Add notes and allow env var configuration

Author: znicholls

.github/workflows/ci.yaml

@@ -113,6 +113,12 @@ jobs:
           # TODO: figure out whether we need/want to use other compilers too
           compiler: "gcc"
           version: "13"
+      - name: Set DLL directory environment variable
+        # Used when running the tests, see `tests/conftest.py` for details
+        if: ${{ startsWith(matrix.os, 'windows') }}
+        run: |
+          echo "PYTHON_ADD_DLL_DIRECTORY=C:\\mingw64\\bin" >> $GITHUB_ENV
+
       - uses: ./.github/actions/setup
         with:
           python-version: ${{ matrix.python-version }}

tests/conftest.py

@@ -5,7 +5,16 @@
 """
 
 import os
-import sys
 
-if os.environ.get("CI", "false") == "true" and sys.platform == "win32":
-    os.add_dll_directory("C:\\mingw64\\bin")
+if dll_directory_to_add := os.environ.get("PYTHON_ADD_DLL_DIRECTORY", None):
+    # Add the directory which has the libgfortran.dll file.
+    #
+    # A super deep dive into this is here:
+    # https://stackoverflow.com/a/78276248
+    # The tl;dr is - mingw64's linker can be tricked by windows craziness
+    # into linking a dynamic library even when we wanted only static links,
+    # so if you want to avoid this, link with something else (e.g. the MS linker).
+    # (From what I know, this isn't an issue for the built wheels
+    # thanks to the cleverness of delvewheel)
+    os.add_dll_directory(dll_directory_to_add)
+    # os.add_dll_directory("C:\\mingw64\\bin")