoError(t, handler.Process(&UserRemoved{SequenceNo: 3, Email: "jim@example.org", CompanyID: "abc-123"}))
+	assert.NoError(t, handler.Process(&UserAdded{SequenceNo: 4, Email: "jim@example.org", CompanyID: "abc-123"}))
 
 	// Membership is back, without the privileges the removal took away.
 	assert.Equal(t, []string{"abc-123"}, handler.CompaniesByUser("jim@example.org", func(CompanyPrivileges) bool {
@@ -150,8 +160,9 @@ func TestPrivilegeHandler_GetCompanies_No_Companies_Found(t *testing.T) {
 	handler := New(WithBaseURL("base"))
 
 	err := handler.Process(&UserAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
+		SequenceNo: 1,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
 	})
 	assert.NoError(t, err)
 
@@ -168,8 +179,9 @@ func TestPrivilegeHandler_GetCompanies_No_Companies_Found(t *testing.T) {
 	assert.Equal(t, []string{"abc-123"}, companies)
 
 	err = handler.Process(&UserRemoved{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
+		SequenceNo: 2,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
 	})
 	assert.NoError(t, err)
 
@@ -183,9 +195,10 @@ func TestPrivilegeHandler_GetCompanies_Company_With_Company_Access_Found(t *test
 	handler := New(WithBaseURL("base"))
 
 	err := handler.Process(&PrivilegeAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
-		Privilege: PrivilegeCompany,
+		SequenceNo: 1,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
+		Privilege:  PrivilegeCompany,
 	})
 	assert.NoError(t, err)
 
@@ -200,9 +213,10 @@ func TestPrivilegeHandler_GetCompanies_Company_With_Admin_Access_Found(t *testin
 	handler := New(WithBaseURL("base"))
 
 	err := handler.Process(&PrivilegeAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
-		Privilege: PrivilegeConsumer,
+		SequenceNo: 1,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
+		Privilege:  PrivilegeConsumer,
 	})
 	assert.NoError(t, err)
 
@@ -227,9 +241,10 @@ func TestPrivilegeHandler_IsAllowed_Return_True_If_Privilege_Exists(t *testing.T
 	handler := New(WithBaseURL("base"))
 
 	_ = handler.Process(&PrivilegeAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
-		Privilege: PrivilegeTime,
+		SequenceNo: 1,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
+		Privilege:  PrivilegeTime,
 	})
 
 	result := handler.IsAllowed("jim@example.org", "abc-123", func(privileges CompanyPrivileges) bool {
@@ -239,9 +254,10 @@ func TestPrivilegeHandler_IsAllowed_Return_True_If_Privilege_Exists(t *testing.T
 	assert.True(t, result)
 
 	_ = handler.Process(&PrivilegeAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
-		Privilege: PrivilegeInvoicing,
+		SequenceNo: 2,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
+		Privilege:  PrivilegeInvoicing,
 	})
 
 	result = handler.IsAllowed("jim@example.org", "abc-123", func(privileges CompanyPrivileges) bool {
@@ -251,9 +267,10 @@ func TestPrivilegeHandler_IsAllowed_Return_True_If_Privilege_Exists(t *testing.T
 	assert.True(t, result)
 
 	_ = handler.Process(&PrivilegeAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
-		Privilege: PrivilegeAccounting,
+		SequenceNo: 3,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
+		Privilege:  PrivilegeAccounting,
 	})
 
 	result = handler.IsAllowed("jim@example.org", "abc-123", func(privileges CompanyPrivileges) bool {
@@ -263,9 +280,10 @@ func TestPrivilegeHandler_IsAllowed_Return_True_If_Privilege_Exists(t *testing.T
 	assert.True(t, result)
 
 	_ = handler.Process(&PrivilegeAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
-		Privilege: PrivilegeSupplier,
+		SequenceNo: 4,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
+		Privilege:  PrivilegeSupplier,
 	})
 
 	result = handler.IsAllowed("jim@example.org", "abc-123", func(privileges CompanyPrivileges) bool {
@@ -275,9 +293,10 @@ func TestPrivilegeHandler_IsAllowed_Return_True_If_Privilege_Exists(t *testing.T
 	assert.True(t, result)
 
 	_ = handler.Process(&PrivilegeAdded{
-		Email:     "jim@example.org",
-		CompanyID: "abc-123",
-		Privilege: PrivilegeSalary,
+		SequenceNo: 5,
+		Email:      "jim@example.org",
+		CompanyID:  "abc-123",
+		Privilege:  PrivilegeSalary,
 	})
 
 	result = handler.IsAllowed("jim@example.org", "abc-123", func(privileges CompanyPrivileges) bool {
@@ -386,7 +405,7 @@ func TestPrivilegeHandler_Fetch_Valid(t *testing.T) {
 
 	err := handler.Fetch()
 	assert.NoError(t, err)
-	expectedPrivileges := map[string]map[string]*CompanyPrivileges{
+	expectedPrivileges := map[string]map[string]CompanyPrivileges{
 		"jim@example.org": {
 			"00010203-0405-4607-8809-0a0b0c0d0e0f": {
 				Admin:      false,
@@ -400,7 +419,7 @@ func TestPrivilegeHandler_Fetch_Valid(t *testing.T) {
 			},
 		},
 	}
-	assert.Equal(t, expectedPrivileges, handler.privileges)
+	assert.Equal(t, expectedPrivileges, privilegesOf(handler))
 }
 
 func TestPrivilegeHandler_Fetch_Concurrent_Fetches(t *testing.T) {
@@ -450,7 +469,7 @@ func TestPrivilegeHandler_Fetch_Concurrent_Fetches(t *testing.T) {
 	}
 
 	// Verify privileges were set correctly
-	expectedPrivileges := map[string]map[string]*CompanyPrivileges{
+	expectedPrivileges := map[string]map[string]CompanyPrivileges{
 		"jim@example.org": {
 			"00010203-0405-4607-8809-0a0b0c0d0e0f": {
 				Admin:      false,
@@ -464,7 +483,7 @@ func TestPrivilegeHandler_Fetch_Concurrent_Fetches(t *testing.T) {
 			},
 		},
 	}
-	assert.Equal(t, expectedPrivileges, handler.privileges)
+	assert.Equal(t, expectedPrivileges, privilegesOf(handler))
 }
 
 func TestPrivilegeHandler_Concurrent_Fetch_And_Read(t *testing.T) {
@@ -559,9 +578,10 @@ func TestPrivilegeHandler_Concurrent_Process_And_Read(t *testing.T) {
 		go func(id string) {
 			defer wg.Done()
 			_ = handler.Process(&PrivilegeAdded{
-				Email:     "jim@example.org",
-				CompanyID: id,
-				Privilege: PrivilegeAdmin,
+				SequenceNo: 1,
+				Email:      "jim@example.org",
+				CompanyID:  id,
+				Privilege:  PrivilegeAdmin,
 			})
 		}(companyID)
 	}
@@ -635,9 +655,10 @@ func TestPrivilegeHandler_Concurrent_Multiple_Operations(t *testing.T) {
 		go func(idx int) {
 			defer wg.Done()
 			_ = handler.Process(&PrivilegeAdded{
-				Email:     "jane@example.org",
-				CompanyID: fmt.Sprintf("company-%d", idx%5),
-				Privilege: PrivilegeCompany,
+				SequenceNo: 1,
+				Email:      "jane@example.org",
+				CompanyID:  fmt.Sprintf("company-%d", idx%5),
+				Privilege:  PrivilegeCompany,
 			})
 		}(i)
 	}
@@ -711,22 +732,333 @@ func TestPrivilegeHandler_process(t *testing.T) {
 	admin := func(p CompanyPrivileges) bool { return p.Admin }
 
 	assert.NoError(t, process[UserAdded](handler)(ctx, spec.ConsumableEvent[UserAdded]{
-		Payload: UserAdded{Email: "jim@example.org", CompanyID: "abc-123"},
+		Payload: UserAdded{SequenceNo: 1, Email: "jim@example.org", CompanyID: "abc-123"},
 	}))
 	assert.False(t, handler.IsAllowed("jim@example.org", "abc-123", admin))
 
 	assert.NoError(t, process[PrivilegeAdded](handler)(ctx, spec.ConsumableEvent[PrivilegeAdded]{
-		Payload: PrivilegeAdded{Email: "jim@example.org", CompanyID: "abc-123", Privilege: PrivilegeAdmin},
+		Payload: PrivilegeAdded{SequenceNo: 2, Email: "jim@example.org", CompanyID: "abc-123", Privilege: PrivilegeAdmin},
 	}))
 	assert.True(t, handler.IsAllowed("jim@example.org", "abc-123", admin))
 
 	assert.NoError(t, process[PrivilegeRemoved](handler)(ctx, spec.ConsumableEvent[PrivilegeRemoved]{
-		Payload: PrivilegeRemoved{Email: "jim@example.org", CompanyID: "abc-123", Privilege: PrivilegeAdmin},
+		Payload: PrivilegeRemoved{SequenceNo: 3, Email: "jim@example.org", CompanyID: "abc-123", Privilege: PrivilegeAdmin},
 	}))
 	assert.False(t, handler.IsAllowed("jim@example.org", "abc-123", admin))
 
 	assert.NoError(t, process[UserRemoved](handler)(ctx, spec.ConsumableEvent[UserRemoved]{
-		Payload: UserRemoved{Email: "jim@example.org", CompanyID: "abc-123"},
+		Payload: UserRemoved{SequenceNo: 4, Email: "jim@example.org", CompanyID: "abc-123"},
 	}))
 	assert.Empty(t, handler.CompaniesByUser("jim@example.org", func(CompanyPrivileges) bool { return true }))
 }
+
+// privilegesOf returns the privileges of every membership in the handler.
+func privilegesOf(h *PrivilegeHandler) map[string]map[string]CompanyPrivileges {
+	h.RLock()
+	defer h.RUnlock()
+	result := map[string]map[string]CompanyPrivileges{}
+	for email, companies := range h.state {
+		for companyID, s := range companies {
+			if !s.member {
+				continue
+			}
+			if result[email] == nil {
+				result[email] = map[string]CompanyPrivileges{}
+			}
+			result[email][companyID] = s.privileges
+		}
+	}
+	return result
+}
+
+func TestPrivilegeHandler_Process_Orders_By_SequenceNo(t *testing.T) {
+	const email, company = "jim@example.org", "abc-123"
+	userAdded := func(seq int) any { return &UserAdded{Email: email, CompanyID: company, SequenceNo: seq} }
+	userRemoved := func(seq int) any { return &UserRemoved{Email: email, CompanyID: company, SequenceNo: seq} }
+	added := func(seq int) any {
+		return &PrivilegeAdded{Email: email, CompanyID: company, Privilege: PrivilegeAdmin, SequenceNo: seq}
+	}
+	removed := func(seq int) any {
+		return &PrivilegeRemoved{Email: email, CompanyID: company, Privilege: PrivilegeAdmin, SequenceNo: seq}
+	}
+	tests := []struct {
+		name      string
+		delivered []any
+		member    bool
+		admin     bool
+	}{
+		{name: "in order", delivered: []any{userAdded(1), added(2)}, member: true, admin: true},
+		{name: "privilege before older user added keeps the grant", delivered: []any{added(2), userAdded(1)}, member: true, admin: true},
+		{name: "late privilege added after user removed", delivered: []any{userAdded(1), userRemoved(3), added(2)}, member: false, admin: false},
+		{name: "late privilege added after privilege removed", delivered: []any{userAdded(1), removed(3), added(2)}, member: true, admin: false},
+		{name: "late user added does not revive privileges removed with the user", delivered: []any{userRemoved(3), added(2), userAdded(1)}, member: false, admin: false},
+		{name: "re-added user starts without the removed privileges", delivered: []any{added(2), userRemoved(3), userAdded(4)}, member: true, admin: false},
+		{name: "privilege granted after re-add", delivered: []any{userAdded(4), userRemoved(3), added(5), added(2)}, member: true, admin: true},
+		{name: "late user removed does not remove a newer grant", delivered: []any{added(4), userRemoved(3)}, member: true, admin: true},
+		{name: "late user removed does not remove a newer membership", delivered: []any{userAdded(4), userRemoved(3)}, member: true, admin: false},
+		{name: "late privilege added before re-add stays revoked", delivered: []any{userAdded(1), userRemoved(3), added(2), userAdded(4)}, member: true, admin: false},
+		{name: "privilege removed after user removed keeps the user out", delivered: []any{userRemoved(2), removed(3)}, member: false, admin: false},
+		{name: "duplicate delivery is idempotent", delivered: []any{added(2), removed(3), added(2)}, member: true, admin: false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			handler := New()
+			for _, event := range tt.delivered {
+				require.NoError(t, handler.Process(event))
+			}
+			all := func(CompanyPrivileges) bool { return true }
+			assert.Equal(t, tt.member, handler.IsAllowed(email, company, all))
+			assert.Equal(t, tt.admin, handler.IsAllowed(email, company, func(p CompanyPrivileges) bool { return p.Admin }))
+			if tt.member {
+				assert.Equal(t, []string{company}, handler.CompaniesByUser(email, all))
+			} else {
+				assert.Equal(t, []string{}, handler.CompaniesByUser(email, all))
+			}
+		})
+	}
+}
+
+func TestPrivilegeHandler_Process_Without_SequenceNo_Fails_Closed(t *testing.T) {
+	const email, company = "jim@example.org", "abc-123"
+	admin := func(p CompanyPrivileges) bool { return p.Admin }
+	handler := New()
+
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: company, Privilege: PrivilegeAdmin}))
+	require.NoError(t, handler.Process(&UserAdded{Email: email, CompanyID: company}))
+	assert.False(t, handler.IsAllowed(email, company, func(CompanyPrivileges) bool { return true }), "an unordered addition is dropped")
+	require.NoError(t, handler.Process(&UserAdded{Email: email, CompanyID: company, SequenceNo: 1}))
+	assert.False(t, handler.IsAllowed(email, company, admin), "the dropped grant doesn't appear with a later membership")
+
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: company, Privilege: PrivilegeAdmin, SequenceNo: 5}))
+	require.NoError(t, handler.Process(&PrivilegeRemoved{Email: email, CompanyID: company, Privilege: PrivilegeAdmin}))
+	assert.False(t, handler.IsAllowed(email, company, admin), "an unordered privilege removal applies")
+
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: company, Privilege: PrivilegeTime, SequenceNo: 6}))
+	require.NoError(t, handler.Process(&UserRemoved{Email: email, CompanyID: company}))
+	assert.False(t, handler.IsAllowed(email, company, func(CompanyPrivileges) bool { return true }), "an unordered user removal applies")
+}
+
+// snapshotServer serves body as the /authz snapshot at seq; a negative seq sends no header.
+func snapshotServer(t *testing.T, seq int, body string) *httptest.Server {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if seq >= 0 {
+			w.Header().Set(SequenceHeader, fmt.Sprint(seq))
+		}
+		_, _ = w.Write([]byte(body))
+	}))
+	t.Cleanup(server.Close)
+	return server
+}
+
+func TestPrivilegeHandler_Fetch_Merges_Snapshot_By_SequenceNo(t *testing.T) {
+	const email = "jim@example.org"
+	admin := func(p CompanyPrivileges) bool { return p.Admin }
+	all := func(CompanyPrivileges) bool { return true }
+	snapshot := `{"jim@example.org": {"in-snapshot": {"admin": true}, "stale-in-snapshot": {"admin": true}}}`
+
+	handler := New(WithBaseURL(snapshotServer(t, 10, snapshot).URL))
+	// Delivered while the snapshot was read.
+	require.NoError(t, handler.Process(&PrivilegeRemoved{Email: email, CompanyID: "in-snapshot", Privilege: PrivilegeAdmin, SequenceNo: 11}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: "stale-in-snapshot", Privilege: PrivilegeTime, SequenceNo: 9}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: "new", Privilege: PrivilegeAdmin, SequenceNo: 12}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: "revoked", Privilege: PrivilegeAdmin, SequenceNo: 8}))
+	require.NoError(t, handler.Process(&UserAdded{Email: email, CompanyID: "revoked-later", SequenceNo: 7}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: "revoked-later", Privilege: PrivilegeAdmin, SequenceNo: 13}))
+
+	require.NoError(t, handler.Fetch())
+
+	assert.True(t, handler.IsAllowed(email, "in-snapshot", all))
+	assert.False(t, handler.IsAllowed(email, "in-snapshot", admin), "a removal newer than the snapshot wins")
+	assert.False(t, handler.IsAllowed(email, "stale-in-snapshot", func(p CompanyPrivileges) bool { return p.Time }), "the snapshot replaces an older fact")
+	assert.True(t, handler.IsAllowed(email, "new", admin), "a grant newer than the snapshot is kept")
+	assert.False(t, handler.IsAllowed(email, "revoked", all), "a pair missing from the snapshot is removed")
+	assert.True(t, handler.IsAllowed(email, "revoked-later", admin), "a grant newer than the snapshot survives the removal")
+
+	// Events the snapshot already covers are stale.
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: "revoked", Privilege: PrivilegeAdmin, SequenceNo: 10}))
+	assert.False(t, handler.IsAllowed(email, "revoked", all), "an event at the snapshot sequence is stale")
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: "revoked", Privilege: PrivilegeAdmin, SequenceNo: 14}))
+	assert.True(t, handler.IsAllowed(email, "revoked", admin))
+}
+
+func TestPrivilegeHandler_Fetch_Replaces_Pairs_Missing_From_Snapshot(t *testing.T) {
+	handler := New(WithBaseURL(snapshotServer(t, 10, `{"jim@example.org": {"kept": {"company": true}}}`).URL))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: "jim@example.org", CompanyID: "gone", Privilege: PrivilegeAdmin, SequenceNo: 3}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: "jane@example.org", CompanyID: "gone", Privilege: PrivilegeAdmin, SequenceNo: 4}))
+
+	require.NoError(t, handler.Fetch())
+
+	assert.Equal(t, map[string]map[string]CompanyPrivileges{"jim@example.org": {"kept": {Company: true}}}, privilegesOf(handler))
+	handler.RLock()
+	defer handler.RUnlock()
+	assert.NotContains(t, handler.state, "jane@example.org", "an email without companies is dropped")
+}
+
+func TestPrivilegeHandler_Fetch_Without_SequenceHeader_Keeps_Sequenced_Facts(t *testing.T) {
+	handler := New(WithBaseURL(snapshotServer(t, -1, `{"jim@example.org": {"abc-123": {"admin": true}}}`).URL))
+	require.NoError(t, handler.Process(&PrivilegeRemoved{Email: "jim@example.org", CompanyID: "abc-123", Privilege: PrivilegeAdmin, SequenceNo: 3}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: "jim@example.org", CompanyID: "other", Privilege: PrivilegeAdmin, SequenceNo: 4}))
+
+	require.NoError(t, handler.Fetch())
+
+	assert.Equal(t, map[string]map[string]CompanyPrivileges{
+		"jim@example.org": {"abc-123": {}, "other": {Admin: true}},
+	}, privilegesOf(handler))
+}
+
+func TestPrivilegeHandler_Fetch_Rejects_Bad_Responses(t *testing.T) {
+	tests := []struct {
+		name   string
+		status int
+		header string
+		err    string
+	}{
+		{name: "status", status: http.StatusUnauthorized, err: "fetch privileges: unexpected status 401 Unauthorized"},
+		{name: "invalid header", status: http.StatusOK, header: "abc", err: `fetch privileges: invalid X-Authz-Sequence header "abc"`},
+		{name: "negative header", status: http.StatusOK, header: "-1", err: `fetch privileges: invalid X-Authz-Sequence header "-1"`},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				if tt.header != "" {
+					w.Header().Set(SequenceHeader, tt.header)
+				}
+				w.WriteHeader(tt.status)
+				_, _ = w.Write([]byte(`{"jim@example.org": {"abc-123": {"admin": true}}}`))
+			}))
+			defer server.Close()
+			handler := New(WithBaseURL(server.URL))
+			require.NoError(t, handler.Process(&PrivilegeAdded{Email: "jane@example.org", CompanyID: "abc-123", Privilege: PrivilegeTime, SequenceNo: 1}))
+
+			assert.EqualError(t, handler.Fetch(), tt.err)
+			assert.Equal(t, map[string]map[string]CompanyPrivileges{"jane@example.org": {"abc-123": {Time: true}}}, privilegesOf(handler), "state is untouched")
+		})
+	}
+}
+
+func TestPrivilegeHandler_Revocation_During_Fetch_Wins(t *testing.T) {
+	// The consumers are bound before Fetch, so a revocation published after authz-service
+	// read the snapshot is processed while the response is still on its way.
+	handler := New()
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// assert, not require: this runs on the server's goroutine.
+		assert.NoError(t, handler.Process(&UserRemoved{Email: "jim@example.org", CompanyID: "abc-123", SequenceNo: 21}))
+		w.Header().Set(SequenceHeader, "20")
+		_, _ = w.Write([]byte(`{"jim@example.org": {"abc-123": {"admin": true}}}`))
+	}))
+	defer server.Close()
+	handler.baseURL = server.URL
+
+	require.NoError(t, handler.Fetch())
+
+	assert.False(t, handler.IsAllowed("jim@example.org", "abc-123", func(CompanyPrivileges) bool { return true }))
+}
+
+func TestPrivilegeHandler_Fetch_Ignores_Snapshot_Older_Than_Merged_One(t *testing.T) {
+	const email = "jim@example.org"
+	all := func(CompanyPrivileges) bool { return true }
+	handler := New(WithBaseURL(snapshotServer(t, 12, `{}`).URL))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: "abc-123", Privilege: PrivilegeAdmin, SequenceNo: 5}))
+	require.NoError(t, handler.Process(&UserRemoved{Email: email, CompanyID: "abc-123", SequenceNo: 11}))
+	require.NoError(t, handler.Fetch())
+
+	// A concurrent Fetch read at seq 10, before the removal, merges last.
+	handler.baseURL = snapshotServer(t, 10, `{"jim@example.org": {"abc-123": {"admin": true}}}`).URL
+	require.NoError(t, handler.Fetch())
+	assert.False(t, handler.IsAllowed(email, "abc-123", all))
+
+	// So does a snapshot without a sequence number.
+	handler.baseURL = snapshotServer(t, -1, `{"jim@example.org": {"abc-123": {"admin": true}}}`).URL
+	require.NoError(t, handler.Fetch())
+	assert.False(t, handler.IsAllowed(email, "abc-123", all))
+}
+
+func TestPrivilegeHandler_Unordered_Removal_Holds_Until_Next_Snapshot(t *testing.T) {
+	const email, company = "jim@example.org", "abc-123"
+	all := func(CompanyPrivileges) bool { return true }
+	admin := func(p CompanyPrivileges) bool { return p.Admin }
+	handler := New(WithBaseURL(snapshotServer(t, 30, `{"jim@example.org": {"abc-123": {"admin": true}, "def-456": {"time": true}}}`).URL))
+	require.NoError(t, handler.Process(&UserAdded{Email: email, CompanyID: company, SequenceNo: 10}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: company, Privilege: PrivilegeAdmin, SequenceNo: 11}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: "def-456", Privilege: PrivilegeTime, SequenceNo: 12}))
+
+	require.NoError(t, handler.Process(&UserRemoved{Email: email, CompanyID: company}))
+	require.NoError(t, handler.Process(&PrivilegeRemoved{Email: email, CompanyID: "def-456", Privilege: PrivilegeTime}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: company, Privilege: PrivilegeSalary, SequenceNo: 15}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: company, Privilege: PrivilegeAdmin, SequenceNo: 16}))
+	require.NoError(t, handler.Process(&UserAdded{Email: email, CompanyID: company, SequenceNo: 17}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: email, CompanyID: "def-456", Privilege: PrivilegeTime, SequenceNo: 18}))
+	assert.False(t, handler.IsAllowed(email, company, all), "no event overrides an unordered user removal")
+	assert.False(t, handler.IsAllowed(email, "def-456", func(p CompanyPrivileges) bool { return p.Time }), "no event overrides an unordered privilege removal")
+
+	require.NoError(t, handler.Fetch())
+	assert.True(t, handler.IsAllowed(email, company, admin), "the next snapshot replaces the unordered removal")
+	assert.True(t, handler.IsAllowed(email, "def-456", func(p CompanyPrivileges) bool { return p.Time }))
+}
+
+func TestPrivilegeHandler_Fetch_Deletes_Unordered_Removal_Missing_From_Snapshot(t *testing.T) {
+	handler := New(WithBaseURL(snapshotServer(t, 30, `{}`).URL))
+	require.NoError(t, handler.Process(&UserRemoved{Email: "jim@example.org", CompanyID: "abc-123"}))
+
+	require.NoError(t, handler.Fetch())
+
+	handler.RLock()
+	defer handler.RUnlock()
+	assert.Empty(t, handler.state)
+}
+
+func TestPrivilegeHandler_Process_Unknown_Privilege_Creates_No_State(t *testing.T) {
+	handler := New()
+	require.NoError(t, handler.Process(&PrivilegeRemoved{Email: "jim@example.org", CompanyID: "abc-123", Privilege: "UNKNOWN", SequenceNo: 1}))
+	require.NoError(t, handler.Process(&PrivilegeAdded{Email: "jim@example.org", CompanyID: "abc-123", Privilege: "UNKNOWN", SequenceNo: 2}))
+
+	handler.RLock()
+	defer handler.RUnlock()
+	assert.Empty(t, handler.state)
+}
+
+func TestPrivilegeHandler_Fetch_Retries_While_ReadView_Not_Ready(t *testing.T) {
+	var calls atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if calls.Add(1) < 3 {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			return
+		}
+		w.Header().Set(SequenceHeader, "7")
+		_, _ = w.Write([]byte(`{"jim@example.org": {"abc-123": {"admin": true}}}`))
+	}))
+	defer server.Close()
+	handler := New(WithBaseURL(server.URL))
+	handler.retryDelay = time.Millisecond
+
+	require.NoError(t, handler.Fetch())
+
+	assert.Equal(t, int32(3), calls.Load())
+	assert.True(t, handler.IsAllowed("jim@example.org", "abc-123", func(p CompanyPrivileges) bool { return p.Admin }))
+}
+
+func TestPrivilegeHandler_Fetch_Gives_Up_When_ReadView_Stays_Behind(t *testing.T) {
+	var calls atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls.Add(1)
+		w.WriteHeader(http.StatusServiceUnavailable)
+	}))
+	defer server.Close()
+	handler := New(WithBaseURL(server.URL))
+	handler.retryDelay = time.Millisecond
+	handler.retries = 2
+
+	assert.EqualError(t, handler.Fetch(), "fetch privileges: authz-service read view not ready")
+	assert.Equal(t, int32(3), calls.Load())
+}
+
+func TestPrivilegeHandler_Process_Drops_Invalid_SequenceNo(t *testing.T) {
+	for _, seq := range []int{-1, math.MaxInt} {
+		handler := New()
+		require.NoError(t, handler.Process(&PrivilegeAdded{Email: "jim@example.org", CompanyID: "abc-123", Privilege: PrivilegeAdmin, SequenceNo: seq}))
+		require.NoError(t, handler.Process(&UserRemoved{Email: "jim@example.org", CompanyID: "abc-123", SequenceNo: seq}))
+		handler.RLock()
+		assert.Empty(t, handler.state, "seq %d", seq)
+		handler.RUnlock()
+	}
+}