fix(runner,sample-app): surface silent errors and demo a failing property (#15)

* fix(runner): surface non-deadline WaitForIdle errors

Previously the WaitForIdle return value was discarded entirely, hiding
real driver failures (gRPC transport errors, sidecar crashes) behind
the expected deadline-exceeded case. Log non-deadline errors so they
are visible without changing control flow.

* chore(sample-app): drop unused uptime_millis extractor

Registered in SampleApplication but never consumed by spec.ts.

* fix(sample-app): drop trivial appIsRunning property

app_state was hardcoded to 'running' so the property was a tautology
that could never fail. Removing both the extractor and the property
is the simplest fix; demo-grade properties that can fail land next.

* feat(sample-app): add Reset button that zeroes clickCount

Pairs with the next commit's tap-reset action so the fuzzer can
violate clickCountNeverDecreases and demonstrate uatu actually
finding a property violation.

* feat(sample-app): add tap-reset action to exercise Reset button

Weighted at 10/122, fuzzer reaches it within a short run. Pairs with
the Reset button to demonstrate uatu detecting the
clickCountNeverDecreases violation.

* fix(runner): filter WaitForIdle errors via context state, not errors.Is

errors.Is(err, context.DeadlineExceeded) misses gRPC's wrapped
status.DeadlineExceeded, so every step under the maestro driver
logged a spurious warning. Check idleCtx.Err() instead — captures
both deadline-fired and parent-canceled cases regardless of how the
driver wraps them.

* chore(sample-app): tune action weights so demo violates in ~30s

Prior weights left tap-reset rare enough that short demo runs missed
the violation by chance. Bumped to 30/107, with typeUsername reduced
since username noise doesn't help exercise clickCount.

* refactor(runner): route warnings through slog

Adds Options.Logger (defaults to slog.Default()) and converts the
three warning sites that were using fmt.Printf. Progress line stays
on Printf since it's user-facing UI, not a log. Makes the warnings
testable via a capturing handler.

* test(runner): assert WaitForIdle driver errors are logged

Captures slog output via TextHandler into a buffer and asserts the
warning message + injected error text appear when the mock driver
returns a non-context error from WaitForIdle. Guards against a
regression of the silent-error swallow.

examples/sample-app/android/src/main/kotlin/dev/uatu/sample/MainActivity.kt

@@ -47,6 +47,16 @@ class MainActivity : Activity() {
         }
         layout.addView(button)
 
+        val resetButton = Button(this).apply {
+            text = "Reset"
+            textSize = 18f
+            setOnClickListener {
+                clickCount = 0
+                label.text = "Clicks: $clickCount"
+            }
+        }
+        layout.addView(resetButton)
+
         val usernameLabel = TextView(this).apply {
             text = "Username: "
             textSize = 20f

examples/sample-app/android/src/main/kotlin/dev/uatu/sample/SampleApplication.kt

@@ -7,11 +7,7 @@ class SampleApplication : Application() {
     override fun onCreate() {
         super.onCreate()
         Uatu.start(this)
-        Uatu.extract("app_state") { "running" }
         Uatu.extract("click_count") { MainActivity.clickCount }
         Uatu.extract("username") { MainActivity.username }
-        Uatu.extract("uptime_millis") { System.currentTimeMillis() - startedAt }
     }
-
-    private val startedAt: Long = System.currentTimeMillis()
 }

examples/sample-app/spec.ts

@@ -11,9 +11,6 @@ import {
 
 // ── Snapshot extractors (fed by SampleApplication.kt) ──────────
 // See ./android/src/main/kotlin/dev/uatu/sample/SampleApplication.kt
-const appState = extract<string>(
-  (state) => (state.snapshots.app_state as string) ?? "",
-);
 const clickCount = extract<number>(
   (state) => (state.snapshots.click_count as number) ?? 0,
 );
@@ -23,11 +20,11 @@ const username = extract<string>(
 
 // ── UI elements ────────────────────────────────────────────────
 const clickButton = extract((state) => state.ax.find("text:Click me"));
+const resetButton = extract((state) => state.ax.find("text:Reset"));
 const usernameField = extract((state) => state.ax.find("desc:username_field"));
 
 // ── Properties ─────────────────────────────────────────────────
 export const properties = {
-  appIsRunning: always(() => appState.current === "running"),
   clickCountNonNegative: always(() => clickCount.current >= 0),
   clickCountNeverDecreases: always(() => {
     const previous = clickCount.previous;
@@ -50,10 +47,15 @@ const typeUsername = actions(() => {
     : [];
 });
 
+const tapReset = actions(() => {
+  return resetButton.current ? [Tap({ on: resetButton.current })] : [];
+});
+
 export const actionsRoot = weighted(
   [50, tapClickMe],
-  [50, typeUsername],
-  [10, taps],
+  [20, typeUsername],
+  [30, tapReset],
+  [5, taps],
   [2, swipes],
 );
 

internal/runner/runner.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"log/slog"
 	"time"
 
 	"github.com/priyanshujain/uatu/internal/agent"
@@ -24,6 +25,7 @@ type Options struct {
 	Driver      driver.Driver
 	Verifier    *verifier.Verifier
 	TraceWriter *trace.Writer
+	Logger      *slog.Logger
 }
 
 type Summary struct {
@@ -46,6 +48,10 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 	if err := validate(options); err != nil {
 		return Summary{}, err
 	}
+	logger := options.Logger
+	if logger == nil {
+		logger = slog.Default()
+	}
 
 	summary := Summary{StartTime: time.Now()}
 	deadline := summary.StartTime.Add(options.Duration)
@@ -64,7 +70,7 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 		// a hierarchy that matches the snapshots captured a moment later.
 		tree, hierarchyErr := fetchHierarchy(ctx, options.Driver)
 		if hierarchyErr != nil {
-			fmt.Printf("warning: step %d hierarchy: %v\n", stepIndex, hierarchyErr)
+			logger.Warn("hierarchy fetch failed", "step", stepIndex, "err", hierarchyErr)
 		}
 		treeSize := 0
 		if tree != nil {
@@ -81,7 +87,7 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 		}
 		screen, screenErr := screenFromSnapshot(snapshot.Snapshots)
 		if screenErr != nil {
-			fmt.Printf("warning: step %d screen: %v\n", stepIndex, screenErr)
+			logger.Warn("screen snapshot decode failed", "step", stepIndex, "err", screenErr)
 		}
 		fmt.Printf("step %d: screen=%q hierarchy=%d nodes\n",
 			stepIndex, screen, treeSize)
@@ -126,7 +132,10 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 		}
 
 		idleCtx, idleCancel := context.WithTimeout(ctx, options.IdleTimeout)
-		_ = options.Driver.WaitForIdle(idleCtx, options.IdleTimeout)
+		idleErr := options.Driver.WaitForIdle(idleCtx, options.IdleTimeout)
+		if idleErr != nil && idleCtx.Err() == nil {
+			logger.Warn("wait_for_idle failed", "step", stepIndex, "err", idleErr)
+		}
 		idleCancel()
 	}
 

internal/runner/runner_test.go

@@ -1,9 +1,11 @@
 package runner
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
+	"log/slog"
 	"net"
 	"os"
 	"path/filepath"
@@ -267,6 +269,41 @@ func TestScreenFromSnapshot(t *testing.T) {
 	})
 }
 
+func TestRunner_LogsWaitForIdleDriverErrors(t *testing.T) {
+	snapshots := []map[string]json.RawMessage{
+		{"balance": json.RawMessage(`100`)},
+	}
+	state := newHarness(t, snapshots)
+	state.startSDK(t)
+	state.acceptConnection(t)
+	state.mock.Failures[mockdriver.ActionWaitForIdle] = errors.New("sidecar lost gRPC stream")
+
+	var logBuf bytes.Buffer
+	logger := slog.New(slog.NewTextHandler(&logBuf, &slog.HandlerOptions{Level: slog.LevelWarn}))
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	if _, err := Run(ctx, Options{
+		Duration:        100 * time.Millisecond,
+		SnapshotTimeout: 2 * time.Second,
+		IdleTimeout:     50 * time.Millisecond,
+		Connection:      state.conn,
+		Driver:          state.mock,
+		Verifier:        state.verifier,
+		TraceWriter:     state.writer,
+		Logger:          logger,
+	}); err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	output := logBuf.String()
+	if !strings.Contains(output, "wait_for_idle failed") {
+		t.Errorf("expected wait_for_idle warning, got: %q", output)
+	}
+	if !strings.Contains(output, "sidecar lost gRPC stream") {
+		t.Errorf("expected driver error message in warning, got: %q", output)
+	}
+}
+
 func TestApplyAction_InputTextSurfacesFocusTapError(t *testing.T) {
 	t.Run("selector focus tap fails", func(t *testing.T) {
 		driverMock := mockdriver.New()