Re: git 0.99.7b doesn't build on Cygwin

[Davide Libenzi <davidel@xmailserver.org>]

On Fri, 23 Sep 2005, Linus Torvalds wrote:

>
>
> On Fri, 23 Sep 2005, Johannes Schindelin wrote:
>>
>> It seems that the fixup of the mmap()ed regions after a fork() does not
>> work properly in cygwin. Remember that cygwin just wraps the non-POSIX
>> Win32API and tries to make it sort of POSIX compliant. The problem is that
>> Win32API lacks a proper fork(). This is therefore emulated, and after
>> that, all the mmap()ed regions have to be mapped again. That fails.
>
> Now, I'm not a big fan of windows ("No, really? Tell us more!") but I'd
> actually like it if the _core_ git stuff worked in as wide a variety of
> situations as possible. Screw the shell scripts and the daemon or
> secondary things like that which windows users might as well generate
> their own stuff for, but I'd hope the really core stuff would work.
>
> If I understood correctly, you said that "git-diff-tree" doesn't work due
> to the fork/mmap issue. Now, I assume that means that it's the builtin
> diff that has problems.

Stay away from Cygwin if you use extensively fork(), since it becomes dog 
slow (the fork() implementation on Cygwin involves creating a new 
suspended task, *copy* - not COW - the *whole* VM space to the child, 
and resuming it). Actually, the whole Cygwin in general is dog slow, 
especially on FS operations (and git likes them). If you really like to 
have Windows support (uuu hoo WinTorvalds) you might be better using a 
small compat layer (should be fairly small for git).



> As far as I can tell, we can solve that two ways:
>
> - make Windows always use the external diff program. That may be the
>   right thing to do, since then the fork() just turns into a regular
>   fork+exec, which is how windows works anyway.
>
> - look at doing the diff internally.
>
> I'm wondering if there is some stupid way to turn a diff generated by
> diff_delta() into a line-based one? If you have the original file and the
> xdiff, I think we should be able to just walk the original file and output
> a unified diff.
>
> Davide, maybe I'm being stupid, but I'm thinking that it might be possible
> to generate a -u3 diff by basically walking the xdiff file in a linear
> fashion: if the edits are in strictly ascending order, we could walk the
> original file one line at a time, and keeping a buffer of the three last
> lines. Then, when the file offset hits the next "edit" in the xdiff, we
> start generating a line-based diff (and use the previous three lines as
> the context).
>
> Does that sound possible? Maybe somebody has even done it? Is it a stupid
> idea?
>
> I realize that it might not generate the same diff as GNU diff would do,
> and maybe it's really nasty, but it sounds like it _could_ be a "cheap"
> way of generating diffs, considering that we have something that already
> generates xdiffs..

Hehe, the same library from where Nicolas lifted the code for the binary 
diff, has a totally portable diff/patch APIs (on top of xdiff/xpach):

http://www.xmailserver.org/xdiff.html

Generating text diffs, unfortunately is quite more complex than binary 
ones. Libxdiff uses the same algorithm of GNU diff (Eugene W. Myers). The 
library has zero dependency other than ANSI C. Another alternative, IIRC 
someone made a library by wrapping the diffutil stuff, but I do not 
remeber where it was.



- Davide