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Add visual timing diagrams for InvokeLeakTest analysis
Co-authored-by: tig <585482+tig@users.noreply.github.com>
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copilot-swe-agent[bot]
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InvokeLeakTest_Timing_Diagram.md
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InvokeLeakTest_Timing_Diagram.md
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# InvokeLeakTest Timing Diagram
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## Normal Operation (Without Debugger)
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```
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Timeline (milliseconds):
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0ms 10ms 20ms 30ms 40ms 50ms 60ms 70ms 80ms 90ms 100ms
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|------|------|------|------|------|------|------|------|------|------|
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│
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│ Background Thread 1: Task.Run → Sleep(2-4ms) → Invoke()
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│ ↓
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│ Background Thread 2: Task.Run → Sleep(2-4ms) → Invoke()
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│ ↓
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│ Background Thread 3: Task.Run → Sleep(2-4ms) → Invoke()
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│ ↓
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│ [All added to _timeouts]
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│
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│ Main Loop: ──────────[Iter]───────[Iter]───────[Iter]───────[Iter]────
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│ ↓ ↓ ↓ ↓
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│ RunTimers RunTimers RunTimers RunTimers
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│ ↓ ↓ ↓ ↓
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│ Execute 0 Execute 5 Execute 10 Execute 15
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│
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│ Counter: 0 ───────→ 0 ─────→ 5 ───────→ 15 ────→ 30 ────→ 45 ─────→ 50
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│
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│ Test Check: ✓ PASS
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│ └──────────────100ms window────────────────┘
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```
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**Result**: All invocations execute within 100ms → Test passes
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---
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## Problem Scenario (With Debugger - @BDisp's Machine)
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```
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Timeline (milliseconds):
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0ms 10ms 20ms 30ms 40ms 50ms 60ms 70ms 80ms 90ms 100ms 110ms
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|------|------|------|------|------|------|------|------|------|------|------|
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│
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│ Background Threads: 500 Tasks launch rapidly
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│ ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓ (burst of invocations)
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│ All added to _timeouts within same DateTime.UtcNow tick
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│ Timestamps: T-100, T-99, T-98, T-97, ... (NudgeToUniqueKey)
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│
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│ Main Loop (SLOW due to debugger overhead):
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│ ────────────────[Iter 1]────────────────────[Iter 2]──────────────
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│ 25ms 60ms
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│ ↓ ↓
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│ RunTimers RunTimers
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│ ↓ ↓
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│ DateTime.UtcNow DateTime.UtcNow
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│ = T0 = T1
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│ ↓ ↓
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│ Check: k < now? Check: k < now?
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│ ↓ ↓
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│ ┌─────────────────┴──────────────┐ │
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│ │ Some timeouts: k >= now ! │ │ Execute some
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│ │ These are NOT executed │ │ timeouts
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│ │ Added back to _timeouts │ │
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│ └────────────────────────────────┘ ↓
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│ Counter += 300
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│ Counter: 0 ────────────────→ 0 ──────────────────────→ 300 ────────────→ 450
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│
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│ Test Check at 100ms: ✗ FAIL
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│ └──────────100ms window──────┘
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│ Counter = 300, Expected = 500
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│ Missing 200 invocations!
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│
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│ (Those 200 invocations execute later, around 110ms)
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```
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**Result**: Not all invocations execute within 100ms → TimeoutException
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---
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## The DateTime.UtcNow Resolution Problem
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```
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Real Time: 0.000ms 0.001ms 0.002ms 0.003ms ... 15.6ms 15.7ms
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│ │ │ │ │ │
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DateTime.UtcNow: T0───────────────────────────────────────→T1─────→T2
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└─────────────15.6ms tick──────────────────┘
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All invocations here get T0
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```
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**When 100 invocations happen within 15.6ms:**
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```
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Invoke #1: k = T0 - 100 ticks
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Invoke #2: k = T0 - 99 ticks (NudgeToUniqueKey increments)
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Invoke #3: k = T0 - 98 ticks
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...
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Invoke #100: k = T0 + 0 ticks (This is T0!)
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```
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**When RunTimers() checks at time T0 + 50 ticks:**
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```
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Invoke #1-50: k < now → Execute ✓
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Invoke #51-100: k >= now → NOT executed! Added back to queue ✗
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```
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---
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## Why Debugger Makes It Worse
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### Without Debugger
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```
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Main Loop Iteration Time: ~10-20ms
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│ Invoke batch: 10 tasks ────→ Execute 10 ────→ Next batch: 10 tasks
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│ 10ms 10ms
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│ Small batches processed quickly
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```
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### With Debugger
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```
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Main Loop Iteration Time: ~25-50ms (2-5x slower!)
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│ Invoke batch: 100 tasks (burst!) ────────→ Execute 50 ──→ 50 still queued
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│ 50ms ↓
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│ Need another 50ms!
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│ Large batches accumulate, processing delayed
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```
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**Effect**: More invocations queue up between iterations, increasing likelihood of timestamp collisions and race conditions.
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---
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## The Race Condition Explained
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```
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Thread Timeline:
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Background Thread Main Thread
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───────────────── ───────────
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[Call Invoke()]
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↓
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[Lock _timeoutsLockToken]
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↓
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k = DateTime.UtcNow.Ticks
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= 1000
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↓
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k -= 100 (= 900)
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↓
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[Add to _timeouts with k=900]
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↓
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[Release lock] [Lock _timeoutsLockToken]
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↓
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[Copy _timeouts]
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↓
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[Release lock]
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↓
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now = DateTime.UtcNow.Ticks
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= 850 ⚠️ (Timer hasn't updated!)
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↓
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Check: k < now?
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900 < 850? → FALSE!
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↓
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[Timeout NOT executed]
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[Added back to _timeouts]
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```
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**Problem**: Between when `k` is calculated (900) and when it's checked (now=850), the system timer hasn't updated! This can happen because:
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1. DateTime.UtcNow has coarse resolution (~15ms)
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2. Thread scheduling can cause the check to happen "early"
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3. Debugger makes timing less predictable
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---
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## Solution Comparison
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### Current: DateTime.UtcNow
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```
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Resolution: ~15.6ms (Windows), varies by platform
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Precision: Low
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Stability: Affected by system time changes
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Debugger: Timing issues
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Time: 0ms ────────────→ 15.6ms ────────────→ 31.2ms
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Reading: T0 T1 T2
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└─────All values here are T0─────┘
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```
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### Proposed: Stopwatch.GetTimestamp()
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```
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Resolution: ~1 microsecond (typical)
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Precision: High
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Stability: Not affected by system time changes
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Debugger: More reliable
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Time: 0ms → 0.001ms → 0.002ms → 0.003ms → ...
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Reading: T0 T1 T2 T3 ...
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Each reading is unique and monotonic
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```
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**Benefit**: With microsecond resolution, even 1000 rapid invocations get unique timestamps, eliminating the NudgeToUniqueKey workaround and race conditions.
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---
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## Test Scenarios
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### Scenario 1: Fast Machine, No Debugger
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```
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Iteration time: 5-10ms
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Invoke rate: 20-30/ms
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Result: ✓ PASS (plenty of time margin)
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```
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### Scenario 2: Normal Machine, No Debugger
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```
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Iteration time: 10-20ms
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Invoke rate: 10-20/ms
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Result: ✓ PASS (adequate time margin)
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```
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### Scenario 3: VM/Slow Machine, Debugger (ARM Mac)
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```
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Iteration time: 20-30ms
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Invoke rate: 15-20/ms
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Result: ✓ PASS (close but within 100ms)
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```
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### Scenario 4: @BDisp's Machine, Debugger (Intel x64 VM)
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```
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Iteration time: 30-50ms
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Invoke rate: 10-15/ms
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DateTime.UtcNow resolution: 15-20ms (VM virtualization affect)
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Result: ✗ FAIL (exceeds 100ms window)
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```
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---
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## Recommendations
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### Immediate Fix (Test-Level)
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```csharp
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// Increase tolerance for debugger scenarios
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#if DEBUG
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private const int POLL_MS = Debugger.IsAttached ? 500 : 100;
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#else
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private const int POLL_MS = 100;
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#endif
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```
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### Long-Term Fix (Production Code)
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```csharp
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// In TimedEvents.cs, replace DateTime.UtcNow with Stopwatch
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private static long GetTimestampTicks()
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{
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return Stopwatch.GetTimestamp() * (TimeSpan.TicksPerSecond / Stopwatch.Frequency);
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}
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// Use in AddTimeout:
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long k = GetTimestampTicks() + time.Ticks;
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```
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This provides microsecond resolution and eliminates the race condition entirely.
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