[NSXML + libxml2] external retains

[Niels Grewe]

Hi Doug, Greg and all,

I've just been digging into the new libxml2 powered NSXML code and got a
bit confused about the code path that takes care of external references.
I get that we want to increase the retain count of the top node in a
tree if any object outside the tree makes reference to some node in the
tree. Otherwise, we'd get dangling parent pointers when nothing
references the top node and it gets deallocated.

But from looking at the code, I somehow think that we should also be
decreasing the top node's external retain count when a node gets
detached from the tree (which we don't do presently). But with the way
things are done now, I don't see how we would do that because once the
node is attached the record keeping for external references is taken
over by the top node. The situation basically looks like this ("|"
denotes the parent-child-relation where the parent is written above the
child and "<-" denotes an external object holding a reference to the node):

A       (externalRetains == 2)
|
B<-X    (externalRetains == 0)
|
C       (externalRetains == 0)
|
D<-Y    (externalRetains == 0)

Now if we detach, say, C, then we have no way to tell:

a) that we should send C and extra -retain in order to prevent D from
ending up with a dangling reference to C.

b) that we should decrease the externalRetains of A by 1. And we need to
do that, because if we don't, and X stops retaining B, we start leaking
A and B.

This would be much easier to handle if we were tracking the extra
refcount all the way through the tree:


A       (externalRetains == 2)
|
B<-X    (externalRetains == 2)
|
C       (externalRetains == 1)
|
D<-X    (externalRetains == 1)

Would that be about right? Also: Couldn't we just do away with this
whole contraption if we had the children retain their parents while the
parents only hold weak references to them?

Also there is another thing I cannot quite wrap my head around right now
and it would be nice if someone could elaborate on that as well: Why
does NSXMLNode's -release method decrease the externalRetains only "if
([self retainCount] == [internal->subNodes count])" (NSXMLNode.m:590)?

I already apologise if I'm just overlooking the obvious here. Also, I
hope that I don't sound overly cirtical about this code, which I think
is really a nice new feature. I already turned on some bits in DBusKit
that require NSXML, and it turns out that it's working quite well in
general :-)

Cheers,

Niels

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