[lldb] Add build option to specify the libxml 2 version

[Steelskin]

The Swift Windows toolchain uses its own static build of libxml 2, which is more recent than 2.8, resulting in the provided libxml 2 to be rejected. This change allows to specify a custom version for libxml 2, while defaulting to 2.8.

[llvmbot]

@llvm/pr-subscribers-lldb

Author: Fabrice de Gans (Steelskin)

Changes

The Swift Windows toolchain uses its own static build of libxml 2, which is more recent than 2.8, resulting in the provided libxml 2 to be rejected. This change allows to specify a custom version for libxml 2, while defaulting to 2.8.

---

Full diff: https://github.com/llvm/llvm-project/pull/142183.diff

1 Files Affected:

• (modified) lldb/cmake/modules/LLDBConfig.cmake (+3-1)

diff --git a/lldb/cmake/modules/LLDBConfig.cmake b/lldb/cmake/modules/LLDBConfig.cmake
index cfd626c9358a1..f25ad1333468c 100644
--- a/lldb/cmake/modules/LLDBConfig.cmake
+++ b/lldb/cmake/modules/LLDBConfig.cmake
@@ -51,13 +51,15 @@ macro(add_optional_dependency variable description package found)
   message(STATUS "${description}: ${${variable}}")
 endmacro()
 
+option(LLDB_LIBXML2_VERSION "Sepcify the version of libxml 2 to use with LLDB" "2.8")
+
 add_optional_dependency(LLDB_ENABLE_SWIG "Enable SWIG to generate LLDB bindings" SWIG SWIG_FOUND VERSION 4)
 add_optional_dependency(LLDB_ENABLE_LIBEDIT "Enable editline support in LLDB" LibEdit LibEdit_FOUND)
 add_optional_dependency(LLDB_ENABLE_CURSES "Enable curses support in LLDB" CursesAndPanel CURSESANDPANEL_FOUND)
 add_optional_dependency(LLDB_ENABLE_LZMA "Enable LZMA compression support in LLDB" LibLZMA LIBLZMA_FOUND)
 add_optional_dependency(LLDB_ENABLE_LUA "Enable Lua scripting support in LLDB" LuaAndSwig LUAANDSWIG_FOUND)
 add_optional_dependency(LLDB_ENABLE_PYTHON "Enable Python scripting support in LLDB" PythonAndSwig PYTHONANDSWIG_FOUND)
-add_optional_dependency(LLDB_ENABLE_LIBXML2 "Enable Libxml 2 support in LLDB" LibXml2 LIBXML2_FOUND VERSION 2.8)
+add_optional_dependency(LLDB_ENABLE_LIBXML2 "Enable Libxml 2 support in LLDB" LibXml2 LIBXML2_FOUND VERSION ${LLDB_LIBXML2_VERSION})
 add_optional_dependency(LLDB_ENABLE_FBSDVMCORE "Enable libfbsdvmcore support in LLDB" FBSDVMCore FBSDVMCore_FOUND QUIET)
 
 option(LLDB_USE_ENTITLEMENTS "When codesigning, use entitlements if available" ON)

[JDevlieghere]

Could the problem be that the statically linked libxml2 is reporting the wrong version or something? We don't pass the EXACT flag, which means that CMake will find a version that "is compatible" [1]. The latter is intentionally vague and I couldn't immediately find the corresponding logic for libxml2. However, on my linux machine, CMake happily picks up libxml 2.12.7 with the existing logic.

[1] https://cmake.org/cmake/help/latest/command/find_package.html#config-mode-version-selection

[Steelskin]

This is how the version file is configured in libxml2: https://github.com/GNOME/libxml2/blob/0740a367a570c40246fe8077c2d172b63693f105/CMakeLists.txt#L596

write_basic_package_version_file(
	${CMAKE_CURRENT_BINARY_DIR}/libxml2-config-version.cmake
	VERSION ${PROJECT_VERSION}
	COMPATIBILITY ExactVersion
)

The problem is the COMPATIBILITY ExactVersion part. I have been told there are also ABI incompatibilities with different libxml 2 versions for the Swift build but I am not familiar with the details.

[JDevlieghere]

Ack. My apt package doesn't install libxml2-config-version.cmake so I assume CMake is falling back on some standard logic for finding it and that would explain why CMake is happy to use 2.12 on my system.

That said, it seems like this is done intentionally, and this is working around that (e.g. the ABI incompatibility you mentioned). That might be fine for how we use libxml2 in lldb but let's make it clear that folks are doing this at their own risk. Can you update the description or add a comment with something along those lines?

[Steelskin]

Ack. My apt package doesn't install libxml2-config-version.cmake so I assume CMake is falling back on some standard logic for finding it and that would explain why CMake is happy to use 2.12 on my system.

That said, it seems like this is done intentionally, and this is working around that (e.g. the ABI incompatibility you mentioned). That might be fine for how we use libxml2 in lldb but let's make it clear that folks are doing this at their own risk. Can you update the description or add a comment with something along those lines?

I updated the comment and marked the new cache variable as advanced.

[JDevlieghere]

LGTM