On Sun, 11 Mar 2018 05:43:13 -0400, Yuya Nishihara <y...@tcha.org> wrote:

On Sun, 11 Mar 2018 00:04:45 -0500, Matt Harbison wrote:
# HG changeset patch
# User Matt Harbison <matt_harbi...@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.

Yep, fileobjectobserver flushes worked without this. I've got no idea how this flush broke things free in the first place. Maybe the server was waiting for more client requests, this gave it to the server, which then wrote enough more output to flush the server buffers?
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