Re: Preserving git history across repositories

On 12/29/13 5:30 AM, Aristotle Pagaltzis wrote:
> * James E Keenan<[email protected]>  [2013-12-28 19:15]:
>> At this point, the top-level directory merely contained the
>> directories formerly found in ext/Pod-Html/. And 'git log' and 'git
>> blame' indicated that the history had been preserved.
>
> Except, at one time it had lived in lib/ and you are completely missing
> that history. Yours effectively starts at the point where Nick moved it:
> https://github.com/jkeenan/opodhtml/commit/75e62e6c1c6203daf034df38b525a6428d419b19
> But you then have a couple of commits before that, at the beginning of
> your local history… which are completely empty.
>
>> git filter-branch --subdirectory-filter ext/Pod-Html/ -- --all
>
> So first, make sure there are no useless commits:
>
>      git filter-branch --prune-empty \
>          --subdirectory-filter ext/Pod-Html/ -- --all
>
> Next, you REALLY don’t want --all, which makes some 63 THOUSAND commits
> that will take forEVER to process (hours). You want to look at only the
> commits that touch relevant paths, which is some 530 total. On an SSD
> that can be index-filtered in half a minute. MUCH better.
>
>      git filter-branch --prune-empty \
>          --subdirectory-filter ext/Pod-Html/ -- -- lib/Pod ext/Pod-Html
>
> Note the double `--` – that is not a typo, the first `--` is for telling
> git-filter-branch that the rest of the arguments are for git-rev-list,
> so the second one gets passed through to git-rev-list, which takes it to
> mean that only paths follow.
>
> Next, since unfortunately --subdirectory-filter cannot extract multiple
> directories at once, this job will need --index-filter.
>
>      git filter-branch --prune-empty --index-filter '
>          git rm --cached -r -q -- . ;
>          git reset -q $GIT_COMMIT -- ext/Pod-Html/ lib/Pod/
>      ' -- -- ext/Pod-Html/ lib/Pod/
>
> The first line will clear out the index entirely. The next line restores
> the relevant directories from the original commit undergoing rewriting.
>
> Now comes the hard part, because lib/Pod/ alone is both too much (there
> have been a number of other modules in there over time) as well as too
> little (the relevant files used to be strewn all over the place before
> there were consolidated into ext/Pod-Html/). This requires sleuthing.
> I started with a full clone of perl5.git and did
>
>      git log --name-status --full-diff -- ext/Pod-Html | egrep ^R
>
> to find out all the files that were ever moved into ext/Pod-Html from
> elsewhere:
>
>      lib/Pod/Html.pm
>      lib/Pod/t/eol.t
>      lib/Pod/t/htmlescp.pod
>      lib/Pod/t/htmlescp.t
>      lib/Pod/t/htmllink.pod
>      lib/Pod/t/htmllink.t
>      lib/Pod/t/htmlview.pod
>      lib/Pod/t/htmlview.t
>      lib/Pod/t/pod2html-lib.pl
>      pod/pod2html.PL
>
> That’s not necessarily sufficient since those files may have chequered
> histories of their own that may need tracking. E.g. it turns out that
> pod/pod2html.PL had a predecessor called pod/pod2html.SH very early on,
> which had been called pod/pod2html before even that. Are they relevant?
> Maybe. In this case it turns out the answer is yes: they were not actual
> shell scripts, but wrappers that generated a Perl pod2html script whose
> code became the installed pod2html became the module plus stub script…
> so you don’t want to miss them.
>
> The other files turn out to be boring and obvious. (Thankfully!)
>
> A potential complication is that Pod::Html used to load Pod::Functions.
> But it turns out that it never actually used anything from that module…
> as far as I can tell. So I’d take the easy way out: simply ignore that.
>
> In the final analysis, you get this:
>
>      export L='ext/Pod-Html/ lib/Pod/Html.pm ...' # all the files listed above
>      git filter-branch --prune-empty --index-filter '
>          git rm --cached -r -q -- . ;
>          git reset -q $GIT_COMMIT -- $L
>      ' -- -- $L
>
> This extracts 287 commits from perl5.git, including the far beginning of
> history. It leaves everything in the subdirectories it was in while it
> was in perl5.git, but I figure that’s better here since the history of
> splits and moves among files becomes unintelligible otherwise.
>
> I’ve put up the result:
>
>      https://github.com/ap/opodhtml
>
> Feel free to clone that as a basis for the rest of your work.
>
> I figure a commit that moves ext/Pod-Html/* to the root of the repo is
> a clean cut to document “today begins the rest of life for this module”.
> I have not done this, figuring I’ll leave it to you to do the honours.
>
> (I want to get rid of that repo once it has served its purpose so please
> let me know either way. If you do choose to use it, ideally, do not fork
> it on GitHub (since that would record it as a fork), just git-clone it,
> change the origin URL in to your existing GH repo, and force-push.)
>

Aristotle,

Thanks for the git work; it's years beyond my own git fu and that of 
almost everyone I know.

Can you check out that the 'master' branch at 
https://github.com/jkeenan/opodhtml has the correct history?

Thank you very much.
Jim Keenan