The --limit-concurrency seems to be off by one (specifying 1 means 0; specifying 2, means 1)

[anton-daneyko-ultramarin]

The --limit-concurrency seems to be off by one (specifying 1 means 0; specifying 2, means 1). Here's a repro:

# Save to sleep_app_handler.py 
import asyncio
import json


class SleepApp:
    async def __call__(self, scope, receive, send):
        # Ensure it is handling an HTTP request
        if scope["type"] != "http":
            await self.send_response(send, 500, {"error": "Unsupported scope type"})
            return

        method = scope["method"]
        path = scope["path"]

        if method == "GET" and path.startswith("/sleep/"):
            try:
                # Extract seconds from the path (e.g., /sleep/5)
                seconds = float(path.split("/")[-1])

                # Validate seconds (ensure it's not negative)
                if seconds < 0:
                    await self.send_response(send, 400, {"error": "Seconds cannot be negative"})
                    return

                # Perform asynchronous sleep
                await asyncio.sleep(seconds)

                # Return a success message
                await self.send_response(send, 200, {"message": f"Slept for {seconds} seconds"})
            except ValueError:
                await self.send_response(send, 400, {"error": "Invalid seconds parameter"})
        else:
            # Handle unknown paths
            await self.send_response(send, 404, {"error": "Not found"})

    @staticmethod
    async def send_response(send, status, body: dict):
        body_bytes = json.dumps(body).encode("utf-8")
        # Send the response start information
        await send({
            "type": "http.response.start",
            "status": status,
            "headers": [
                (b"content-type", b"application/json"),
                (b"content-length", str(len(body_bytes)).encode("utf-8")),
            ],
        })
        # Send the response body
        await send({
            "type": "http.response.body",
            "body": body_bytes,
        })


app = SleepApp()

Then run it with uvicorn sleep_app_handler:app --host 127.0.0.1 --port 8000 --limit-concurrency 1
Observe that curl http://127.0.0.1:8000/sleep/1 always returns HTTP 503. I checked different --limit-concurrency and I see off by one behavior. I see this has been already pointed out (on Dec 29, 2023) in #2074 (comment).