New submission from John Snow :
Hi, asyncio.create_unix_server appears to treat the "backlog" parameter as
where 0 means that *no connection will ever possibly be pending*, which (at the
very least for UNIX sockets on my machine) is untrue.
Consider a (non-asyncio) server:
```python
import os, socket, sys, time
sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sock.bind('test.sock')
sock.listen(backlog=0)
while True:
print('.', end='', file=sys.stderr)
time.sleep(1)
```
This server never calls accept(), and uses a backlog of zero. However, a client
can actually still successfully call connect against such a server:
```python
import os, socket, time
sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sock.setblocking(False)
sock.connect('test.sock')
print("Connected!")
```
When run against the server example, the first invocation of this client will
actually connect successfully (Surprising, but that's how the C syscalls work
too, so... alright) but the second invocation of this client will raise
BlockingIOError (EAGAIN).
Further, if we amend the first server example to actually call accept(), it
will succeed when the first client connects -- demonstrating that the actual
total queue length here was actually effectively 1, not 0.
(i.e. there's always room for at least one connection to be considered, and the
backlog counts everybody else.)
However, in asyncio.BaseSelectorEventLoop._accept_connection(...), the code
uses `for _ in range(backlog)` to determine the maximum number of accept calls
to make. When backlog is set to zero, this means we will *never* call accept,
even when there are pending connections.
Note that when backlog=1, this actually allows for *two* pending connections
before clients are rejected, but this loop will only fire once. This behavior
is surprising, because backlog==0 means we'll accept no clients, but backlog==1
means we will allow for two to enqueue before accepting both. There is
seemingly no way with asyncio to actually specify "Exactly one pending
connection".
I think this loop should be amended to reflect the actual truth of the backlog
parameter, and it should iterate over `backlog + 1`. This does necessitate a
change to `Lib/test/test_asyncio/test_selector_events.py` which believes that
backlog=100 means that accept() should be called 100 times (instead of 101.)
A (very) simple fix is attached here; if it seems sound, I can spin a real PR
on GitHub.
--
components: asyncio
files: issue.patch
keywords: patch
messages: 413025
nosy: asvetlov, jnsnow, yselivanov
priority: normal
severity: normal
status: open
title: asyncio.create_unix_server has an off-by-one error concerning the
backlog parameter
type: behavior
versions: Python 3.10, Python 3.11, Python 3.7, Python 3.8, Python 3.9
Added file: https://bugs.python.org/file50618/issue.patch
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