Copy conda DLLs to build dir at configure time instead of manipulating DLL search paths

Author: akenmorris

Libs/Application/Job/PythonWorker.cpp

@@ -237,34 +237,14 @@ bool PythonWorker::init() {
 
 #ifdef _WIN32
   if (using_bundled) {
-    // Register DLL directories via AddDllDirectory BEFORE Python init.
-    // We do NOT call SetDefaultDllDirectories here — Python 3.8+ calls
-    // SetDefaultDllDirectories(LOAD_LIBRARY_SEARCH_DEFAULT_DIRS) internally
-    // during Py_Initialize(), which enables LOAD_LIBRARY_SEARCH_USER_DIRS.
-    // Directories registered via AddDllDirectory before that point will then
-    // be searched for implicit DLL dependencies of .pyd extensions.
+    // Conda's .pyd extensions need DLLs from Library/bin/ (e.g. libexpat.dll).
+    // In the build tree, these are copied next to the executables by CMake.
+    // In the installed app, critical DLLs are in bin/ from InstallBundledPython.
+    // Prepend key directories to PATH as a safety net.
     QString dlls_dir = python_home + "/DLLs";
     QString app_dir = QCoreApplication::applicationDirPath();
-    AddDllDirectory(dlls_dir.toStdWString().c_str());
-    AddDllDirectory(python_home.toStdWString().c_str());
-    AddDllDirectory(app_dir.toStdWString().c_str());
-
-    // In the build tree, PYTHONHOME points to conda's prefix. Conda's .pyd
-    // files need DLLs from conda's Library/bin/ (e.g. libexpat.dll for pyexpat).
-    QString conda_library_bin = python_home + "/Library/bin";
-    if (QDir(conda_library_bin).exists()) {
-      AddDllDirectory(conda_library_bin.toStdWString().c_str());
-    }
-
-    // Also prepend to PATH for any DLL loading that happens before Python's
-    // SetDefaultDllDirectories takes effect.
     QString current_path = qgetenv("PATH");
-    QString new_path = dlls_dir + ";" + python_home + ";" + app_dir;
-    if (QDir(conda_library_bin).exists()) {
-      new_path += ";" + conda_library_bin;
-    }
-    new_path += ";" + current_path;
-    qputenv("PATH", new_path.toUtf8());
+    qputenv("PATH", (dlls_dir + ";" + python_home + ";" + app_dir + ";" + current_path).toUtf8());
 
     SW_LOG("Registered bundled Python DLL directories");
   } else {

cmake/BundledPython.cmake

@@ -106,6 +106,21 @@ if(WIN32)
   # On Windows, Python3_EXECUTABLE is directly in the conda prefix (no bin/ subdir)
   set(_conda_prefix "${_conda_prefix}")
   message(STATUS "Windows: skipping conda extension copy (bundled Python has its own DLLs)")
+
+  # Copy conda's Library/bin DLLs to the build output directory so the embedded
+  # interpreter can find them. Conda's .pyd extensions (pyexpat, _ssl, etc.) have
+  # implicit DLL dependencies (libexpat, libssl, etc.) in Library/bin/. In the
+  # build tree, the application directory is always searched for DLLs, so placing
+  # them next to the executables is the most reliable approach.
+  set(_conda_lib_bin "${_conda_prefix}/Library/bin")
+  if(EXISTS "${_conda_lib_bin}")
+    file(GLOB _conda_dlls "${_conda_lib_bin}/*.dll")
+    list(LENGTH _conda_dlls _num_dlls)
+    message(STATUS "Copying ${_num_dlls} DLLs from conda Library/bin to build output")
+    file(COPY ${_conda_dlls} DESTINATION "${CMAKE_BINARY_DIR}/bin/Release")
+    # Also copy to bin/ for NMake builds
+    file(COPY ${_conda_dlls} DESTINATION "${CMAKE_BINARY_DIR}/bin")
+  endif()
 else()
   get_filename_component(_conda_prefix "${_conda_prefix}" DIRECTORY)       # .../
   set(_conda_lib_dynload "${_conda_prefix}/lib/python3.12/lib-dynload")