Generating fonts from Designspace failed: list index out of range (encore)

[driehuis]

Just like in #1071, the OpenType build of DINish fails, where the TrueType build from the same sources succeeds:

$ fontmake --verbose DEBUG --validate-ufo --flatten-components DINish-Variable.designspace -o variable-cff2
[...]
INFO:fontTools.varLib:Generating HVAR
ERROR:root:In 'DINish-Variable.designspace': Generating fonts from Designspace failed: list index out of range

I tried throwing in --validate-ufo and --verbose DEBUG, but unlike in #1071, this did not produce a light bulb moment.

I've attached the log.

The traceback looks like this:

Traceback (most recent call last):
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontmake/font_project.py", line 1208, in run_from_designspace
    self._run_from_designspace_interpolatable(
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontmake/font_project.py", line 1326, in _run_from_designspace_interpolatable
    self.build_variable_fonts(
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontmake/font_project.py", line 470, in build_variable_fonts
    fonts = ufo2ft.compileVariableCFF2s(
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/ufo2ft/__init__.py", line 295, in compileVariableCFF2s
    return VariableCFF2sCompiler(**kwargs).compile_variable(designSpaceDoc)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/ufo2ft/_compilers/baseCompiler.py", line 420, in compile_variable
    vfNameToTTFont = self._merge(designSpaceDoc, excludeVariationTables)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/ufo2ft/_compilers/interpolatableOTFCompiler.py", line 32, in _merge
    return varLib.build_many(
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/varLib/__init__.py", line 1141, in build_many
    vf = build(
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/varLib/__init__.py", line 1241, in build
    _add_CFF2(vf, model, master_fonts)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/varLib/__init__.py", line 869, in _add_CFF2
    merge_region_fonts(varFont, model, ordered_fonts_list, glyphOrder)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/varLib/cff.py", line 266, in merge_region_fonts
    cvData = merge_charstrings(glyphOrder, num_masters, top_dicts, model)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/varLib/cff.py", line 335, in merge_charstrings
    region_charstring.draw(var_pen)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/misc/psCharStrings.py", line 1168, in draw
    extractor.execute(self)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/misc/psCharStrings.py", line 636, in execute
    super().execute(charString)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/misc/psCharStrings.py", line 360, in execute
    rv = handler(index)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/misc/psCharStrings.py", line 727, in op_rlineto
    self.rLineTo(point)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/misc/psCharStrings.py", line 654, in rLineTo
    self.pen.lineTo(self._nextPoint(point))
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/pens/basePen.py", line 328, in lineTo
    self._lineTo(pt)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/varLib/cff.py", line 538, in _lineTo
    self.add_point("rlineto", pt_coords)
  File "/home/bert/.venv/dinish-tools/lib/python3.10/site-packages/fontTools/varLib/cff.py", line 488, in add_point
    cmd = self._commands[self.pt_index]
IndexError: list index out of range

Issue reproduces after updating to today's fontmake and fonttools. I tried reading the source code and even digging in with pdb, but this is way over my head...

The issue raised its head after I made extensive changes to the font, including rewriting the designspace from scratch. As I also struggled with toolchain updates in the meantime it is very hard for me to pin the issue on a particular change. Do the traceback or the log give any indication of where I could start looking? Is there any other diagnostic that could turn up the source of the issue?