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Text-base upgrades

From: Philip Martin <philip_at_codematters.co.uk>
Date: Tue, 29 Jun 2010 18:44:34 +0100

I've been looking at the code to upgrade from .svn/text-base to
.svn/pristine. There are two ways to invoke it:

- For a 1.6 wc the user must execute an explicit "svn upgrade". Any
  outstanding log files must first be cleared using a 1.6 client.
  
- For a 1.7 wc the upgrade is automatical for most operations. At
  present this happens when the wcroot object is created. It doesn't
  run any outstanding workqueues, but if outstanding workqueues
  interrupt the operation the wc is only partially upgraded.

The current text-base upgrade code involves no database schema change
beyond the user_version number. It excutes as a single transaction
per directory. It calculates the checksum of each text-base, adds a
row to the PRISTINE table and copies the file into the new location.
Both db operations and file IO occur during the transaction.

There are a few issues:

- I think upgrading with outstanding workqueue items is wrong. If
  there are items that will modify the text-base then the upgrade will
  install the wrong pristine with the wrong checksum and the change
  will get applied to an obsolete file and be lost.

- From a performance point of view it would be better to move, rather
  than copy, the text-base.

- We have to allow the upgrade to be interrupted and restarted. This
  works if we copy the text-base, but if we move instead we have to
  track the moves.

To fix these I think the basic upgrade operation should be changed to
the following: first create workqueue items to move the text-base
files. Then, in one transaction, add all the rows to the PRISTINE
table and install the workqueue items. Finally run the workqueue.

Then the automatic upgrade process for 1.7 working copies should be
changed so that it only runs if the workqueues are empty. If there
are outstanding workqueues the user is forced to run "svn cleanup".

-- 
Philip
Received on 2010-06-29 19:45:17 CEST

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