From c76bdbd43e7f28f4f323f0024a98aa8a0e51699c Mon Sep 17 00:00:00 2001
From: Charles Zablit <c_zablit@apple.com>
Date: Mon, 6 Oct 2025 16:30:14 +0100
Subject: [PATCH] [lldb][windows] delay loading python.dll

---
 lldb/CMakeLists.txt              |  2 ++
 lldb/cmake/modules/AddLLDB.cmake |  6 ++++
 lldb/tools/driver/CMakeLists.txt |  4 +++
 lldb/tools/driver/Driver.cpp     | 50 ++++++++++++++++++++++++++++++++
 4 files changed, 62 insertions(+)

diff --git a/lldb/CMakeLists.txt b/lldb/CMakeLists.txt
index daa0b437f9d5d..ca721a1fdc8b6 100644
--- a/lldb/CMakeLists.txt
+++ b/lldb/CMakeLists.txt
@@ -84,6 +84,8 @@ if (LLDB_ENABLE_PYTHON)
     "Path to python interpreter exectuable, relative to python's install prefix")
   set(cachestring_LLDB_PYTHON_EXT_SUFFIX
     "Filename extension for native code python modules")
+  set(cachestring_LLDB_PYTHON_DLL_RELATIVE_PATH
+    "Path where python.dll is installed, relative to LLDB's install prefix")
 
   foreach(var LLDB_PYTHON_RELATIVE_PATH LLDB_PYTHON_EXE_RELATIVE_PATH LLDB_PYTHON_EXT_SUFFIX)
     if(NOT DEFINED ${var} AND NOT CMAKE_CROSSCOMPILING)
diff --git a/lldb/cmake/modules/AddLLDB.cmake b/lldb/cmake/modules/AddLLDB.cmake
index c91b08f371112..d20145536a135 100644
--- a/lldb/cmake/modules/AddLLDB.cmake
+++ b/lldb/cmake/modules/AddLLDB.cmake
@@ -283,6 +283,12 @@ function(add_lldb_executable name)
   )
 
   target_link_libraries(${name} PRIVATE ${ARG_LINK_LIBS})
+  if(WIN32)
+    list(FIND ARG_LINK_LIBS liblldb LIBLLD_INDEX)
+    if(NOT LIBLLD_INDEX EQUAL -1)
+      target_link_options(${name} PRIVATE "/DELAYLOAD:$<TARGET_FILE_BASE_NAME:liblldb>.dll")
+    endif()
+  endif()
   if(CLANG_LINK_CLANG_DYLIB)
     target_link_libraries(${name} PRIVATE clang-cpp)
   else()
diff --git a/lldb/tools/driver/CMakeLists.txt b/lldb/tools/driver/CMakeLists.txt
index cd304a047dea6..569a5abe32af8 100644
--- a/lldb/tools/driver/CMakeLists.txt
+++ b/lldb/tools/driver/CMakeLists.txt
@@ -28,6 +28,10 @@ add_dependencies(lldb
   ${tablegen_deps}
 )
 
+if(DEFINED LLDB_PYTHON_DLL_RELATIVE_PATH)
+  target_compile_definitions(lldb PRIVATE LLDB_PYTHON_DLL_RELATIVE_PATH="${LLDB_PYTHON_DLL_RELATIVE_PATH}")
+endif()
+
 if(LLDB_BUILD_FRAMEWORK)
   # In the build-tree, we know the exact path to the framework directory.
   # The installed framework can be in different locations.
diff --git a/lldb/tools/driver/Driver.cpp b/lldb/tools/driver/Driver.cpp
index 4f9a920e1136b..7436a2e1e0f42 100644
--- a/lldb/tools/driver/Driver.cpp
+++ b/lldb/tools/driver/Driver.cpp
@@ -20,11 +20,15 @@
 #include "lldb/API/SBStructuredData.h"
 #include "lldb/Host/Config.h"
 
+#include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/ConvertUTF.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Signals.h"
+#include "llvm/Support/Windows/WindowsSupport.h"
 #include "llvm/Support/WithColor.h"
 #include "llvm/Support/raw_ostream.h"
 
@@ -423,6 +427,48 @@ SBError Driver::ProcessArgs(const opt::InputArgList &args, bool &exiting) {
   return error;
 }
 
+#ifdef _WIN32
+// Returns the full path to the lldb.exe executable
+inline std::wstring GetPathToExecutableW() {
+  // Iterate until we reach the Windows max path length (32,767).
+  std::vector<WCHAR> buffer;
+  buffer.resize(MAX_PATH);
+  while (buffer.size() < 32767) {
+    if (GetModuleFileNameW(NULL, buffer.data(), buffer.size()) < buffer.size())
+      return std::wstring(buffer.begin(), buffer.end());
+    buffer.resize(buffer.size() * 2);
+  }
+  return L"";
+}
+
+// Resolve the Embeddable Python directory bundled with the Swift toolchain.
+// If it exists, add it to the list of DLL search directories.
+// The installation of Embeddable Python is optional when installing the Swift
+// toolchain, therefore the directory might not exist.
+void AddPythonDLLToSearchPath() {
+  std::wstring modulePath = GetPathToExecutableW();
+  if (modulePath.empty()) {
+    WithColor::error() << "Unable to find python: " << GetLastError() << '\n';
+    return;
+  }
+
+  SmallVector<char, MAX_PATH> utf8Path;
+  if (sys::windows::UTF16ToUTF8(modulePath.c_str(), modulePath.length(),
+                                utf8Path))
+    return;
+  sys::path::remove_filename(utf8Path);
+  sys::path::append(utf8Path, LLDB_PYTHON_DLL_RELATIVE_PATH);
+  sys::fs::make_absolute(utf8Path);
+
+  SmallVector<wchar_t, 1> widePath;
+  if (sys::windows::widenPath(utf8Path.data(), widePath))
+    return;
+
+  if (sys::fs::exists(utf8Path))
+    SetDllDirectoryW(widePath.data());
+}
+#endif
+
 std::string EscapeString(std::string arg) {
   std::string::size_type pos = 0;
   while ((pos = arg.find_first_of("\"\\", pos)) != std::string::npos) {
@@ -753,6 +799,10 @@ int main(int argc, char const *argv[]) {
                         "~/Library/Logs/DiagnosticReports/.\n");
 #endif
 
+#ifdef _WIN32
+  AddPythonDLLToSearchPath();
+#endif
+
   // Parse arguments.
   LLDBOptTable T;
   unsigned MissingArgIndex;