test(mock): one failure plan replaces the per-call wrapper drivers

mock.Failures already made a method fail on every call. FailurePlan adds the
call numbers, which is the shape of nearly every device fault the runner has to
survive: the first read times out, the next one works. Expressing that by
embedding the mock in a one-off wrapper put a different seven-line method in
every test that needed one.

internal/driver/mock/mock.go

@@ -3,6 +3,7 @@ package mock
 
 import (
 	"context"
+	"slices"
 	"sync"
 	"time"
 
@@ -49,19 +50,31 @@ type Action struct {
 	Idle           time.Duration
 }
 
+// FailurePlan makes one method fail. OnCalls names the 1-based calls that fail
+// and every other call succeeds; an empty OnCalls fails every call. It exists
+// because "the first read times out and the next one works" is the shape of
+// nearly every device fault the runner has to survive, and expressing it by
+// embedding the mock in a one-off wrapper puts a different seven-line method in
+// every test that needs one.
+type FailurePlan struct {
+	Err     error
+	OnCalls []int
+}
+
 // Driver is an in-memory Driver implementation for unit tests.
 // Tests can program HierarchyJSON, ImageData, HealthInfo, and per-method
 // Failures, and read back Actions to assert what the runner asked for.
 type Driver struct {
-	mutex   sync.Mutex
+	mutex      sync.Mutex
-	actions []Action
+	actions    []Action
+	callCounts map[ActionKind]int
 
 	HierarchyJSON string
 	ImageData     driver.Image
 	HealthInfo    driver.Health
 	LogEntries    []driver.LogEntry
 	MetricsData   driver.Metrics
-	Failures      map[ActionKind]error
+	Failures      map[ActionKind]FailurePlan
 
 	// ReplacesText makes the mock assert the TextReplacer capability, so
 	// tests cover both the erase-before-type and replace-on-input paths.
@@ -90,7 +103,8 @@ type Driver struct {
 
 func New() *Driver {
 	return &Driver{
-		Failures: map[ActionKind]error{},
+		Failures:   map[ActionKind]FailurePlan{},
+		callCounts: map[ActionKind]int{},
 		HealthInfo: driver.Health{
 			Ready:    true,
 			Version:  "mock",
@@ -116,7 +130,15 @@ func (d *Driver) record(action Action) {
 func (d *Driver) failure(kind ActionKind) error {
 	d.mutex.Lock()
 	defer d.mutex.Unlock()
-	return d.Failures[kind]
+	d.callCounts[kind]++
+	plan, planned := d.Failures[kind]
+	if !planned {
+		return nil
+	}
+	if len(plan.OnCalls) == 0 || slices.Contains(plan.OnCalls, d.callCounts[kind]) {
+		return plan.Err
+	}
+	return nil
 }
 
 func (d *Driver) Launch(_ context.Context, bundleID string, clearState bool, _ map[string]string) error {

internal/driver/mock/mock_test.go

@@ -100,7 +100,7 @@ func TestProgrammableScreenshotIsReturned(t *testing.T) {
 func TestFailureInjection(t *testing.T) {
 	boom := errors.New("boom")
 	mock := New()
-	mock.Failures[ActionTap] = boom
+	mock.Failures[ActionTap] = FailurePlan{Err: boom}
 
 	if err := mock.Tap(context.Background(), 0, 0); !errors.Is(err, boom) {
 		t.Fatalf("expected boom, got %v", err)

internal/runner/runner_test.go

@@ -365,9 +365,9 @@ func TestRenderSummary_CountsTheStepsNothingJudged(t *testing.T) {
 // green iOS summary is indistinguishable from one over a silent app.
 func TestRunner_ReadsTheDriverCannotMakeAreNotPassedChecks(t *testing.T) {
 	state := newHarnessWithSpec(t, logErrorSpec)
-	state.mock.Failures[mockdriver.ActionRecentLogs] = driver.ErrNotSupported
+	state.mock.Failures[mockdriver.ActionRecentLogs] = mockdriver.FailurePlan{Err: driver.ErrNotSupported}
-	state.mock.Failures[mockdriver.ActionMetrics] = driver.ErrNotSupported
+	state.mock.Failures[mockdriver.ActionMetrics] = mockdriver.FailurePlan{Err: driver.ErrNotSupported}
-	state.mock.Failures[mockdriver.ActionHealth] = driver.ErrNotSupported
+	state.mock.Failures[mockdriver.ActionHealth] = mockdriver.FailurePlan{Err: driver.ErrNotSupported}
 
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
@@ -877,7 +877,7 @@ func TestTraceActionFor_RecordsKindSpecificFields(t *testing.T) {
 
 func TestRunner_LogsWaitForIdleDriverErrors(t *testing.T) {
 	state := newHarness(t)
-	state.mock.Failures[mockdriver.ActionWaitForIdle] = errors.New("sidecar lost gRPC stream")
+	state.mock.Failures[mockdriver.ActionWaitForIdle] = mockdriver.FailurePlan{Err: errors.New("sidecar lost gRPC stream")}
 
 	var logBuf bytes.Buffer
 	logger := slog.New(slog.NewTextHandler(&logBuf, &slog.HandlerOptions{Level: slog.LevelWarn}))
@@ -976,7 +976,7 @@ func TestApplyAction_InputTextSkipsEraseWhenTargetEmpty(t *testing.T) {
 func TestApplyAction_InputTextSurfacesFocusTapError(t *testing.T) {
 	t.Run("selector focus tap fails", func(t *testing.T) {
 		driverMock := mockdriver.New()
-		driverMock.Failures[mockdriver.ActionTapSelector] = errors.New("adb unreachable")
+		driverMock.Failures[mockdriver.ActionTapSelector] = mockdriver.FailurePlan{Err: errors.New("adb unreachable")}
 		action := verifier.Action{Kind: verifier.ActionKindInputText, On: "id:username", Text: "alice"}
 
 		_, err := applyAction(context.Background(), driverMock, action, nil)
@@ -989,7 +989,7 @@ func TestApplyAction_InputTextSurfacesFocusTapError(t *testing.T) {
 	})
 	t.Run("coordinate focus tap fails", func(t *testing.T) {
 		driverMock := mockdriver.New()
-		driverMock.Failures[mockdriver.ActionTap] = errors.New("tap driver error")
+		driverMock.Failures[mockdriver.ActionTap] = mockdriver.FailurePlan{Err: errors.New("tap driver error")}
 		action := verifier.Action{Kind: verifier.ActionKindInputText, X: 10, Y: 20, Text: "alice"}
 
 		_, err := applyAction(context.Background(), driverMock, action, nil)
@@ -1639,7 +1639,7 @@ func TestRunner_ASourceAskedAndHandedNothingSaysSo(t *testing.T) {
 // here has an action to offer, and no step of this run gets to hear it.
 func TestRunner_AHeldStepIsNotRecordedAsASourceThatDeclined(t *testing.T) {
 	state := newHarness(t)
-	state.mock.Failures[mockdriver.ActionSnapshot] = errors.New("adb: device offline")
+	state.mock.Failures[mockdriver.ActionSnapshot] = mockdriver.FailurePlan{Err: errors.New("adb: device offline")}
 
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()

internal/runner/web_last_action_test.go

@@ -130,7 +130,7 @@ func TestRunner_WebInstallsTheStepsLogsInThePage(t *testing.T) {
 // be dropped in silence, under a comment claiming it was warned about.
 func TestRunner_ReportsALogFetchItCouldNotMake(t *testing.T) {
 	state := newHarnessWithSpec(t, lastActionSpec)
-	state.mock.Failures[mockdriver.ActionRecentLogs] = errors.New("adb: device offline")
+	state.mock.Failures[mockdriver.ActionRecentLogs] = mockdriver.FailurePlan{Err: errors.New("adb: device offline")}
 
 	var buffer bytes.Buffer
 	logger := slog.New(slog.NewTextHandler(&buffer, &slog.HandlerOptions{Level: slog.LevelWarn}))