[Seaside] Backtracking with collections

[Lukas Renggli]

Backtracking used to be an area with many bugs in versions prior to
Seaside 2.7. I believe that most of these bugs have been fixed in
Seaside 2.7 for Squeak. I added tests for all of these regressions and
special implementations of #snapshotCopy and #restoreFromSnapshot:
that were necessary in Squeak. Since these fixtures are mostly related
to implementation details of the Squeak collection hierarchy other
platforms probably need other tests to reliably detect backtracking
problems (and we would like to integrate those into the core tests).
Thats why Seaside 2.8 moves #snapshotCopy and #restoreFromSnapshot: to
platform specific code (I just did some more cleanup right now).

>  | array |
>  array := #(1 2 3 4 5) copy.
>  array restoreFrom: #(4 5 6) copy.
>  array
>
>  --->  #(4 5 6 nil nil)
>
>
>  | array |
>  array := #(1 2 3) copy.
>  array restoreFrom: #(4 5 6 7 8) copy.
>  array
>
>
>  --> Index out of bounds exception

This is not how the backtracking API works, #copy is not part of it.
Have a look at WABacktrackingTest:

	original := Array with: 1 with: 2 with: 3.
	snapshot1 := original snapshotCopy.

	original at: 1 put: #a.
	snapshot2 := original snapshotCopy.

	original at: 2 put: #b.
	snapshot3 := original snapshotCopy.

	original restoreFromSnapshot: snapshot1.
	self assert: original = #( 1 2 3 ).

	original restoreFromSnapshot: snapshot2.
	self assert: original = #( a 2 3 ).

	original restoreFromSnapshot: snapshot3.
	self assert: original = #( a b 3 )

>  Michael Lucas-Smith tried it in Squeak and the first example produced
>  #(4 5 6 4 5) and the second one produced: #(4 5 6).

Arrays are considered to be immutable in size in Squeak.

>  To do this properly, you'd first have to create a new collection of the
>  proper size, use a become: to change the original collection back, then
>  restore it to the proper values using the existing restore mechanism.
>
>  Am I out to lunch on this one or does this change need to be made?

You need to implement #snapshotCopy and #restoreFromSnapshot: in
OrderedCollection according to your internal implementation. I guess
#snapshotCopy  just returns a shallow copy (the default), while
#restoreFromSnapshot: can do a #become: of a copy of the snapshot to
the one you want to restore. Or something like that.

Lukas

-- 
Lukas Renggli
http://www.lukas-renggli.ch