freecad: use python from environment

FreeCAD is pedantic about the source of its python packages. It has
been unsetting the PYTHONPATH environment variable for some time. With
the recent support for Python 3.11 they are initializing the Python
environment using an "isolated" configuration. That means our previous
approach of removing the `putenv` line from the MainGui doesn't cut it
anymore. In this commit I'm replacing the isolated configuration with
a regular configuration. This means that FreeCAD will again pickup our
PYTHONPATH.

An alternative approach would have been to pass each of our (runtime)
python dependencies with the `-P` argument to the FreeCAD binary
using (yet another) wrapper.

Perhaps the best approach, one that I might explore, is to figure out
where FreeCAD usually expects the python dependencies to be. It might
be that we can just symlink them into the package itself. One escape
hatch that has been there for a while but hasn't been used (by us at
least) is the `VIRTUAL_ENV` environment variable. We could provide an
environment to the binary and get away without patching the source
code. This requires some (new) machinery to produce virtualenv-alike
folders.

pkgs/applications/graphics/freecad/0001-NIXOS-don-t-ignore-PYTHONPATH.patch

@@ -0,0 +1,60 @@
+From c4f452ef6ae083ed21095313582f6d1bd775cbf3 Mon Sep 17 00:00:00 2001
+From: Andreas Rammhold <andreas@rammhold.de>
+Date: Thu, 2 Nov 2023 17:32:07 +0100
+Subject: [PATCH] NIXOS: don't ignore PYTHONPATH
+
+On NixOS or rather within nixpkgs we provide the runtime Python
+packages via the PYTHONPATH environment variable. FreeCAD tries its
+best to ignore Python environment variables that are being inherited
+from the environment. For Python versions >=3.11 it also tries to
+initialize the interpreter config without any environmental data. We
+have to initialize the configuration *with* the information from the
+environment for our packaging to work.
+
+Upstream has purposely isolated the environments AFAIK and thus
+shouldn't accept this patch (as is). What they might accept (once
+support for older Python versions has been dropped) is removing the
+PYTHONPATH specific putenv calls.
+---
+ src/Base/Interpreter.cpp | 2 +-
+ src/Main/MainGui.cpp     | 3 ---
+ 2 files changed, 1 insertion(+), 4 deletions(-)
+
+diff --git a/src/Base/Interpreter.cpp b/src/Base/Interpreter.cpp
+index 52c47168af..9966bd0013 100644
+--- a/src/Base/Interpreter.cpp
++++ b/src/Base/Interpreter.cpp
+@@ -554,7 +554,7 @@ void initInterpreter(int argc,char *argv[])
+ {
+     PyStatus status;
+     PyConfig config;
+-    PyConfig_InitIsolatedConfig(&config);
++    PyConfig_InitPythonConfig(&config);
+ 
+     status = PyConfig_SetBytesArgv(&config, argc, argv);
+     if (PyStatus_Exception(status)) {
+diff --git a/src/Main/MainGui.cpp b/src/Main/MainGui.cpp
+index 48ae847ef4..28813df383 100644
+--- a/src/Main/MainGui.cpp
++++ b/src/Main/MainGui.cpp
+@@ -112,17 +112,14 @@ int main( int argc, char ** argv )
+     // See https://forum.freecad.org/viewtopic.php?f=18&t=20600
+     // See Gui::Application::runApplication()
+     putenv("LC_NUMERIC=C");
+-    putenv("PYTHONPATH=");
+ #elif defined(FC_OS_MACOSX)
+     (void)QLocale::system();
+-    putenv("PYTHONPATH=");
+ #elif defined(__MINGW32__)
+     const char* mingw_prefix = getenv("MINGW_PREFIX");
+     const char* py_home = getenv("PYTHONHOME");
+     if (!py_home && mingw_prefix)
+         _putenv_s("PYTHONHOME", mingw_prefix);
+ #else
+-    _putenv("PYTHONPATH=");
+     // https://forum.freecad.org/viewtopic.php?f=4&t=18288
+     // https://forum.freecad.org/viewtopic.php?f=3&t=20515
+     const char* fc_py_home = getenv("FC_PYTHONHOME");
+-- 
+2.42.0
+

pkgs/applications/graphics/freecad/default.nix

@@ -108,6 +108,10 @@ stdenv.mkDerivation (finalAttrs: {
     qtx11extras
   ];
 
+  patches = [
+    ./0001-NIXOS-don-t-ignore-PYTHONPATH.patch
+  ];
+
   cmakeFlags = [
     "-Wno-dev" # turns off warnings which otherwise makes it hard to see what is going on
     "-DBUILD_FLAT_MESH:BOOL=ON"
@@ -127,10 +131,7 @@ stdenv.mkDerivation (finalAttrs: {
     export NIX_LDFLAGS="-L${gfortran.cc}/lib64 -L${gfortran.cc}/lib $NIX_LDFLAGS";
   '';
 
-  # Their main() removes PYTHONPATH=, and we rely on it.
   preConfigure = ''
-    sed '/putenv("PYTHONPATH/d' -i src/Main/MainGui.cpp
-
     qtWrapperArgs+=(--prefix PYTHONPATH : "$PYTHONPATH")
   '';