Freecad: fix cursor.pcf font load crash

[julian1]

See,

#346740
#284880
https://discourse.nixos.org/t/freecad-failed-to-compute-left-right-minimum-bearings-for-cursor-pcf/35266
FreeCAD/FreeCAD#10514

This change blacklists the cursor.pcf font from being background saved into the preferences serialization file.
When this font is loaded, extents cannot be computed, and Freecad freezes at 100% cpu, with errors streamed to stderr.

A proposed workaround, that involves manually editing ~/.config/FreeCAD/user.cfg, to clear the cursor.pcf field when modified, only goes so far.

Since some actions that trigger the behavior - such as DXF import and export - don't provide any opportunity to save work, before one has to perform the pkil, and re-edit the user.cfg, so it is not possible to progress the design.

The change is similar to
FreeCAD/FreeCAD#10555

The proposed fix,
FreeCAD/FreeCAD@9806a46
(now reverted) did not work for me.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• Nixpkgs 25.11 Release Notes (or backporting 24.11 and 25.05 Nixpkgs Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
• NixOS 25.11 Release Notes (or backporting 24.11 and 25.05 NixOS Release notes)
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[LordGrimmauld]

Might an upstream PR make sense if it crashes not just on NixOS?

[LordGrimmauld]

i also can't reproduce the issue

[julian1]

cursor.pcf is used for X11's mouse cursor.
It should not be loaded by the app as a general purpose monospace font.
Depending on the set of installed system fonts, other fonts may be loaded in priority and according to QTs font enumeration, and in the loop code of DlgSettingsEditor.cpp.
So the OS/windowing system/set of installed fonts will all factor in whether the bad behavior is triggered.

[LordGrimmauld]

Right. What about upstreaming the patch? Your changes itself do look fine, i am not opposed against this patch. But opening an upstream PR might fix it for more than just nix users while also getting "official" reviews.
(I'd be happy if there'd be an upstream PR that is linked in a comment, no need to get that upstream PR merged immediately)

[julian1]

Good points. done.

[LordGrimmauld]

After a backlink in nix (so the next person updating the package can check whether it was merged) this is ready

[nixpkgs-ci]

Successfully created backport PR for release-25.05:

• [Backport release-25.05] Freecad: fix cursor.pcf font load crash #417848