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 #48
Also fixes #141

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

.github/workflows/mingw.yml

@@ -126,15 +126,15 @@ jobs:
           cp "${pkgdir}${MINGW_PREFIX}"/bin/idle3 "${pkgdir}${MINGW_PREFIX}"/bin/idle
           cp "${pkgdir}${MINGW_PREFIX}"/bin/pydoc3 "${pkgdir}${MINGW_PREFIX}"/bin/pydoc
 
+          # copy to /
+          cp -rf "${pkgdir}"/* /
+
       - name: Run Smoke Test (installed)
         shell: msys2 {0}
         run: |
-          export PYTHONTZPATH="${MINGW_PREFIX}/share/zoneinfo"
           SMOKETESTS="$(pwd)/mingw_smoketests.py"
-          cd _build
-          cd python_pkgdir/${MINGW_PREFIX}/bin
-          ./python.exe "$SMOKETESTS"
-          MSYSTEM= ./python.exe "$SMOKETESTS"
+          ${MINGW_PREFIX}/bin/python.exe "$SMOKETESTS"
+          MSYSTEM= ${MINGW_PREFIX}/bin/python.exe "$SMOKETESTS"
 
       - name: Compress
         if: always()

Lib/test/__main__.py

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

Python/dynload_win.c

@@ -4,6 +4,7 @@
 #include "Python.h"
 #include "pycore_fileutils.h"     // _Py_add_relfile()
 #include "pycore_pystate.h"       // _PyInterpreterState_GET()
+#include "pycore_initconfig.h"
 
 #ifdef HAVE_DIRECT_H
 #include <direct.h>
@@ -225,6 +226,18 @@ dl_funcptr _PyImport_FindSharedFuncptrWindows(const char *prefix,
     dl_funcptr p;
     char funcname[258], *import_python;
 
+    int use_legacy = 0;
+    DWORD load_library_flags = 0;
+
+    _Py_get_env_flag(1, &use_legacy, "PYTHONLEGACYWINDOWSDLLLOADING");
+
+    if (use_legacy) {
+        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 /* _MSC_VER */
@@ -249,8 +262,7 @@ 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
         PyMem_Free(wpathname);
 

Tools/build/check_extension_modules.py

@@ -60,6 +60,10 @@
     "winsound",
 }
 
+if sys.platform == "win32":
+    # Enable DLL loading from PATH.
+    os.environ["PYTHONLEGACYWINDOWSDLLLOADING"] = "1"
+
 
 logger = logging.getLogger(__name__)
 

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')