Font machinery crashes hard when no fonts are available on a machine

[rahulporuri]

Users have reported a hard crash in an application running on a headless machine. The subset of code that triggers the exception is

gc = GraphicsContext(temp_frame, pix_format="rgb24")
gc.set_font(Font(face_name="Verdana", size=self.size))

The full traceback is

Traceback (most recent call last):
  File "[edm environment]/lib/python3.6/site-packages/kiva/fonttools/font_manager.py", line 412, in _load_from_cache_or_rebuild
    fontManager = _pickle_load(cache_file)
  File "[edm environment]/lib/python3.6/site-packages/kiva/fonttools/font_manager.py", line 460, in _pickle_load
    fh = open(filename, "rb")
FileNotFoundError: [Errno 2] No such file or directory: '/home/centos/.enthought/kiva/fontList.cache'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
[Traceback from private package]
    gc.set_font(Font(face_name="Verdana", size=self.size))
  File "[edm environment]/lib/python3.6/site-packages/kiva/agg/agg.py", line 1216, in set_font
    spec = font.findfont()
  File "[edm environment]/lib/python3.6/site-packages/kiva/fonttools/font.py", line 130, in findfont
    return default_font_manager().findfont(query)
  File "[edm environment]/lib/python3.6/site-packages/kiva/fonttools/font_manager.py", line 65, in default_font_manager
    fontManager = _load_from_cache_or_rebuild(_get_font_cache_path())
  File "[edm environment]/lib/python3.6/site-packages/kiva/fonttools/font_manager.py", line 419, in _load_from_cache_or_rebuild
    fontManager = _new_font_manager(cache_file)
  File "[edm environment]/lib/python3.6/site-packages/kiva/fonttools/font_manager.py", line 437, in _new_font_manager
    fontManager = FontManager()
  File "[edm environment]/lib/python3.6/site-packages/kiva/fonttools/font_manager.py", line 110, in __init__
    self.default_font["ttf"] = ttffiles[0]
IndexError: list index out of range

Note that the user also reported seeing the following warning on the machine.

In [2]: from kiva.image import GraphicsContext 
   ...:                                                                                                                                                                                                          
/home/msmarte/.edm/envs/kiva-test/lib/python3.6/site-packages/kiva/image.py:19: Warning: Error initializing Agg: No module named '_plat_support'
  from .agg import GraphicsContextArray as GraphicsContext

At the moment, the culprit seems to be

enable/kiva/fonttools/font_manager.py

Lines 101 to 110 in 7b9eff4

	# Load TrueType fonts and create font database.
	ttffiles = scan_system_fonts(paths) + scan_system_fonts()
	for fname in ttffiles:
	logger.debug("trying fontname %s", fname)
	if fname.lower().find("vera.ttf") >= 0:
	self.default_font["ttf"] = fname
	break
	else:
	# use anything
	self.default_font["ttf"] = ttffiles[0]

[rahulporuri]

@mdsmarte can you please correct the issue title/description if necessary.

[mdsmarte]

LGTM I would just add that it seems to be triggered by set_font, not the instantiation of a Font.

[mdsmarte]

Note that the user also reported seeing the following warning on the machine.

Observed on a headless machine, CentOS 7.9.2009, EDM 3.0.1. Can reproduce the warning in a fresh environment with just enable 5.2.0-1 (and its dependencies) installed.

[rahulporuri]

@jwiggins i'm not sure what the right behavior here should be. Should we be raising a runtime warning instead of an exception - stating that the font manager was created without any fonts?

[mdsmarte]

Adding a note that this also breaks add_application_fonts, since this function initializes a font manager, and the bug is in FontManager.__init__.