Re: svn:externals using git submodules

> Hello,
> 
> I've done this by hand as a proof of concept I suspect it would need loads of 
> work in git-svn to do it properly.  However, I thought I'd mention as part of 
> my "success with submodules" reports.

> ffmpeg is managed with svn; I like to track its development with git-svn.  
> Works wonderfully except for one problem: they've made use of svn:externals 
> for one component, libswscale.  Previously I just regularly updated the 
> libswscale subdirectory by checking out the latest copy (which is all that 
> subversion does) and committing it to my own branch off upstream.
> 
> With submodule support in git, it makes it possible to do a much better job.  
> What I did was have two svn-remote sections in the config:
> 
> [svn-remote "ffmpeg"]
>     url = svn://svn.mplayerhq.hu/ffmpeg
>     fetch = trunk:refs/remotes/ffmpeg-svn
> 
> [svn-remote "libswscale"]
>     url = svn://svn.mplayerhq.hu/mplayer
>     fetch = trunk/libswscale:refs/remotes/libswscale-svn
> 
> After running git-svn fetch; there are two independent branches in my 
> repository:
> 
>   -- * -- * -- * -- * -- * (ffmpeg-svn)
>   ---- * ----- * ------- * (libswscale-svn)  
> 
> Now, we fork from ffmpeg-svn and libswscale-svn to make non-tracking branches 
> that can be committed to:
> 
>  $ git checkout -b master-ffm ffmpeg-svn
>  $ git branch master-sws libswscale-svn
> 
> Next, we create a shared clone of the repository as a subdirectory in that 
> repository.
> 
>  $ git clone -s . libswscale
> 
> Now we want that clone to be even more strongly linked to the parent - to the 
> extent that they share the same refs, etc:
> 
>  $ cd libswscale
>  $ rm -rf .git/refs .git/logs .git/info description config
>  $ ln -s ../../.git/refs .git/refs
>  $ ln -s ../../.git/logs .git/logs
>  $ ln -s ../../.git/info .git/info
>  $ ln -s ../../.git/config .git/config
>  $ ln -s ../../.git/description .git/description
> 
> Only HEAD and index are independent.  Next we switch from the ffmpeg branch to 
> the libswscale branch in this subdirectory:
> 
>  $ git checkout master-sws
> 
> Now, we make the subdirectory a submodule in the parent:
> 
>  $ cd ..
>  $ git add libswscale
>  $ git commit -m "libswscale is now a submodule"
> 
> How dangerous is this?  I've made the repository it's own submodule and it 
> shares the same refs, info and logs.  LIVING ON THE EDGE MAN!
> 
> You have to run two git-svn commands to sync with upstream:
> 
>  $ git-svn fetch ffmpeg
>  $ git-svn fetch libswscale
> 
> Then of course you would merge
> 
>  $ git merge ffmpeg-svn
>  $ cd libswscale; git merge libswscale-svn; cd ..
>  $ git commit -m "Sync with upstream"
> 
> Personally I think that's pretty cool, this is significantly better than 
> svn:externals because the particular revision of libswscale in use is 
> recorded.  Seriously - someone show me another VCS that can do that - I think 
> git has actual magic powers :-)
> 
> I dare say that git-svn could do much better because it could reconstruct the 
> submodule history based on the repository dates and create the link in the 
> tracking branch rather than having to do it manually at the end as I've done 
> here.  That would mean that the recorded submodule was right for all 
> history - again, not the case for svn:externals, if you check out a previous 
> version the external remains current.
> 
> 
> 
> Andy
> -- 
> Dr Andy Parkins, M Eng (hons), MIET
> andyparkins@gmail.com
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