Re: [PATCH v4 00/13] New remote-hg helper

> On Fri, Nov 2, 2012 at 5:41 PM, Felipe Contreras
> <felipe.contreras@gmail.com> wrote:
>> On Fri, Nov 2, 2012 at 3:48 PM, Jeff King <peff@peff.net> wrote:
>>> On Thu, Nov 01, 2012 at 05:08:52AM +0100, Felipe Contreras wrote:
>>>
>>>>> Turns out msysgit's remote-hg is not exporting the whole repository,
>>>>> that's why it's faster =/
>>>>
>>>> It seems the reason is that it would only export to the point where
>>>> the branch is checked out. After updating the to the tip I noticed
>>>> there was a performance difference.
>>>>
>>>> I investigated and found two reasons:
>>>>
>>>> 1) msysgit's version doesn't export files twice, I've now implemented the same
>>>> 2) msysgit's version uses a very simple algorithm to find out file changes
>>>>
>>>> This second point causes msysgit to miss some file changes. Using the
>>>> same algorithm I get the same performance, but the output is not
>>>> correct.
>>>
>>> Do you have a test case that demonstrates this? It would be helpful for
>>> reviewers, but also helpful to msysgit people if they want to fix their
>>> implementation.
>>
>> Cloning the mercurial repo:
>>
>> % hg log --stat -r 131
>> changeset:   131:c9d51742471c
>> parent:      127:44538462d3c8
>> user:        jake@edge2.net
>> date:        Sat May 21 11:35:26 2005 -0700
>> summary:     moving hgweb to mercurial subdir
>>
>>  hgweb.py           |  377
>> ------------------------------------------------------------------------------------------
>>  mercurial/hgweb.py |  377
>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 377 insertions(+), 377 deletions(-)
>>
>> % git show --stat 1f9bcfe7cc3d7af7b4533895181acd316ce172d8
>> commit 1f9bcfe7cc3d7af7b4533895181acd316ce172d8
>> Author: jake@edge2.net <none@none>
>> Date:   Sat May 21 11:35:26 2005 -0700
>>
>>     moving hgweb to mercurial subdir
>>
>>  mercurial/hgweb.py | 377
>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 377 insertions(+)
> 
> I talked with some people in #mercurial, and apparently there is a
> concept of a 'changelog' that is supposed to store these changes, but
> since the format has changed, the content of it is unreliable. That's
> not a big problem because it's used mostly for reporting purposes
> (log, query), not for doing anything reliable.

> That's what I was doing already, but I found a more efficient way to
> do it. msysGit is using the changelog, which is quite fast, but not
> reliable.
> 
> Unfortunately while going trough mercurial's code, I found an issue,
> and it turns out that 1) is not correct.
> 
> In mercurial, a file hash contains also the parent file nodes, which
> means that even if two files have the same content, they would not
> have the same hash, so there's no point in keeping track of them to
> avoid extracting the data unnecessarily, because in order to make sure
> they are different, you need to extract the data anyway, defeating the
> purpose.

> Which means mercurial doesn't really behave as one would expect:
> 
> # add files with the same content
> 
>  $ echo a > a
>   $ hg ci -Am adda
>   adding a
>   $ echo a >> a
>   $ hg ci -m changea
>   $ echo a > a
>   $ hg st --rev 0
>   $ hg ci -m reverta
>   $ hg log -G --template '{rev} {desc}\n'
>   @  2 reverta
>   |
>   o  1 changea
>   |
>   o  0 adda
> 
> # check the difference between the first and the last revision
> 
>   $ hg st --rev 0:2
>   M a
>   $ hg cat -r 0 a
>   a
>   $ hg cat -r 2 a
>   a