condition-based-waiting

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

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

// ❌ BEFORE: Guessing at timing
await new Promise(r => setTimeout(r, 50));
const result = getResult();
expect(result).toBeDefined();

// ✅ AFTER: Waiting for condition
await waitFor(() => getResult() !== undefined);
const result = getResult();
expect(result).toBeDefined();

Quick Patterns

Scenario	Pattern
Wait for event	waitFor(() => events.find(e => e.type === 'DONE'))
Wait for state	waitFor(() => machine.state === 'ready')
Wait for count	waitFor(() => items.length >= 5)
Wait for file	waitFor(() => fs.existsSync(path))
Complex condition	waitFor(() => obj.ready && obj.value > 10)

Implementation

async function waitFor<T>(
  condition: () => T | undefined | null | false,
  description: string,
  timeoutMs = 5000
): Promise<T> {
  const startTime = Date.now();

  while (true) {
    const result = condition();
    if (result) return result;

    if (Date.now() - startTime > timeoutMs) {
      throw new Error(`Timeout waiting for ${description} after ${timeoutMs}ms`);
    }

    await new Promise(r => setTimeout(r, 10)); // Poll every 10ms
  }
}

Python Implementation

import time

def wait_for(condition, description, timeout=5.0, poll_interval=0.01):
    start = time.time()
    while True:
        result = condition()
        if result:
            return result
        if time.time() - start > timeout:
            raise TimeoutError(f"Timeout waiting for {description}")
        time.sleep(poll_interval)

Common Mistakes

Mistake	Fix
Polling too fast (1ms)	Poll every 10ms
No timeout	Always include timeout with clear error
Stale data (cache before loop)	Call getter inside loop for fresh data
Vague error message	Include what was expected

When Arbitrary Timeout IS Correct

// Tool ticks every 100ms - need 2 ticks to verify partial output
await waitForEvent(manager, 'TOOL_STARTED'); // First: wait for condition
await new Promise(r => setTimeout(r, 200));   // Then: wait for timed behavior
// 200ms = 2 ticks at 100ms intervals - documented and justified

Requirements:

1. First wait for triggering condition
2. Based on known timing (not guessing)
3. Comment explaining WHY

Real-World Impact

From debugging session:

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