_ProactorReadPipeTransport pause_reading()/resume_reading() broken if called before any read is perfored

[FulvioEsposito]

BPO	26819
Nosy	@asvetlov, @1st1, @miss-islington
PRs	bpo-26819: Prevent proactor double read on resume #6921 [3.7] bpo-26819: Prevent proactor double read on resume (GH-6921) #7004 [3.6] bpo-26819: Prevent proactor double read on resume (GH-6921) #7110
Files	pause_resume_test.py: Minimal test case to reproduce the issue

^{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 2018-05-25.08:03:58.176>
created_at = <Date 2016-04-21.17:34:00.917>
labels = ['3.8', 'type-bug', '3.7', 'expert-asyncio']
title = '_ProactorReadPipeTransport pause_reading()/resume_reading() broken if called before any read is perfored'
updated_at = <Date 2018-05-25.08:03:58.176>
user = 'https://bugs.python.org/FulvioEsposito'

bugs.python.org fields:

activity = <Date 2018-05-25.08:03:58.176>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2018-05-25.08:03:58.176>
closer = 'asvetlov'
components = ['asyncio']
creation = <Date 2016-04-21.17:34:00.917>
creator = 'Fulvio Esposito'
dependencies = []
files = ['42559']
hgrepos = []
issue_num = 26819
keywords = ['patch']
message_count = 4.0
messages = ['263927', '317161', '317164', '317661']
nosy_count = 4.0
nosy_names = ['asvetlov', 'yselivanov', 'Fulvio Esposito', 'miss-islington']
pr_nums = ['6921', '7004', '7110']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue26819'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[FulvioEsposito]

Calling pause_reading()/resume_reading() on a _ProactorReadPipeTransport will result in an InvalidStateError('Result is not ready.') from a future if no read has been issued yet. The reason is that resume_reading() will schedule _loop_reading() a second time on the event loop. For example, currently aiomysql always fails to connect using a ProactorEventLoop on Windows because it calls pause_reading()/resume_reading() to set TCP_NODELAY on the socket just after connecting and before any read is performed.

[asvetlov]

New changeset 4151061 by Andrew Svetlov (CtrlZvi) in branch 'master':
bpo-26819: Prevent proactor double read on resume (bpo-6921)
4151061

[miss-islington]

New changeset 28ea38b by Miss Islington (bot) in branch '3.7':
bpo-26819: Prevent proactor double read on resume (GH-6921)
28ea38b

[asvetlov]

New changeset 749afe8 by Andrew Svetlov (CtrlZvi) in branch '3.6':
[3.6] bpo-26819: Prevent proactor double read on resume (GH-6921) (bpo-7110)
749afe8