Subject: Re: new platform & S_IFGITLINK problem

From: Andreas Ericsson <[email-masked]>
Date: Tue, 04 May 2010 08:13:44 +0200

On 05/04/2010 05:52 AM, Linus Torvalds wrote:
> 
> 
> On Mon, 3 May 2010, Linus Torvalds wrote:
>> On Sat, 1 May 2010, Junio C Hamano wrote:
>>>
>>>   (3) write MODE_SYSTEM_TO_GIT() macro to convert from S_IF{REG,DIR,LNK} we
>>>       read from struct stat to the "canonical" GIT_S_IF{REG,DIR,LNK}
>>>       values; and
>>>
>>>   (4) change all the code that read mode from struct stat and use it to
>>>       first use MODE_SYSTEM_TO_GIT().
>>>
>>> Currently 'git grep -e "S_IF[A-Z]" -e "struct stat"' reports around 250
>>> hits, so it is not infeasible amount of work, but it is not a trivial and
>>> mechanical replacement, either.  I or somebody need to set aside a block
>>> of time to do this clean-up and audit the result.
>>
>> Ugh. And since nobody sane has different values from the system ones, if
>> we miss some case we'll never notice on any sane platform ;(
> 
> As to your (3) and (4), I actually think that the best option would be to
> stop using '[lf]stat()' directly for "working tree stat", and instead just
> introduce a 'git_[lf]stat()' function that just always does the conversion
> (when necessary - the "conversion" can be a no-op on sane architectures)
> for us.
> 
> Right now, a _lot_ of the functions work either directly on 'st->st_mode'
> _or_ on random index entries, and that would be a major pain if they might
> ever be in "different number domains". So the easiest way to make sure
> that we _always_ use the GIT_IFxyz numbers would be to never ever have
> anything that uses the native ones even by mistake.
> 
> I've actually wanted to have a 'git_lstat()' wrapper for other reasons: it
> would have made it _so_ much easier to do breakpoints etc when doing the
> whole name lookup optimizations.
> 
> Note: there are various cases of '[lf]stat()' being used not for working
> tree entries, but for things like the pack files etc internal git files.
> Those should _not_ do the conversion - they should use the "native" stat()
> functions. It's only the working tree accesses we should need to do any
> conversion on, since those are the ones that are relevant for the index
> (and tree) comparisons.
> 

There are other benefits to the git_[fl]stat as well. Windows people would
probably be delighted if they were introduced. We have them in libgit2 for
that precise reason, since stat() sucks arse but some fileGetInfo() call is
at least reasonably fast on windows.

So why not make even pack file access work with the git macros? After all,
the conversion is a no-op on sane platforms, and it would probably be
easier to review patches if one doesn't have to remember that we have two
stat() implementations and which one to use where. Especially when we add
utility functions like read_file_chunk() (no, it doesn't exist) that could
operate on either internal files or work-tree files.

It would also make code-stealing to libgit2 a bit easier ;)

-- 
Andreas Ericsson                   [email-masked]
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

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