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fix master push overwrite race when updating adjusted branch, by maintaining basis ref

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Joey Hess 2016-04-09 14:12:25 -04:00
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doc/design/adjusted_branches.mdwn
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@ -256,6 +256,28 @@ in [[todo/deferred_update_mode]]. Arguably, it's just as confusing for the
file to remain visible but have its content temporarily replaced with a
annex pointer.
### master push overwrite race (fixed)
There are potentially races in code that assumes a branch like
master is not being changed by someone else.
In particular, if propigateAdjustedCommits rebases the adjusted branch on
top of master. That is called by sync. The assumption is that any changes
in master have already been handled by updateAdjustedBranch. But, if
another remote pushed a new master at just the right time, the adjusted
branch could be rebased on top of a master that it doesn't incorporate,
which is wrong.
Best fix seems to be to maintain a basis ref, that is not a branch,
like refs/adjusted/master(unlocked). Copy master's ref to it when
entering the view branch. Then, make all adjustments via the basis
ref, and propigate back to refs/heads/master.
It's fine to overwrite changes that were pushed to master when
propigating from the adjusted branch. Synced changes also go to
synced/master so won't be lost. Pushes not made using git-annex sync
of master are not really desired, just a possibility.
## integration with view branches
Entering a view from an adjusted branch should probably carry the adjusting
@ -279,21 +301,3 @@ into adjusted view worktrees.]
will make copies of the content of annexed files, so this would need
to checkout the adjusted branch some other way. Maybe generalize so this
more efficient checkout is available as a git-annex command?
* There are potentially races in code that assumes a branch like
master is not being changed by someone else.
In particular, propigateAdjustedCommits rebases the adjusted branch on
top of master. That is called by sync. The assumption is that any changes
in master have already been handled by updateAdjustedBranch. But, if
another remote pushed a new master at just the right time, the adjusted
branch could be rebased on top of a master that it doesn't incorporate,
which is wrong.
Best fix seems to be to use a hidden ref, like refs/annex/adjusted/master
and copy master's ref to it when entering the view branch. Then, make
all adjustments via that ref, and propigate back to refs/heads/master.
It's fine to overwrite changes that were pushed to master when
propigating from the adjusted branch. Synced changes also go to
synced/master so won't be lost. Pushes not made using git-annex sync
of master are not really desired, just a possibility.