[Lldb-commits] [lldb] [lldb][windows] print an error if python.dll is not in the DLL search path (PR #164893)

[Charles Zablit via lldb-commits]

https://github.com/charles-zablit created https://github.com/llvm/llvm-project/pull/164893

This is a follow up to https://github.com/llvm/llvm-project/pull/162509.

Using the `SearchPathW` API, we can ensure that the correct version of Python is installed before `liblldb` is loaded (and `python.dll` subsequently). If it's not, we try to add it to the search path with the methods introduced in https://github.com/llvm/llvm-project/pull/162509. If that fails or if that method is `#ifdef`'d out, we print an error which will appear before lldb crashes due to the missing dll.

Before https://github.com/llvm/llvm-project/pull/162509, when invoked from Powershell, lldb would silently crash (no error message/crash report). After https://github.com/llvm/llvm-project/pull/162509, it crashes without any indications that the root cause is the missing python.dll. With this patch, we print the error before crashing.

>From 3d2976b3e251bee88b9dee88fef0dcd9460dded2 Mon Sep 17 00:00:00 2001
From: Charles Zablit <c_zablit at apple.com>
Date: Thu, 23 Oct 2025 13:48:29 -0700
Subject: [PATCH] [lldb][windows] print an error if python.dll is not in the
 DLL search path

---
 lldb/CMakeLists.txt              |  4 +++
 lldb/tools/driver/CMakeLists.txt |  3 +++
 lldb/tools/driver/Driver.cpp     | 45 +++++++++++++++++++++++++-------
 3 files changed, 42 insertions(+), 10 deletions(-)

diff --git a/lldb/CMakeLists.txt b/lldb/CMakeLists.txt
index e3b72e94d4beb..7c85c6fa8b825 100644
--- a/lldb/CMakeLists.txt
+++ b/lldb/CMakeLists.txt
@@ -61,6 +61,8 @@ 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_SHARED_LIBRARY_NAME
+    "Filename of Python's shared library")
 
   foreach(var LLDB_PYTHON_RELATIVE_PATH LLDB_PYTHON_EXE_RELATIVE_PATH LLDB_PYTHON_EXT_SUFFIX)
     if(NOT DEFINED ${var} AND NOT CMAKE_CROSSCOMPILING)
@@ -87,6 +89,8 @@ if (LLDB_ENABLE_PYTHON)
       set(LLDB_PYTHON_EXT_SUFFIX "_d${LLDB_PYTHON_EXT_SUFFIX}")
     endif()
   endif()
+  set(LLDB_PYTHON_SHARED_LIBRARY_FILENAME
+    "python${Python3_VERSION_MAJOR}${Python3_VERSION_MINOR}${CMAKE_SHARED_LIBRARY_SUFFIX}")
 endif ()
 
 if (LLDB_ENABLE_LUA)
diff --git a/lldb/tools/driver/CMakeLists.txt b/lldb/tools/driver/CMakeLists.txt
index 67956af7fe3fb..467ca9f91b3c1 100644
--- a/lldb/tools/driver/CMakeLists.txt
+++ b/lldb/tools/driver/CMakeLists.txt
@@ -37,6 +37,9 @@ add_dependencies(lldb
 if(DEFINED LLDB_PYTHON_DLL_RELATIVE_PATH)
   target_compile_definitions(lldb PRIVATE LLDB_PYTHON_DLL_RELATIVE_PATH="${LLDB_PYTHON_DLL_RELATIVE_PATH}")
 endif()
+if(DEFINED LLDB_PYTHON_SHARED_LIBRARY_FILENAME)
+  target_compile_definitions(lldb PRIVATE LLDB_PYTHON_SHARED_LIBRARY_FILENAME="${LLDB_PYTHON_SHARED_LIBRARY_FILENAME}")
+endif()
 
 if(LLDB_BUILD_FRAMEWORK)
   # In the build-tree, we know the exact path to the framework directory.
diff --git a/lldb/tools/driver/Driver.cpp b/lldb/tools/driver/Driver.cpp
index ba0041111045b..96157525f3703 100644
--- a/lldb/tools/driver/Driver.cpp
+++ b/lldb/tools/driver/Driver.cpp
@@ -433,7 +433,8 @@ SBError Driver::ProcessArgs(const opt::InputArgList &args, bool &exiting) {
   return error;
 }
 
-#if defined(_WIN32) && defined(LLDB_PYTHON_DLL_RELATIVE_PATH)
+#ifdef _WIN32
+#ifdef LLDB_PYTHON_DLL_RELATIVE_PATH
 /// Returns the full path to the lldb.exe executable.
 inline std::wstring GetPathToExecutableW() {
   // Iterate until we reach the Windows API maximum path length (32,767).
@@ -447,32 +448,51 @@ inline std::wstring GetPathToExecutableW() {
   return L"";
 }
 
-/// Resolve the full path of the directory defined by
+/// \brief Resolve the full path of the directory defined by
 /// LLDB_PYTHON_DLL_RELATIVE_PATH. If it exists, add it to the list of DLL
 /// search directories.
-void AddPythonDLLToSearchPath() {
+/// \return `true` if the library was added to the search path.
+/// `false` otherwise.
+bool AddPythonDLLToSearchPath() {
   std::wstring modulePath = GetPathToExecutableW();
   if (modulePath.empty()) {
-    llvm::errs() << "error: unable to find python.dll." << '\n';
-    return;
+    return false;
   }
 
   SmallVector<char, MAX_PATH> utf8Path;
   if (sys::windows::UTF16ToUTF8(modulePath.c_str(), modulePath.length(),
                                 utf8Path))
-    return;
+    return false;
   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;
+    return false;
 
   if (sys::fs::exists(utf8Path))
-    SetDllDirectoryW(widePath.data());
+    return SetDllDirectoryW(widePath.data());
+  return false;
+}
+#endif
+
+#ifdef LLDB_PYTHON_SHARED_LIBRARY_FILENAME
+/// Returns whether `python3x.dll` is in the DLL search path.
+bool IsPythonDLLInPath() {
+#define WIDEN2(x) L##x
+#define WIDEN(x) WIDEN2(x)
+  WCHAR foundPath[MAX_PATH];
+  DWORD result =
+      SearchPathW(nullptr, WIDEN(LLDB_PYTHON_SHARED_LIBRARY_FILENAME), nullptr,
+                  MAX_PATH, foundPath, nullptr);
+#undef WIDEN2
+#undef WIDEN
+
+  return result > 0;
 }
 #endif
+#endif
 
 std::string EscapeString(std::string arg) {
   std::string::size_type pos = 0;
@@ -776,8 +796,13 @@ int main(int argc, char const *argv[]) {
                         "~/Library/Logs/DiagnosticReports/.\n");
 #endif
 
-#if defined(_WIN32) && defined(LLDB_PYTHON_DLL_RELATIVE_PATH)
-  AddPythonDLLToSearchPath();
+#if defined(_WIN32) && defined(LLDB_PYTHON_SHARED_LIBRARY_FILENAME)
+  if (!IsPythonDLLInPath())
+#ifdef LLDB_PYTHON_DLL_RELATIVE_PATH
+    if (!AddPythonDLLToSearchPath())
+#endif
+      llvm::errs() << "error: unable to find "
+                   << LLDB_PYTHON_SHARED_LIBRARY_FILENAME << ".\n";
 #endif
 
   // Parse arguments.