Merge lp:~spiv/bzr/push-relock into lp:bzr

This won't work with older bzr formats that use a mutex across all processes
on write locks. I don't know if that matters, but it should be at least
documented, I think. Other than that it seems that perhaps its worth doing
an api to upgrade to a write lock?

Hmm. I see what you mean (about older formats). I suppose the thing to
do is to catch LockContention, and if it occurs emit a warning to the
user that the push location could not be set due lock contention, and
then let the rest of the code proceed as normal. I think even for older
formats that's probably an improvement in behaviour: I'd rather the
overall push worked immediately even if bzr can't remember the location
because the branch is locked.

I'd love an API to upgrade to a write lock — and I think we'll need one
if we ever decide to pursue a zero relocks as a goal. But it looks like
a moderately large chunk of work. Maybe I'm wrong?

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Andrew Bennetts wrote:
> Hmm. I see what you mean (about older formats). I suppose the thing to
> do is to catch LockContention, and if it occurs emit a warning to the
> user that the push location could not be set due lock contention, and
> then let the rest of the code proceed as normal. I think even for older
> formats that's probably an improvement in behaviour: I'd rather the
> overall push worked immediately even if bzr can't remember the location
> because the branch is locked.
>
> I'd love an API to upgrade to a write lock — and I think we'll need one
> if we ever decide to pursue a zero relocks as a goal. But it looks like
> a moderately large chunk of work. Maybe I'm wrong?

Well, we already have *some* of that in the code today. Because Dirstate
can be updated within a logical 'read' lock. (status updating the hash
cache). So it has some "try to switch to a write lock" code.

I don't know what that would imply for something like Branch, though.
For Repository, it is trivial, because a write-lock is *still* a no-op
until you 'commit_write_group'. Branch and WT need to actually do
something to write-lock, though.

And we'd also need to decide what to do when the write-lock is already
taken.

John
=:->

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