[subset] FeatureVariations subsetting is wrong

[behdad]

We cannot drop FeatureVariationsRecords that have empty substitutions. That changes the behavior. A FeatureVariationsRecords stops the search when the conditions match; so even if there's no susbstitutions we cannot drop the record as that will keep the search going.

The only optimizations we can do are 1. dropping from the end of the array any records with no susbtitutions, and 2. dropping records earlier with no susbstitution only after analysing the conditions and proving that the dropping does not change functionality.

Both of those are unnecessary right now. We should fix the functionality.

This seems to be wrong in FontTools as well: https://github.com/fonttools/fonttools/blob/master/Lib/fontTools/subset/__init__.py#L1325

cc @anthrotype @qxliu76

[behdad]

The spec is here: https://docs.microsoft.com/en-us/typography/opentype/spec/chapter2

[qxliu76]

@anthrotype please let me know when this is fixed in fonttools, I'll then fix it in harfbuzz

[ebraminio]

@qxliu76 and colleagues, hadn't the chance to state this somewhere else, thanks for all the tireless efforts you've put on implementing this and other in progress layout subset works, hopefully we will soon fix all the mentioned issues providing layout subset as a quality works to the users. Thanks again :)

[behdad]

Here's fonttools fix. We don't need to implement the pruning here initially. fonttools/fonttools#1882

[qxliu76]

Now fonttools is pruning empty record at the end, while harfbuzz is not doing the same thing. Is it an issue in testing?

[qxliu76]

@ebraminio, thank you!

[behdad]

Now fonttools is pruning empty record at the end, while harfbuzz is not doing the same thing. Is it an issue in testing?

We'll deal with that when we get to.