From 8807d68aca042ea348ee1fb0f1ef6419c667dd6e 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/cmake/modules/AddLLDB.cmake |  6 ++++
 lldb/tools/driver/CMakeLists.txt |  4 +++
 lldb/tools/driver/Driver.cpp     | 61 ++++++++++++++++++++++++++++++++
 3 files changed, 71 insertions(+)

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..e393dc84792fc 100644
--- a/lldb/tools/driver/CMakeLists.txt
+++ b/lldb/tools/driver/CMakeLists.txt
@@ -28,6 +28,10 @@ add_dependencies(lldb
   ${tablegen_deps}
 )
 
+if(DEFINED Python3_VERSION)
+  target_compile_definitions(lldb PRIVATE Python3_VERSION=${Python3_VERSION})
+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..6a5e6afe819cd 100644
--- a/lldb/tools/driver/Driver.cpp
+++ b/lldb/tools/driver/Driver.cpp
@@ -20,7 +20,10 @@
 #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"
@@ -423,6 +426,60 @@ SBError Driver::ProcessArgs(const opt::InputArgList &args, bool &exiting) {
   return error;
 }
 
+#ifdef _WIN32
+inline std::wstring GetPathToExecutableW() {
+  // Iterate until we max out the Windows max path length (256*2^7 > 32,767).
+  static const size_t maximumIterations = 7;
+  std::vector<WCHAR> buffer;
+  DWORD bufferSize = MAX_PATH;
+  for (size_t i = 0; i < maximumIterations; i++) {
+    buffer.resize(bufferSize);
+    if (GetModuleFileNameW(NULL, buffer.data(), buffer.size()) < buffer.size())
+      return std::wstring(buffer.begin(), buffer.end());
+    bufferSize *= 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;
+
+  SmallString<MAX_PATH> pythonDir(utf8Path.begin(), utf8Path.end());
+  // Python-${Version} is located in the Swift directory and lldb.exe is in:
+  // `Swift/Toolchains/x.x.x/usr/bin/lldb.exe`.
+  for (int i = 0; i < 5; i++) {
+    sys::path::remove_filename(pythonDir);
+  }
+#define TO_STRING_IMPL(x) #x
+#define TO_STRING(x) TO_STRING_IMPL(x)
+  std::string pythonDirName = "Python-" TO_STRING(Python3_VERSION);
+#undef TO_STRING_IMPL
+#undef TO_STRING
+
+  sys::path::append(pythonDir, pythonDirName);
+
+  SmallVector<wchar_t, 1> widePythonDir;
+  if (sys::windows::UTF8ToUTF16(pythonDir.str(), widePythonDir))
+    return;
+
+  if (sys::fs::exists(pythonDir))
+    SetDllDirectoryW(widePythonDir.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 +810,10 @@ int main(int argc, char const *argv[]) {
                         "~/Library/Logs/DiagnosticReports/.\n");
 #endif
 
+#ifdef _WIN32
+  AddPythonDLLToSearchPath();
+#endif
+
   // Parse arguments.
   LLDBOptTable T;
   unsigned MissingArgIndex;