feat: add ResettableTimer for simplified timer management

x/resettabletimer/readme.md

@@ -0,0 +1,225 @@
+### Resettable Timers in Cadence Workflows
+
+#### Status
+
+November 4, 2025
+
+This is experimental and the API may change in future releases.
+
+#### Background
+
+In Cadence workflows, timers are a fundamental building block for implementing timeouts and delays. However, standard timers cannot be reset once created - you must cancel the old timer and create a new one, which can lead to complex code patterns.
+
+The resettable timer provides a simple way to implement timeout patterns that need to restart based on external events.
+
+#### Getting Started
+
+Import the package:
+
+```go
+import (
+    "go.uber.org/cadence/workflow"
+    "go.uber.org/cadence/x/resettabletimer"
+)
+```
+
+#### Basic Usage
+
+Create a timer that can be reset:
+
+```go
+func MyWorkflow(ctx workflow.Context) error {
+    // Create a timer that fires after 30 seconds
+    timer := resettabletimer.New(ctx, 30*time.Second)
+
+    // Wait for the timer
+    err := timer.Future.Get(ctx, nil)
+    if err != nil {
+        return err
+    }
+
+    // Timer fired - handle timeout
+    workflow.GetLogger(ctx).Info("Timeout occurred")
+    return nil
+}
+```
+
+#### Resetting the Timer
+
+```go
+func MyWorkflow(ctx workflow.Context) error {
+    timer := resettabletimer.New(ctx, 30*time.Second)
+    activityChan := workflow.GetSignalChannel(ctx, "activity")
+
+    selector := workflow.NewSelector(ctx)
+
+    // Add timer to selector
+    selector.AddFuture(timer.Future, func(f workflow.Future) {
+        workflow.GetLogger(ctx).Info("User inactive for 30 seconds")
+    })
+
+    // Add signal channel to selector
+    selector.AddReceive(activityChan, func(c workflow.Channel, more bool) {
+        var signal string
+        c.Receive(ctx, &signal)
+
+        // Reset the timer when activity is detected
+        timer.Reset(30 * time.Second)
+        workflow.GetLogger(ctx).Info("Activity detected, timer reset")
+    })
+
+    selector.Select(ctx)
+    return nil
+}
+```
+
+#### Example: Inactivity Timeout with Dynamic Duration
+
+```go
+func InactivityTimeoutWorkflow(ctx workflow.Context) error {
+    // Start with 5 minute timeout
+    timeout := 5 * time.Minute
+    timer := resettabletimer.New(ctx, timeout)
+
+    activityChan := workflow.GetSignalChannel(ctx, "user_activity")
+    stopChan := workflow.GetSignalChannel(ctx, "stop")
+
+    done := false
+    for !done {
+        selector := workflow.NewSelector(ctx)
+
+        selector.AddFuture(timer.Future, func(f workflow.Future) {
+            workflow.GetLogger(ctx).Info("User inactive - logging out")
+            done = true
+        })
+
+        selector.AddReceive(activityChan, func(c workflow.Channel, more bool) {
+            var activity struct {
+                Type    string
+                Timeout time.Duration
+            }
+            c.Receive(ctx, &activity)
+
+            // Reset with possibly different duration
+            if activity.Timeout > 0 {
+                timeout = activity.Timeout
+            }
+            timer.Reset(timeout)
+
+            workflow.GetLogger(ctx).Info("Activity detected", 
+                "type", activity.Type, 
+                "new_timeout", timeout)
+        })
+
+        selector.AddReceive(stopChan, func(c workflow.Channel, more bool) {
+            var stop bool
+            c.Receive(ctx, &stop)
+            done = true
+        })
+
+        selector.Select(ctx)
+    }
+
+    return nil
+}
+```
+
+#### API Reference
+
+##### Types
+
+```go
+type ResettableTimer interface {
+    workflow.Future
+
+    // Reset cancels the current timer and starts a new one with the given duration.
+    // If the timer has already fired, Reset has no effect.
+    Reset(d time.Duration)
+}
+```
+
+##### Functions
+
+```go
+// New creates a new resettable timer that fires after duration d.
+func New(ctx workflow.Context, d time.Duration) *ResettableTimer
+```
+
+##### Methods
+
+```go
+// Reset cancels the current timer and starts a new one with the given duration
+timer.Reset(newDuration time.Duration)
+
+// Future is the underlying Future field for use with workflow.Selector
+timer.Future workflow.Future
+
+// Get blocks until the timer fires (convenience method)
+timer.Future.Get(ctx, nil)
+
+// IsReady returns true if the timer has fired (convenience method)
+timer.Future.IsReady()
+```
+
+#### Important Notes
+
+1. **Use with Selector**: When using the timer with `workflow.Selector`, you access the Future field directly:
+   ```go
+   selector.AddFuture(timer.Future, func(f workflow.Future) {
+       // timer fired
+   })
+   ```
+
+2. **Reset After Fire**: Once a timer has fired, calling `Reset()` has no effect. The timer is considered "done" after it fires.
+
+3. **Determinism**: Like all workflow code, timer operations are deterministic and will replay correctly during workflow replay.
+
+4. **Resolution**: Timer resolution is in seconds using `math.Ceil(d.Seconds())`, consistent with standard Cadence timers.
+
+#### Testing
+
+The resettable timer works seamlessly with Cadence's workflow test suite:
+
+```go
+func TestMyWorkflow(t *testing.T) {
+    testSuite := &testsuite.WorkflowTestSuite{}
+    env := testSuite.NewTestWorkflowEnvironment()
+
+    // Register delayed callback to simulate activity
+    env.RegisterDelayedCallback(func() {
+        env.SignalWorkflow("activity", "user_action")
+    }, 10*time.Second)
+
+    env.ExecuteWorkflow(MyWorkflow)
+
+    require.True(t, env.IsWorkflowCompleted())
+    require.NoError(t, env.GetWorkflowError())
+}
+```
+
+#### Comparison with Standard Timers
+
+**Standard Timer Pattern:**
+```go
+// Must manage timer cancellation and recreation manually
+var timerCancel workflow.CancelFunc
+timerCtx, timerCancel := workflow.WithCancel(ctx)
+timer := workflow.NewTimer(timerCtx, 30*time.Second)
+
+// On activity - must cancel and recreate
+timerCancel()
+timerCtx, timerCancel = workflow.WithCancel(ctx)
+timer = workflow.NewTimer(timerCtx, 30*time.Second)
+```
+
+**Resettable Timer Pattern:**
+```go
+// Simple creation and reset
+timer := resettabletimer.New(ctx, 30*time.Second)
+
+// On activity - just reset
+timer.Reset(30 * time.Second)
+```
+
+The resettable timer encapsulates the cancellation and recreation logic, making timeout patterns much cleaner and easier to reason about.
+

x/resettabletimer/resettable_timer.go

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+package resettabletimer
+
+import (
+	"time"
+
+	"go.uber.org/cadence"
+	"go.uber.org/cadence/workflow"
+)
+
+type (
+	// ResettableTimer represents a timer that can be reset to restart its countdown.
+	ResettableTimer interface {
+		workflow.Future
+
+		// Reset - Cancels the current timer and starts a new one with the given duration.
+		// If the timer has already fired, Reset has no effect.
+		Reset(d time.Duration)
+	}
+
+	Timer struct {
+		ctx         workflow.Context
+		timerCtx    workflow.Context
+		cancelTimer workflow.CancelFunc
+		// This is suboptimal, but we cannot implement the internal asyncFuture interface because it is not exported. It is what it is.
+		Future   workflow.Future
+		settable workflow.Settable
+		duration time.Duration
+		isReady  bool
+	}
+)
+
+// New returns a timer that can be reset to restart its countdown. The timer becomes ready after the
+// specified duration d. The timer can be reset using timer.Reset(duration) with a new duration. This is useful for
+// implementing timeout patterns that should restart based on external events. The workflow needs to use this
+// New() instead of creating new timers repeatedly. The current timer resolution implementation is in
+// seconds and uses math.Ceil(d.Seconds()) as the duration. But is subjected to change in the future.
+func New(ctx workflow.Context, d time.Duration) *Timer {
+	rt := &Timer{
+		ctx:      ctx,
+		duration: d,
+	}
+	rt.Future, rt.settable = workflow.NewFuture(ctx)
+	rt.startTimer(d)
+	return rt
+}
+
+func (rt *Timer) startTimer(d time.Duration) {
+	rt.duration = d
+
+	if rt.cancelTimer != nil {
+		rt.cancelTimer()
+	}
+
+	rt.timerCtx, rt.cancelTimer = workflow.WithCancel(rt.ctx)
+
+	timer := workflow.NewTimer(rt.timerCtx, d)
+
+	workflow.Go(rt.ctx, func(ctx workflow.Context) {
+		err := timer.Get(ctx, nil)
+
+		if !cadence.IsCanceledError(err) && !rt.isReady {
+			rt.isReady = true
+			rt.settable.Set(nil, err)
+		}
+	})
+}
+
+// Reset - Cancels the current timer and starts a new one with the given duration.
+// If the timer has already fired, Reset has no effect.
+func (rt *Timer) Reset(d time.Duration) {
+	if !rt.isReady {
+		rt.startTimer(d)
+	}
+}