branch locking mk2.

[Robert Collins]

On Wed, 2006-02-08 at 14:19 +0200, Nir Soffer wrote:

> Isn't this required for bzr? many clients reading the same branch in 
> the same time, only one can change it?

We're designing so that you can have a bzr client writing to a
repository over sftp and clients reading from the same over http. 

The constraints we have chosen that we believe deliver this:
 * alterations to file contents [not append operations] must be done by
write + rename
 * data present in the repository must only be removed in exceptional
circumstances [it may break readers]
 * all the data dependencies of an object must be written before the
object itself. [i.e. signatures before revisions, file texts before
inventories, inventories before revisions.]
 * format changes should be done by preparing a new .bzr dir and moving
the old one out and the new one in. This is non atomic but as close as
we can get, and leaves a relatively small window for client confusion,
note that obviously we lock the old dir so that no other writers get
involved.

Rob

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