RSTR-PERF-202 — `time.sleep()` inside an `async def`

Summary

time.sleep() is a blocking call. Inside an async def, it blocks the entire event loop for the duration — every other coroutine that should be making progress (HTTP clients, DB pools, background tasks) stops as well. The cooperative-scheduler contract requires every "wait" to go through await.

Severity

High. The bug looks innocuous and the unit test still passes, but production throughput collapses because the loop spends N×sleep_time not doing anything.

Languages

Python.

What rastray flags

async def poll():
    while True:
        check_status()
        time.sleep(5)                    # ← flagged

What rastray deliberately does not flag

time.sleep() in regular def functions (synchronous code; fine).
await asyncio.sleep(...) — the correct form.
await trio.sleep(...), await anyio.sleep(...).

How to fix it

Replace with asyncio.sleep:

import asyncio

async def poll():
    while True:
        check_status()
        await asyncio.sleep(5)

If the sleep is buried in a synchronous library call you cannot modify, run that call in a thread executor so it doesn't block the loop:

import asyncio

async def call_blocking():
    loop = asyncio.get_running_loop()
    return await loop.run_in_executor(None, blocking_thing)

rastray rules