asyncio.streams.FlowControlMixin should be part of the API

[xitop]

BPO	34993
Nosy	@asvetlov, @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 2019-05-27.20:18:17.036>
created_at = <Date 2018-10-15.17:27:55.153>
labels = ['3.7', 'expert-asyncio']
title = 'asyncio.streams.FlowControlMixin should be part of the API'
updated_at = <Date 2019-05-27.20:19:02.813>
user = 'https://bugs.python.org/xitop'

bugs.python.org fields:

activity = <Date 2019-05-27.20:19:02.813>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2019-05-27.20:18:17.036>
closer = 'asvetlov'
components = ['asyncio']
creation = <Date 2018-10-15.17:27:55.153>
creator = 'xitop'
dependencies = []
files = []
hgrepos = []
issue_num = 34993
keywords = []
message_count = 5.0
messages = ['327764', '327765', '327771', '343685', '343686']
nosy_count = 3.0
nosy_names = ['asvetlov', 'yselivanov', 'xitop']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue34993'
versions = ['Python 3.7']

[xitop]

This issue report is based on a SO question "How to create asyncio stream reader/writer for stdin/stdout?", its answer and comments. Link: https://stackoverflow.com/q/52089869/5378816

The key point is that two unidirectional pipes should be used the same way as one bidirectional network socket. The answer (more precisely the Linux/Unix part of it) and some following comments - both created by highly competent SO members - suggest that FlowControlMixin is a useful class required to write a proper code for this and similar usecases.

That's why the FlowControlMixin or an equivalent should be made available to asyncio programmers as a documented part of the library.

[1st1]

asvetlov: need to handle this usecase with the new API; -1 on exposing FlowControlMixin though.

[asvetlov]

1. Trio has *stappled* streams for the case, especially useful for starting TLS session over stdin/stdout.
   We can master something too.
2. FlowControlMixin was always considered a **private** API. Would be nice to add an underscore prefix to the name if not too late. IIRC the official policy is: if you need the class -- copy and paste it into your project.
3. Even a person with very many stars on StackOverflow doesn't become an asyncio expert automatically; while I very respect the sie and so on.

[asvetlov]

FlowControlMixing is a private API class.
The class is not intended for the usage outside of asyncio.
If you found it useful for you please feel free to copy-paste the class from asyncio sources into your project.

The reason is: we want to keep freedom for changing asyncio internals without breaking backward compatibility.
One of key technologies for it is keeping the public API as small as possible.

[asvetlov]

bpo-36889 deprecates using FlowXontrolMixin outside of asyncio