TLDR: in CFF is it legal to call a local subroutine from a global subroutine?
I'm currently working on some software that parses/manipulates charstrings that represent the glyphs in CFF (Compact Font Format) fonts.
While working on the part that handles global and local subroutines, I was initially working under the assumption that:
however, this seems not to be the case: when I ran some tests on a number of font files, I discovered that for example in "Nimbus Roman Italic" (a standard font installed on Ubuntu Linux) global subroutines exist that contain the
callsubr opcode, thereby trying to call local subroutines.
In the context of OTF fonts this can be worked around relatively unambiguously as OTF restricts CFF to contain only one CharSet and therefore only one private dict with local subroutines. But in a general case this seems like a bad idea.