Blender Git Loki

Bugfix [#32017] Infinite recursion in depsgraph material/node driver handling

When initially coding this functionality, I was aware of the potential for
infinite recursion here, just not how frequently such setups are actually
used/created out in the wild (nodetree.ma_node -> ma -> ma.nodetree is all too
common, and often even with several levels of indirection!).

However, the best fix for these problems was not immediately clear. Alternatives
considered included...
1) checking for common recursive cases. This was the solution employed for one
of the early patches committed to try and get around this. However, it's all too
easy to defeat these measures (with all the possible combinations of indirection
node groups bring).
2) arbitrarily restricting recursion to only go down 2/3 levels? Has the risk
of missing some deeply chained/nested drivers, but at least we're guaranteed to
not get too bad. (Plus, who creates such setups anyway ;)
*3) using the generic LIB_DOIT flag (check for tagged items and not recurse down
there). Not as future-proof if some new code suddenly decides to start adding
these tags to materials along the way, but is easiest to add, and should be
flexible enough to catch most cases, since we only care that at some point those
drivers will be evaluated if they're attached to stuff we're interested in.
4) introducing a separate flag for Materials indicating they've been checked
already. Similar to 3) and solves the future-proofing, but this leads to...
5) why bother with remembering to clear flags before traversing for drivers to
evaluate, when they should be tagged for evaluation like everything else?
Downside - requires depsgraph refactor so that we can actually track the fact
that there are dependencies to/from the material datablock, and not just to the
object using said material. (i.e. Currently infeasible)