[PATCH STABLE] win32: backout 1a9ebc83a74c

[Adrian Buehlmann]

On 2014-05-05 21:37, Pierre-Yves David wrote:
> 
> 
> On 05/05/2014 11:56 AM, Steve Borho wrote:
>> On Sat, May 3, 2014 at 3:43 AM, Adrian Buehlmann <adrian at cadifra.com> wrote:
>>> # HG changeset patch
>>> # User Adrian Buehlmann <adrian at cadifra.com>
>>> # Date 1399106034 -7200
>>> # Branch stable
>>> # Node ID b3bceb2a103e9f279f058e7a3bacf272968b6bb6
>>> # Parent  d36440d843284ba546858b241da9cc4817811fe5
>>> win32: backout 1a9ebc83a74c
>>>
>>> This appears to cause havoc on TortoiseHg. While 1a9ebc83a74c may be nice to
>>> have (as soon as someone can prove it has no unwanted side effects), it is not
>>> worth the troubles at this point.
>>
>> To add some context; the troubles are specifically that files which
>> are being "watched" for changes by QFileSystemWatcher() are no longer
>> safely updated by util.rename().  So things like updating .hg/hgrc or
>> perhaps even the changelog are suddenly dangerous.  We're still not
>> completely certain of the exact semantics of the problem so I don't
>> know if this patch is mandatory, but we believe it would resolve the
>> problem.
> 
> Do you have actually example of bugs this changes introduce (I'm curious)

[It's nice to see you being curious about hairy dirty win32 details. :-) ]

Yep. But only together with TortoiseHg on Windows, which uses this layer
of Mercurial:

Steve lost his hgrc file on a local disk (as he reported), which is
enough evidence for me to backout 1a9ebc83a74c.

1a9ebc83a74c was the only change on that layer, which has always been a
delicate matter (Windows file access trying to emulate posix
expectations of Mercurial, combined with more or less evil virus
scanner-like environments).

I really can no longer recommend to inflict 1a9ebc83a74c on everyone.
TortoiseHg replacing win32.unlink is not really an option either, as
that would reduce usage of this part of Mercurial to an almost
negligible quantity.

It wasn't found out earlier, apparently because no one built a
TortoiseHg with 1a9ebc83a74c and used it. Steve was the guinea pig himself.

1a9ebc83a74c is a performance optimization for a non-essential use case.
Not worth the trouble, IMHO.

In fact I strongly recommend against publishing a TortoiseHg binary
containing 1a9ebc83a74c.

The other option would be to find someone who is cute enough to release
such a binary and wait for another one which either proves all works
fine or presents a theory of why 1a9ebc83a74c is harmful exactly.

I won't be neither one.

And Steve has already publicly declared that he will hold off tagging a
TortoiseHg release until this issue is resolved.

So, either we find hard evidence or we just backout.