On Sun, 26 May 2013, Frank Shearar wrote:

> I'm not sure what caused it, but I just hit an emergency evaluator on
> updating a clean 4.5 (as in the Squeak4.5.image in the build scripts).
>
> It goes something like this:
>
> ***System error handling failed***
> Original error: MessageNotUnderstood: UndefinedObject>>at:ifPresent:.
>    Debugger error: MessageNotUnderstood: UndefinedObject>>at:ifPresent::
> Debugger class(Object)>>primitiveError:
> [] in [] in Debugger class>>morphicOpenOn:context:label:contents:fullView:
> BlockContext>>cull:
> [] in MethodContext(ContextPart)>>handleSignal:
> BlockClosure>>ensure:
> MethodContext(ContextPart)>>handleSignal:
> MessageNotUnderstood(Exception)>>signal
> UndefinedObject(Object)>>doesNotUnderstand: #at:ifPresent:
> Environment>>at:ifPresentAndInMemory:
> SmalltalkImage>>at:ifPresentAndInMemory:
> Debugger>>process:controller:context:
> [] in [] in Debugger class>>morphicOpenOn:context:label:contents:fullView:
>
> and so on. I'm strongly tempted to point at Environments-cwp.27
> because of that call to #at:ifPresentAndInMemory: which uses
> Environment's contents ivar prior to this change and the new
> declarations ivar after. A missing migration step over the shape
> change?

I think the cause of the problem is that MC doesn't have atomic loading. I 
guess the variables are renamed first in the class, but that triggers 
recompilation of all existing methods, which will point to nonexistent 
variables.

If I'm right, then there needs to be an intermediate step, where the old 
and the new variables coexist and both point to the same objects.

So, I'd probably use 3 updates (and create 2 update maps, one after all 
but the last point):
1. Add the new variables to the class, and a migration method that assigns 
the value of the old variables to the new ones.
2. In the preamble send the migration method to all Environment instances. 
Then update all methods to use only the new variables
3. Remove the old variables


Levente

>
> frank
>
>