Search DLLs only on paths added using add_dll_directory().

This is the default behavior on upstream Python. We previously
reverted this behavior because it broke some use cases.

The old behavior can be restored by setting the environment variable
PYTHONLEGACYWINDOWSDLLLOADING to 1.

Fixes msys2-contrib#48
Also fixes msys2-contrib#141

Signed-off-by: Naveen M K <naveen521kk@gmail.com>

.github/workflows/mingw.yml

@@ -135,6 +135,7 @@ jobs:
         run: |
           export SETUPTOOLS_USE_DISTUTILS=stdlib
           export PYTHONTZPATH="${MINGW_PREFIX}/share/zoneinfo"
+          export PYTHONLEGACYWINDOWSDLLLOADING=1
           SMOKETESTS="$(pwd)/mingw_smoketests.py"
           cd _build
           cd python_pkgdir/${MINGW_PREFIX}/bin

Lib/test/__main__.py

@@ -1,2 +1,10 @@
+import os
+import sys
+
 from test.libregrtest import main
+
+if sys.platform == "win32":
+    # Enable DLL loading from PATH.
+    os.environ["PYTHONLEGACYWINDOWSDLLLOADING"] = "1"
+
 main()

Python/dynload_win.c

@@ -223,6 +223,16 @@ dl_funcptr _PyImport_FindSharedFuncptrWindows(const char *prefix,
     dl_funcptr p;
     char funcname[258], *import_python;
 
+    wchar_t* use_legacy = _wgetenv(L"PYTHONLEGACYWINDOWSDLLLOADING");
+    DWORD load_library_flags = 0;
+
+    if (use_legacy && wcscmp(use_legacy, L"1") == 0) {
+        load_library_flags = LOAD_WITH_ALTERED_SEARCH_PATH;
+    } else {
+        load_library_flags = LOAD_LIBRARY_SEARCH_DEFAULT_DIRS |
+                              LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR;
+    }
+
 #ifdef _MSC_VER
     _Py_CheckPython3(); 
 #endif
@@ -249,8 +259,7 @@ dl_funcptr _PyImport_FindSharedFuncptrWindows(const char *prefix,
            AddDllDirectory function. We add SEARCH_DLL_LOAD_DIR to
            ensure DLLs adjacent to the PYD are preferred. */
         Py_BEGIN_ALLOW_THREADS
-        hDLL = LoadLibraryExW(wpathname, NULL,
-                              LOAD_WITH_ALTERED_SEARCH_PATH);
+        hDLL = LoadLibraryExW(wpathname, NULL, load_library_flags);
         Py_END_ALLOW_THREADS
 #if !USE_UNICODE_WCHAR_CACHE
         PyMem_Free(wpathname);

mingw_smoketests.py

@@ -34,6 +34,9 @@
 else:
     SEP = "\\"
 
+if sysconfig.is_python_build():
+    os.environ["PYTHONLEGACYWINDOWSDLLLOADING"] = "1"
+
 _UCRT = sysconfig.get_platform() not in ('mingw_x86_64', 'mingw_i686')
 
 

setup.py

@@ -88,6 +88,10 @@ def msys_system(command):
         return os_system(command_in_sh)
     os.system = msys_system
 
+    # set PYTHONLEGACYWINDOWSDLLLOADING to 1 to enable the legacy Windows DLL
+    # loading behavior. This is needed while building Python itself.
+    os.environ["PYTHONLEGACYWINDOWSDLLLOADING"] = "1"
+
 CROSS_COMPILING = ("_PYTHON_HOST_PLATFORM" in os.environ)
 HOST_PLATFORM = get_platform()
 MS_WINDOWS = (HOST_PLATFORM == 'win32' or HOST_PLATFORM == 'mingw')