PR#1163 breaks some font building: AssertionError: Default source not found!

[hosiet]

The merged Pull Request #1163 by @anthrotype , or more specifically, 04df2cb , broke the building of some old font projects. I don't know if it is a bug or feature, so I am looking for help from fontmake developers.

For example, with https://github.com/fontworks-fonts/Train/tree/2972a9857a8332c2af86b221799558c177ba9012 :

With fontmake 3.11.0:

-> % uv pip install fontmake==3.11.0 fonttools
Using Python 3.13.11 environment at: venv2
Resolved 14 packages in 114ms
Prepared 1 package in 32ms
Installed 14 packages in 15ms
 + attrs==25.4.0
 + booleanoperations==0.10.0
 + cffsubr==0.4.0
 + compreffor==0.6.0
 + fontmake==3.11.0
 + fontmath==0.9.4
 + fonttools==4.61.1
 + glyphslib==6.12.6
 + lxml==6.0.2
 + openstep-plist==0.5.1
 + pyclipper==1.4.0
 + ufo2ft==3.7.0
 + ufolib2==0.18.1
 + unicodedata2==17.0.0
-> % python ./build.py
[Train One] Generating TTF
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib.parser:Parsing .glyphs file
INFO:glyphsLib.builder:Running 'align_alternate_layers' transformation
INFO:glyphsLib.builder:Running 'propagate_all_anchors' transformation
INFO:fontmake.font_project:Loading 1 DesignSpace source UFOs
INFO:fontmake.font_project:Building TTF for TrainOne-Regular
INFO:ufo2ft:Pre-processing glyphs
INFO:ufo2ft.filters:Running EraseOpenCornersFilter on TrainOne-Regular
INFO:ufo2ft.filters:Running DecomposeComponentsFilter on TrainOne-Regular
INFO:ufo2ft.filters:Running RemoveOverlapsFilter on TrainOne-Regular
INFO:ufo2ft.filters:Running CubicToQuadraticFilter on TrainOne-Regular
INFO:ufo2ft.filters.cubicToQuadratic:New spline lengths: 1: 7216, 2: 79047, 3: 21622, 4: 7066, 5: 593, 6: 107, 7: 8
INFO:ufo2ft:Building OpenType tables
INFO:ufo2ft.postProcessor:Renaming glyphs to final production names
INFO:fontmake.font_project:Saving master_ttf/TrainOne-Regular.ttf
[TrainOne-Regular] Adding stub DSIG
[TrainOne-Regular] Making other changes

With fontmake 3.11.1:

-> % uv pip install fontmake==3.11.1 fonttools
Using Python 3.13.11 environment at: venv3
Resolved 14 packages in 9ms
Installed 14 packages in 14ms
 + attrs==25.4.0
 + booleanoperations==0.10.0
 + cffsubr==0.4.0
 + compreffor==0.6.0
 + fontmake==3.11.1
 + fontmath==0.9.4
 + fonttools==4.61.1
 + glyphslib==6.12.6
 + lxml==6.0.2
 + openstep-plist==0.5.1
 + pyclipper==1.4.0
 + ufo2ft==3.7.0
 + ufolib2==0.18.1
 + unicodedata2==17.0.0
-> % python ./build.py
[Train One] Generating TTF
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib.parser:Parsing .glyphs file
INFO:glyphsLib.builder:Running 'align_alternate_layers' transformation
INFO:glyphsLib.builder:Running 'propagate_all_anchors' transformation
INFO:fontmake.font_project:Loading 1 DesignSpace source UFOs
Traceback (most recent call last):
  File "/home/hosiet/tmp/Train/./build.py", line 8, in <module>
    __main__.main(("-g","sources/TrainOne.glyphs", "-o","ttf",))
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/hosiet/tmp/venv3/lib/python3.13/site-packages/fontmake/__main__.py", line 719, in main
    project.run_from_glyphs(inputs.glyphs_path, **args)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/hosiet/tmp/venv3/lib/python3.13/site-packages/fontmake/font_project.py", line 931, in run_from_glyphs
    self.run_from_designspace(
    ~~~~~~~~~~~~~~~~~~~~~~~~~^
        designspace,
        ^^^^^^^^^^^^
    ...<4 lines>...
        **kwargs,
        ^^^^^^^^^
    )
    ^
  File "/home/hosiet/tmp/venv3/lib/python3.13/site-packages/fontmake/font_project.py", line 1182, in run_from_designspace
    assert default_source is not None, "Default source not found!"
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Default source not found!

Any help, suggestion or hint would be greatly appreciated.

[lundstroem]

I encountered the same issue in 3.11.1 when building DotGothic16