[BUG] Pull command on Windows: we have 2 problems

[Aaron Bentley]

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John A Meinel wrote:
> I think the basic problem is that there are 2 branches that are being
> grabbed. The merge code is creating its own Branch object, rather than
> re-using the one used by "pull".

Okay, so these are the sort of locking composition problems that
originally led me to suggest a system of lock singletons back when we
were first discussing it.

> Here are possible fixes:
> 
> 1) Change the merge interface to take a Branch object, instead of just a
> path
> 2) Make sure the pull code has destroyed its Branch object before
> calling merge (this is difficult, because I believe it wants to use it
> afterwards, though it could always just grab it again).

I see those as workarounds.  I'd add
3) change Branch so that read-locked operations will work when Branch is
write-locked.

> I think the basic difference is that on Linux, if you already have a
> write lock on a file, it silently ignores the request for a read lock.

We can emulate that by factoring out the lock into a LocationLock
object, and keeping a global dict of LocationLocks.  I think factoring
out LocationLock would be a positive step, no matter what.

> Probably Aaron should do the changes so that the merge is done using an
> actual Branch object, rather than a path.
> I know "merge.merge()" is designed for easy command line interpretation,
> and there is a different function for internal bzr code. 

Actually, pull's already using that internal function.  The problem is
in there.

But the attached patch will probably fix it.

Aaron
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