Re: [gentoo-dev] [PATCH] multiprocessing.eclass: work around Cygwin FIFO shortcoming

[Zac Medico]

On 01/13/2017 02:18 AM, Michael Haubenwallner wrote:
> Hi,
> 
> same patch is about to be applied to portage:
> https://github.com/gentoo/portage/pull/86
> 
> Thanks for comments!
> /haubi/
> 

LGTM.

I've merged the corresponding portage patch (tested on both Linux and
FreeBSD):

https://gitweb.gentoo.org/proj/portage.git/commit/?id=5a53e732efa0ee7e2a3f8afe90d729212ff187fb
-- 
Thanks,
Zac

[gentoo-dev] [PATCH] multiprocessing.eclass: work around Cygwin FIFO shortcoming

[Michael Haubenwallner]

Hi,

same patch is about to be applied to portage:
https://github.com/gentoo/portage/pull/86

Thanks for comments!
/haubi/

>From dc80b87a87f407cece9abad81a5369e2ec5ae2a5 Mon Sep 17 00:00:00 2001
From: Michael Haubenwallner 
Date: Wed, 27 Apr 2016 15:21:52 +
Subject: [PATCH] multiprocessing.eclass: work around Cygwin FIFO shortcoming

Cygwin does not support multiple read-handles for one FIFO (yet).
As we really need just one readonly- and one writeonly-handle, we can
reorder to open one single readwrite- and one writeonly-handle.

X-Gentoo-Bug: 583962
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=583962
---
 eclass/multiprocessing.eclass | 9 +++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/eclass/multiprocessing.eclass b/eclass/multiprocessing.eclass
index 70ca475..67f7e2d 100644
--- a/eclass/multiprocessing.eclass
+++ b/eclass/multiprocessing.eclass
@@ -139,11 +139,16 @@ multijob_init() {
 
 	# Setup a pipe for children to write their pids to when they finish.
 	# We have to allocate two fd's because POSIX has undefined behavior
-	# when you open a FIFO for simultaneous read/write. #487056
+	# when using one single fd for both read and write. #487056
+	# However, opening an fd for read or write only will block until the
+	# opposite end is opened as well. Thus we open the first fd for both
+	# read and write to not block ourselve, but use it for reading only.
+	# The second fd really is opened for write only, as Cygwin supports
+	# just one single read fd per FIFO. #583962
 	local pipe="${T}/multijob.pipe"
 	mkfifo -m 600 "${pipe}"
-	redirect_alloc_fd mj_write_fd "${pipe}"
 	redirect_alloc_fd mj_read_fd "${pipe}"
+	redirect_alloc_fd mj_write_fd "${pipe}" '>'
 	rm -f "${pipe}"
 
 	# See how many children we can fork based on the user's settings.
-- 
2.7.3