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added adjusted_branches design doc, meets at least 2 different needs

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Joey Hess 2016-02-09 12:30:29 -04:00
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doc/design/adjusted_branches.mdwn Normal file
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Consider two use cases:
1. Using a v6 repo with locked files on a crippled filesystem not
supporting symlinks. For the files to be usable, they need to be
unlocked. But, the user may not want to unlock the files everywhere,
just on this one crippled system.
2. [[todo/hide_missing_files]]
Both of these could be met by making `git-annex sync` maintain an adjusted
version of the HEAD branch. This could be a detached HEAD or a branch named
eg `adjusted/master`.
There would be a filter function. For #1 above it would simply convert all
annex symlinks to annex file pointers. For #2 above it would omit files
whose content is not currently in the annex.
When merging changes from a remote, apply the filter to the head of the
remote branch, and merge the result.
When objects are added/removed from the annex, the associated file has to
be looked up, and the filter applied to it. So, dropping a file with the
missing file filter would cause it to be removed from the adjusted branch,
and receiving a file's content would cause it to appear in the adjusted
branch.
When committing changes, the filter has to be reversed somehow. The
commit is fed through the reversing filter to get a commit that should be
the same as if the branch had not been adjusted. This needs to be done by
the pre-commit hook, so that the user can run `git commit`, if possible.
It would be annoying if only `git-annex sync` made such commits.
The reverse-filtered commit becomes the new tip of the master branch, and
so can be pushed out to remotes. The adjusted/master branch is not pushed
to remotes. `git-annex sync` should automatically push master when
adjusted/master is checked out.
Note that reversing filter #1 would mean only converting pointer files to
symlinks when the file was originally a symlink. This is problimatic when a
file is renamed. Would it be ok, if foo is renamed to bar and bar is
committed, for it to be committed as an unlocked file, even if foo was
originally locked?
## problems
Using `git checkout` when in an adjusted branch is problimatic, because a
non-adjusted branch would then be checked out. But, we can just say, if
you want to get into an adjusted branch, you have to run some command.
Tags are bit of a problem. If the user tags an ajusted branch, the tag
includes the local adjustments.
Running `git push` when in adjusted/master won't push anything
(with "matching" push strategy). Users may find that surprising.
Users of `git-annex sync` won't need to worry about it though.

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## ahead
* [[design/caching_database]] for metadata views, direct mode mappings
* [[adjusted_branches]]
* [[design/caching_database]] for metadata views
* [[assistant/deltas]]
* [[assistant/gpgkeys]] management for the assistant
* [[assistant/telehash]] or similar
* [[design/requests_routing]]
* [[design/v6]] repo format
* [[todo/deferred_update_mode]]
* [[use smudge and clean filters|todo/smudge]] like Git LFS
## the rearview

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doc/todo/deferred_update_mode.mdwn
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@ -26,3 +26,8 @@ annex.merge=trydownloadfirst and annex.merge=reqdownloadfirst
Let `git annex merge` be used to force a merge, even when content is not
available.
> I think this is the wrong solution, and [[hide_missing_files]] is a
> better approach to the same problem. See [[design/adjusted_branches]].
>
> Closing as dup. [[done]] --[[Joey]]