Fields in expression evaluator always null if field only referenced inside length([Field])

[Ben Harper]

I've discovered what looks like a bug in Mapnik, but I'm finding it hard to track down where the bug might be.

If you create one or more rules that have filter expressions like this:

length([field]) % 5 = 0

length([field]) % 5 = 1

length([field]) % 5 = 2

Then, during evaluation, [field] is always null. I've verified that by stepping through the evaluation code.

If I change any of the expressions to something:

length([field]) % 5 = 0 or [field] = 'foobar'

then all expressions are evaluated correctly, and [field] is fed through correctly from the DB.

I've verified that this has nothing to do with the DB fetch - the correct [field] is fetched from the DB in both cases.

I'm assuming this bug is due to some piece of code that tries to analyze the expressions in a style, and avoid populating those fields that aren't needed. If I'm correct in this hunch, then where does that code lie?

If I know where it is, then I can probably fix it.

My Mapnik version is 3.0.15.

Thanks,

Ben

[Dane Springmeyer]

Ben,

That does sound like you've found a bug. I think the problem may be that the function call type (https://github.com/mapnik/mapnik/blob/master/include/mapnik/function_call.hpp) needs to be added/handled in the attribute_collector:

https://github.com/mapnik/mapnik/blob/333ef9fde145f88339eaccba810305707bae9b0e/include/mapnik/attribute_collector.hpp#L59-L85

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[Ben Harper]

Thanks Dane!

I'm off work for a few days, but will look into that, and try cook up a unit test and a fix when I'm back.

[Dane Springmeyer]

Sounds good Ben.