asynchat.async_chat and asyncore.dispatcher_with_send are not thread-safe

[ngg]

BPO	26371
Nosy	@gvanrossum, @vstinner, @1st1

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2016-02-16.20:58:05.927>
created_at = <Date 2016-02-16.20:47:24.872>
labels = ['invalid', 'type-bug', 'library', 'expert-asyncio']
title = 'asynchat.async_chat and asyncore.dispatcher_with_send are not thread-safe'
updated_at = <Date 2016-02-17.09:18:23.977>
user = 'https://bugs.python.org/ngg'

bugs.python.org fields:

activity = <Date 2016-02-17.09:18:23.977>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2016-02-16.20:58:05.927>
closer = 'gvanrossum'
components = ['Library (Lib)', 'asyncio']
creation = <Date 2016-02-16.20:47:24.872>
creator = 'ngg'
dependencies = []
files = []
hgrepos = []
issue_num = 26371
keywords = []
message_count = 7.0
messages = ['260367', '260368', '260369', '260380', '260381', '260383', '260384']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'vstinner', 'yselivanov', 'ngg']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue26371'
versions = ['Python 2.7', 'Python 3.2', 'Python 3.3', 'Python 3.4', 'Python 3.5', 'Python 3.6']

[ngg]

The initiate_send() method in both asynchat.async_chat and asyncore.dispatcher_with_send is not a thread-safe function, but it can be called from multiple threads.

For example if asyncore.loop() runs in a background thread, and the main thread sends a message then both threads will call this.

It can result in sending (part of) the message multiple times or not sending part of it.

Possible solution:
asyncore.dispatcher_with_send.out_buffer and asynchat.async_chat.producer_fifo should be guarded with a lock (it should be locked even in writable())

[vstinner]

asynchat is now deprecated in favor of asyncio.

Almost all asyncio functions are not thread-safe, it's now well documented, see the general info:
https://docs.python.org/dev/library/asyncio-dev.html#concurrency-and-multithreading

If we do something, I suggest to only touch asynchat doc to explain well that asyncore and asynchat are not thread-safe.

[gvanrossum]

I don't believe the original issue is a bug. There is nothing in the docs for asyncore or asynchat that suggests they would be thread-safe.

[ngg]

If I want to write a TCP client which communicates back and forth with the server (both parties can send messages anytime) then it would be really easy to use it the following way:
Start a background thread with asyncore.loop(), and you can send messages easily, and handle_read() will be called automatically whenever data is received.
If this usage is not supported and I understand correctly then I can only send messages before starting the loop or callbacks from the loop (handle_accept, handle_read, etc).
This seems to be a much more difficult and error-prone way (at least for pre-Future python versions)

[vstinner]

"If this usage is not supported (...)"

You can use threads, but access concurrently asyncore/asynchat from different threads. You have to build a communicate channel between your threads using thread-safe primitive like queue.Queue. asyncio has a builtin support for that: loop.call_soon_threadsafe().

Why not using asyncio instead of having to rebuild your own implementation?

[ngg]

"Why not using asyncio instead of having to rebuild your own implementation?"

The issue happened with python 2.7 and we don't want that project to depend on additional libraries.
I think we will upgrade that project to python3 soon and will probably use asyncio then.

[vstinner]

"I think we will upgrade that project to python3 soon and will probably use asyncio then."

Great :-) You may have a look at my https://pypi.python.org/pypi/sixer project, it may help :-)