New asyncio streams API

[asvetlov]

BPO	34634
Nosy	@asvetlov, @tirkarthi
PRs	bpo-34634: [WIP]New streams API #9183
Superseder	bpo-36889: Merge StreamWriter and StreamReader into just asyncio.Stream

^{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 2019-09-11.17:54:00.950>
created_at = <Date 2018-09-11.21:18:02.123>
labels = []
title = 'New asyncio streams API'
updated_at = <Date 2019-09-11.17:54:00.950>
user = 'https://github.com/asvetlov'

bugs.python.org fields:

activity = <Date 2019-09-11.17:54:00.950>
actor = 'xtreak'
assignee = 'none'
closed = True
closed_date = <Date 2019-09-11.17:54:00.950>
closer = 'xtreak'
components = []
creation = <Date 2018-09-11.21:18:02.123>
creator = 'asvetlov'
dependencies = []
files = []
hgrepos = []
issue_num = 34634
keywords = ['patch']
message_count = 3.0
messages = ['325064', '351974', '352003']
nosy_count = 2.0
nosy_names = ['asvetlov', 'xtreak']
pr_nums = ['9183']
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '36889'
type = None
url = 'https://bugs.python.org/issue34634'
versions = []

[asvetlov]

Currently, we have separate classes: reader and writer.

It is very inconvenient: these classes are tightly coupled internally, there is no sense to have two objects to the single logical entity.

The second problem is writer.write() synchronous API. To support flow control user should call await writer.drain() after every writer.write() call. But he/she can forget about drain() easy.

writer.write() should be softly deprecated in favor of await writer.send(). writer.writelines() is not very useful, better to merge all buffers before await stream.send(data) call. writelines should be softly deprecated as well without async replacement.

The last issue is writer.close() which should always be used in conjunction with await writer.wait_closed(). Let's return a future from writer.close(). The encouraged usage becomes await writer.close().
To immediate closing let's add writer.abort() sync function which cleans up resources without waiting for actual closing.

[tirkarthi]

Streams API is implemented as part of bpo-36889 in 3.8. I think we can close this as a duplicate. Thanks.

[asvetlov]

Yes, please do