CFF/CFF2 Index read/write with count=0

[behdad]

The OT spec has the following note:

An empty INDEX is represented by a count field with a 0 value and no additional fields. Thus, the total size of an empty INDEX is 4 bytes.

Is this implemented?

https://learn.microsoft.com/en-us/typography/opentype/spec/cff2#common-formats-dict-and-index

The same goes for CFF-1 Index as well. Looks like we read/write Index1 / Index2 fully (offSize, offsets array) even if count is 0:

https://github.com/googlefonts/fontations/blob/main/resources/codegen_inputs/postscript.rs

For writing, this just writes a couple of extra unnecessary bytes. But for read, this can err when the data is correct.

[dfrg]

Yes, reads of empty INDEX objects will fail if they're at the end of the containing blob. I don't think we've seen this in practice but it probably should be fixed either with more sophisticated codegen or by hand writing the FontRead impls.

[behdad]

We had the same bug in HB and we got a report about one font that failed this.

[cmyr]

This is annoying, the only way to handle this in codegen without some special-case would require having the non-count fields be Options, which doesn't feel very good..