Re: [PATCH] config.mak.uname: Cygwin: Use renames for creation

From: Johannes Schindelin <johannes.schindelin@gmx.de>
To: Adam Dinwoodie <adam@dinwoodie.org>
Cc: "brian m. carlson" <sandals@crustytoothpaste.net>,
	Mark Levedahl <mlevedahl@gmail.com>,
	git@vger.kernel.org, Eric Blake <eblake@redhat.com>,
	"Shawn O . Pearce" <spearce@spearce.org>,
	Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Subject: Re: [PATCH] config.mak.uname: Cygwin: Use renames for creation
Date: Sun, 09 Aug 2015 11:01:26 +0200	[thread overview]
Message-ID: <058a7756ada2fa5043ca9b910d6e1543@www.dscho.org> (raw)
In-Reply-To: <CA+kUOa=KRBSKDqWUj2RiO45PqVYGmN+yqG426jtUoXayxGkduw@mail.gmail.com>

Hi Adam,

On 2015-08-09 04:01, Adam Dinwoodie wrote:

> I do not see any difference between the situation here and the situation
> for MinGW, which is fundamentally a Cygwin fork, but which already has
> this build option set for it in config.mak.uname.

This is incorrect. MinGW is distinctly *not* a Cygwin fork. MinGW means "Minimal GNU on Windows" and that in turn means that it provides an environment to build executables that purely use the Win32 API. Read: no POSIX emulation whatsoever. Most notably, MinGW programs cannot use fork(2); It is simply unavailable.

What you *probably* meant is that Git for Windows relies on MSys2 for its shell and Perl scripts, and that MSys2 in turn is a fork of Cygwin. That affects *only* the scripts, though; Git itself (as in `git.exe`) is still a pure MinGW program (and as a consequence, is quite a bit faster than Cygwin Git, at the price of certain quirks that Cygwin Git does not suffer).

>> We've gotten a lot of users on the list who ask why their Git
>> directories on shared drives aren't working (or are broken in some way).
>> Since I don't use Windows, let me ask: does the Cygwin DLL handle
>> link(2) properly on shared drives, and if not, would this patch help it
>> do so?  I can imagine that perhaps SMB doesn't support the necessary
>> operations to make a POSIX link(2) work properly.
> 
> I'd need to go back to the Cygwin list to get a definite answer, but as
> I understand it, yes, this is is exactly the problem -- quoting Corinna,
> one of the Cygwin project leads, "The MS NFS is not very reliable in
> keeping up with changes to metadata."
> 
> We have verified that setting `core.createobject rename` resolves the
> problem for people who are seeing it, which very strongly implies that
> this build option would solve the problem similarly, but would fix it
> for all users, not just those who spend enough time investigating the
> problem to find that setting.

>From my experience, it appears that providing Corinna Vinschen (or better put: the Cygwin developers in general) with a sound patch gets things fixed pretty timely.

And since `core.createObject = rename` seems to work around the problem, it should be possible to patch the Cygwin runtime accordingly. Sure, it will take a little investigation *what* code should be changed, and how, but the obvious benefit to *all* Cygwin applications should make that effort worth your while.

Please note that Cygwin's source code itself is in Git now, too: https://cygwin.com/git.html

Ciao,
Johannes