other forms of LockDir: changing instantiation

[Aaron Bentley]

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Scott Parish wrote:
> I'm working on a new transport for use with bzr (amazon's s3) which
> doesn't have an atomic rename--i'm going to have to make a new
> implementation of LockDir.  Unfortunately there's no good way to 
> plug-in new versions of LockDir.

In my understanding, LockDir doesn't require atomic rename.  It works on
Windows, after all.

There's a reason why we use the use the same form of locking on every
dumb transport, even when faster alternatives are available: a given
directory may be provided over multiple transports.  And if two locking
regimes are in use, two locks can be taken at the same time.

> Right now LockDir is instantiated like:
> 
>    lockdir.LockDir(transport, name, file_modebits=, dir_modebits=)
> 
> My proposal is to change this to:
> 
>    transport.get_lockdir(name, file_modebits=, dir_modebits=)

If you are certain you can't implement lockdir locking on s3, then I
think you need to implement a new bzrdir format.  That way, downlevel
clients cannot accidentally access files using the wron locking regime.

Aaron
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