Change a test store's failure under its lock, which the keeper's goroutine reads

TestAnUnreadableStoreClearsNothing wrote InMemory.Fail and its values unguarded
while the keeper's teller appended to the same store, and the race detector
failed mesh/repo-check on #170 (a test race on main, not the change).

cmd/mesh-controller/conditions_test.go

@@ -86,7 +86,7 @@ func TestASilenceThroughTheVerbHoldsAndTheConditionStaysOpen(t *testing.T) {
 func TestUnreadableConditionsAreNotAWellMesh(t *testing.T) {
 	open := aMesh(t)
 	_, store := withConditionsInMemory(t)
-	store.Fail = errors.New("the bus is away")
+	store.SetFail(errors.New("the bus is away"))
 	asked, err := theThreeQuestions(t.Context(), open)
 	if err != nil {
 		t.Fatal(err)
@@ -98,7 +98,7 @@ func TestUnreadableConditionsAreNotAWellMesh(t *testing.T) {
 	if !strings.HasPrefix(said, "the open conditions could NOT be read") || strings.Contains(said, "no open conditions") {
 		t.Fatalf("%s", said)
 	}
-	store.Fail = nil
+	store.SetFail(nil)
 	asked, _ = theThreeQuestions(t.Context(), open)
 	said = printed(t, func() error { return printStatus(asked) })
 	if asked.well() && !strings.Contains(said, "no open conditions;") {

internal/conditions/memory.go

@@ -61,6 +61,14 @@ func (m *InMemory) Update(_ context.Context, key string, value []byte, revision
 	return nil
 }
 
+// SetFail makes every call fail with err from now on, or none with nil: under the store's lock, so a test may
+// change it while the keeper's own goroutine is telling.
+func (m *InMemory) SetFail(err error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.Fail = err
+}
+
 func (m *InMemory) Delete(_ context.Context, key string, revision uint64) error {
 	m.mu.Lock()
 	defer m.mu.Unlock()

internal/conditions/store_test.go

@@ -199,15 +199,17 @@ func TestAnUnreadableStoreClearsNothing(t *testing.T) {
 	if _, err := k.Observe(ctx, silent("ace")); err != nil {
 		t.Fatal(err)
 	}
-	store.Fail = errors.New("the bus is away")
+	store.SetFail(errors.New("the bus is away"))
 	if err := k.Reconcile(ctx, "S1", nil); err == nil {
 		t.Fatal("reconciled against a store it could not read")
 	}
 	if _, err := k.Open(ctx); err == nil {
 		t.Fatal("an unreadable store answered as read")
 	}
-	store.Fail = nil
+	store.SetFail(nil)
+	store.mu.Lock()
 	store.values["machine.g14.silent"] = Entry{Value: []byte("{not a condition"), Revision: 99}
+	store.mu.Unlock()
 	if _, err := k.Open(ctx); err == nil || !strings.Contains(err.Error(), "machine.g14.silent") {
 		t.Fatalf("an unreadable condition was left out rather than said: %v", err)
 	}