Instantiator support for non-default layer sources

[verbosus]

This PR adds the ability to build from non-default layers. If the default designspace source is set to a non-default layer, then its glyphset is used as reference for all the other sources—glyphs that are not on the layer are discarded. I believe this should also address issue #603

[verbosus]

The build failure seems to be related to some codecov “owner” argument I don’t know much about.

[anthrotype]

hi Antonio! thanks, I left some comments

[anthrotype]

don't worry about codecov, it's flaky at best.. i'm going to make the PR pass even when it fails to upload (which is 50% of the time lately)

[verbosus]

Thanks Cosimo: I believe I fixed all your comments, except the one about reporting the non-existing glyphs in non-default masters. I’d rather have it behave like it does right now: it’s a corner case anyway so the main path isn’t affected, and for the way I’m using it I much prefer it to silently ignore the missing glyphs rather than reporting them. Codecov still isn’t happy, but I don’t think there is anything I can do to help there.

[anthrotype]

this is looking good now, thanks!
Maybe we should change the PR title to better reflect the changes that you made. Only the fontmake.instantiator (which generates static instance UFOs from a DS) has been modified, whereas the PR title suggests building in general (not just static interpolated instances) from non-default layer source isn't supported..

by the way, did it work before this PR to build a VF from the same DS with such non-default layer set as default source?

[anthrotype]

I'm also curious, why not simply mark the desired non-default layer that you wish to use as the default DS source as the default layer in the font editor or via script?

[verbosus]

In some cases it’s handy to have metrically-compatible sources in the same UFO (think: a full outline and a stencil variant, etc.) but on different layers. And yeah, I tried and it did NOT use to work before the PR: fontmake was crashing saying it couldn’t find a base master.

[anthrotype]

I tried and it did NOT use to work before the PR: fontmake was crashing saying it couldn’t find a base master.

was that trying to build interpolated static instance font (via instantiator) or building a VF (which does not go through instantiator)?

I'm trying to understand if this PR fixes other things, or if other parts of the pipeline need similar fixes to adjust expectation that a base source may not necessarily come from a default layer

[verbosus]

Sorry, I misunderstood your previous question. I only tried building static instances so far. I just checked and building a VF fails either with or without this PR. The failure is the same: fontmake: Error: In 'sources/testfamily.designspace': Generating fonts from Designspace failed: "'name' table not found". Unless it raises any flags for obvious known issues, I guess it needs some deeper investigation.

[anthrotype]

@verbosus can you try running fontmake with --verbose DEBUG option so it displays the full exception traceback, so you can see where exactly that error is raised?

[anthrotype]

@verbosus please file an issue about that error when you find out more details, and thanks again for the PR

[verbosus]

Thanks for merging and sure, I just filed #983 to cover the rest of the work on the variable path.