[PATCH] wireproto: explicitly flush stdio to prevent stalls on Windows

[Matt Harbison]

> On Mar 11, 2018, at 5:43 AM, Yuya Nishihara <yuya at tcha.org> wrote:
> 
>> On Sun, 11 Mar 2018 00:04:45 -0500, Matt Harbison wrote:
>> # HG changeset patch
>> # User Matt Harbison <matt_harbison at yahoo.com>
>> # Date 1520744281 18000
>> #      Sat Mar 10 23:58:01 2018 -0500
>> # Node ID 1145671119a40e82932f63f83ad4049727416174
>> # Parent  963b4223d14fa419df2a82fbe47cd55075707b6a
>> wireproto: explicitly flush stdio to prevent stalls on Windows
>> 
>> This is the key to fixing the hangs on Windows in D2720[1].  I put flushes in a
>> bunch of other places that didn't help, but I suspect that's more a lack of test
>> coverage than anything else.
>> 
>> Chasing down stuff like this is pretty painful.  I'm wondering if we can put a
>> proxy around sys.stderr (and sys.stdout?) on Windows (only when daemonized?)
>> that will flush on every write (or at least every write with a '\n').
>> 
>> [1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-March/113352.html
>> 
>> diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
>> --- a/mercurial/debugcommands.py
>> +++ b/mercurial/debugcommands.py
>> @@ -2769,6 +2769,8 @@ def debugwireproto(ui, repo, **opts):
>> 
>>     # Now perform actions based on the parsed wire language instructions.
>>     for action, lines in blocks:
>> +        util.stdout.flush()
> 
> This one is suspicious. Can you clarify which data is flushed? I think
> fileobjectobserver should flush log per method call.

Yeah, this made no sense to me. I would have expected that it needed to be flushed on the server side.  But, here’s where it stalls without this:

https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-March/113388.html

I’ll try the fileobjectobserver flush (I think I have that already, trying to get the stderr data to show up).

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