using WriteBarrier

[Chris Muller]

> Would using WriteBarrier be as simple as "myMagmaSession
> allowWriteBarrier: true"? What drawbacks are there?

Once you load WriteBarrier (which I'm not sure how to do these days),
yes, that should be it.  I would not recommend turning it off and on
while sessions are connected.

I was really excited about WriteBarrier at one time because it improves
the peak commit rate from 24 per second to >100 per second.  It's
another a great idea from Avi.

I put some real work into supporting it with Magma but there is one
test case which can't pass with WriteBarrier.  #testClassSchemaUpgrades
simulates developers changing the shape of classes with existing
instances in Magma, that other users can run their version of the
in-image class "out-of-sync" with the repository and not accidently
truncate data.  If you comment out that test case, all the others
should (and did, last year at least) pass.

It turns out the problem lies within WriteBarrier's interaction with
Squeak's metaclass layer.  It is exposed by changing the instance
variables of a *superclass*, such as:

Object
  ClassA
    ClassB

and changing the instance variables in ClassA will not work with
WriteBarrier turned on (with or without Magma).  I spent several late
nights trying to fix it but I couldn't fix it.  I still have detailed
notes and scripts to reproduce the problem if you or anyone else wants
to give it a shot.

Besides that bug and the fact that I haven't tested it in months, also
know that I don't really have the time (or skill, apparently) to
support it.  If you have problems running with WriteBarrier, I probably
wouldn't be able to help.

 - Chris