Condition-Based Waiting

Overview

Flaky tests often guess at timing with arbitrary delays. This creates race conditions where tests pass on fast machines but fail under load or in CI.

Core principle: Wait for the actual condition you care about, not a guess about how long it takes.

When to Use

dot

digraph when_to_use {
    "Test uses setTimeout/sleep?" [shape=diamond];
    "Testing timing behavior?" [shape=diamond];
    "Document WHY timeout needed" [shape=box];
    "Use condition-based waiting" [shape=box];

    "Test uses setTimeout/sleep?" -> "Testing timing behavior?" [label="yes"];
    "Testing timing behavior?" -> "Document WHY timeout needed" [label="yes"];
    "Testing timing behavior?" -> "Use condition-based waiting" [label="no"];
}

Use when:

•Tests have arbitrary delays (setTimeout, sleep, time.sleep())
•Tests are flaky (pass sometimes, fail under load)
•Tests timeout when run in parallel
•Waiting for async operations to complete

Don't use when:

•Testing actual timing behavior (debounce, throttle intervals)
•Always document WHY if using arbitrary timeout

Core Pattern

csharp

// ❌ BEFORE: Guessing at timing
await Task.Delay(50);
var result = GetResult();
Assert.NotNull(result);

// ✅ AFTER: Waiting for condition
await WaitForAsync(() => GetResult() != null);
var result = GetResult();
Assert.NotNull(result);

Quick Patterns

Scenario	Pattern
Wait for event	`await WaitForAsync(() => events.Any(e => e.Type == EventType.Done))`
Wait for state	`await WaitForAsync(() => machine.State == MachineState.Ready)`
Wait for count	`await WaitForAsync(() => items.Count >= 5)`
Wait for file	`await WaitForAsync(() => File.Exists(path))`
Complex condition	`await WaitForAsync(() => obj.IsReady && obj.Value > 10)`

Implementation

Generic polling function:

csharp

public static async Task<T> WaitForAsync<T>(
    Func<T?> condition,
    string description,
    int timeoutMs = 5000,
    int pollIntervalMs = 10,
    CancellationToken cancellationToken = default)
    where T : class
{
    var startTime = DateTime.UtcNow;
    
    while (!cancellationToken.IsCancellationRequested)
    {
        var result = condition();
        if (result != null) return result;
        
        if ((DateTime.UtcNow - startTime).TotalMilliseconds > timeoutMs)
        {
            throw new TimeoutException($"Timeout waiting for {description} after {timeoutMs}ms");
        }
        
        await Task.Delay(pollIntervalMs, cancellationToken);
    }
    
    throw new OperationCanceledException();
}

// Overload for boolean conditions
public static async Task WaitForAsync(
    Func<bool> condition,
    string description,
    int timeoutMs = 5000,
    int pollIntervalMs = 10,
    CancellationToken cancellationToken = default)
{
    await WaitForAsync(() => condition() ? true : null, description, timeoutMs, pollIntervalMs, cancellationToken);
}

See @example.cs for complete implementation with domain-specific helpers (WaitForEventAsync, WaitForEventCountAsync, WaitForEventMatchAsync) from actual debugging session.

Common Mistakes

❌ Polling too fast: Task.Delay(1) - wastes CPU ✅ Fix: Poll every 10ms

❌ No timeout: Loop forever if condition never met ✅ Fix: Always include timeout with clear error

❌ Stale data: Cache state before loop ✅ Fix: Call getter inside loop for fresh data

When Arbitrary Timeout IS Correct

csharp

// Tool ticks every 100ms - need 2 ticks to verify partial output
await WaitForEventAsync(manager, ToolEventType.Started); // First: wait for condition
await Task.Delay(200);   // Then: wait for timed behavior
// 200ms = 2 ticks at 100ms intervals - documented and justified

Requirements:

•First wait for triggering condition
•Based on known timing (not guessing)
•Comment explaining WHY

Real-World Impact

From debugging session (2025-10-03):

•Fixed 15 flaky tests across 3 files
•Pass rate: 60% → 100%
•Execution time: 40% faster
•No more race conditions