Testing: Missing concurrency stress tests for edge cases

[tanbro]

Severity

High

Location

File: tests/test_threads.py and general test coverage

Description

While there are basic concurrency tests (test_two_threads, test_pool_map), there are no stress tests for critical edge cases:

1. Cache stampede scenarios: Multiple concurrent requests with same arguments hitting cache miss
2. High contention on single keys: Many threads accessing same cache key simultaneously
3. Redis connection failures: Cache behavior during network issues
4. Concurrent eviction during writes: Race conditions during maxsize eviction

Impact

The documented race condition (README.md:1029-1030) is not tested or demonstrated, making it difficult to verify fixes.

Proposed Tests

async def test_cache_stampede_concurrent_misses():
    \"\"\"Test that concurrent misses don't all execute the function.\"\"\"
    cache = RedisFuncCache("stampede", LruTPolicy(), ...)
    
    call_count = 0
    @cache
    async def expensive_func(x):
        nonlocal call_count
        call_count += 1
        await asyncio.sleep(0.5)  # Simulate slow operation
        return x
    
    # 10 concurrent calls with same arguments
    results = await asyncio.gather(*[expensive_func(42) for _ in range(10)])
    
    # With proper locking, call_count should be ~1-2, not 10
    assert call_count < 5  # Allow some tolerance

async def test_high_contention_single_key():
    \"\"\"Test many concurrent accesses to the same key.\"\"\"
    # 100+ concurrent operations on single cache entry
    
async def test_redis_connection_failure():
    \"\"\"Test cache behavior when Redis becomes unavailable.\"\"\"
    # Should fail gracefully or use fallback
    
async def test_concurrent_eviction():
    \"\"\"Test concurrent writes during maxsize eviction.\"\"\"
    # Multiple threads triggering eviction simultaneously

Additional Context

• Use asyncio.gather() or ThreadPoolExecutor for concurrency
• Use slower functions to expose stampede vulnerability
• Consider using pytest-asyncio for async test support
• Tests should validate both correctness and performance