python: introduce MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL.

If enabled, this causes sys.setdlopenflags() being called with
RTLD_GLOBAL before the native Corrade module is loaded, in a hope to
resolve recurring nightmares with static Corrade and Magnum libraries
being linked into multiple dynamic modules.

CMakeLists.txt

@@ -91,6 +91,12 @@ endif()
 
 option(MAGNUM_BUILD_TESTS "Build unit tests" OFF)
 
+if(MAGNUM_BUILD_STATIC AND UNIX)
+    # See src/python/corrade/__init__.py.in and the corresponding CMakeLists
+    # for details
+    option(MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL "Build Python bindings linking to static libraries with RTLD_GLOBAL enabled for loading" ON)
+endif()
+
 # Backwards compatibility for unprefixed CMake options. If the user isn't
 # explicitly using prefixed options in the first run already, accept the
 # unprefixed options, and remember this decision for subsequent runs

doc/python/pages/building.rst

@@ -150,14 +150,31 @@ containing location of all built libraries for use with Python setuptools:
 
 In case Corrade or Magnum is built with :dox:`CORRADE_BUILD_STATIC` /
 :dox:`MAGNUM_BUILD_STATIC`, the corresponding bindings are compiled into a
-single dynamic module instead of one module per Corrade/Magnum library. In this
-case, similarly to linking static plugins to Magnum's own command-line
+single dynamic module instead of one module per Corrade/Magnum library.
+
+In this case, similarly to linking static plugins to Magnum's own command-line
 utilities, you can use the ``MAGNUM_PYTHON_BINDINGS_STATIC_PLUGINS`` CMake
 variable to link static plugins to the Python module, assuming Magnum, Magnum
 Plugins and Magnum Bindings are all CMake subprojects. It's a
 semicolon-separated list of existing CMake targets, for example
 ``Magnum::AnyImageImporter;MagnumPlugins::StbImageImporter``.
 
+On Unix platforms, Python by default loads the modules in isolated namespaces.
+That's a good thing to do from a security point of view, nevertheless with
+static builds of Corrade and Magnum it will result in globals duplicated across
+the Corrade and Magnum modules (and also any other Python modules that
+use Magnum natively inside) unable to see each other in order to deduplicate
+themselves, causing strange issues. To solve that, the
+``MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL`` CMake option, which is enabled by
+default on static builds on Unix platforms, overrides Python's module loading
+code to not load them in isolated namespaces using :ref:`sys.setdlopenflags()`.
+The override then happens inside :py:`import corrade` and is in effect for the
+rest of the interpreter lifetime, to affect also any potential other modules
+depending on Magnum loaded later. See also
+:dox:`CORRADE_BUILD_STATIC_UNIQUE_GLOBALS` and
+:dox:`MAGNUM_BUILD_STATIC_UNIQUE_GLOBALS` in Corrade and Magnum for more
+information.
+
 `Running unit tests`_
 ---------------------
 

doc/python/pages/changelog.rst

@@ -176,6 +176,12 @@ Changelog
 -   Added a ``MAGNUM_PYTHON_BINDINGS_STATIC_PLUGINS`` CMake option for linking
     static plugins to the Python bindings module. See the
     :ref:`building documentation <std:doc:building>` for more information.
+-   Added a ``MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL`` CMake option to make
+    the Python bindings module loaded into the global namespace instead of
+    isolated in order to attempt to solve problems with duplicated globals when
+    static builds of Corrade and Magnum are linked into multiple dynamic
+    modules. See the :ref:`building documentation <std:doc:building>` for more
+    information.
 
 `2020.06`_
 ==========

src/python/corrade/CMakeLists.txt

@@ -84,7 +84,9 @@ if(NOT CORRADE_BUILD_STATIC)
 
 # Otherwise put it all into one library so it's easier to install (which is the
 # point of static builds). It also nicely avoids problems with multiply-defined
-# global data.
+# global data. Unless the static libraries are linked into multiple Python
+# modules, that is, which is (on Unix at least) attempted to be solved by the
+# MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL option below.
 else()
     configure_file(${CMAKE_CURRENT_SOURCE_DIR}/staticconfigure.h.cmake
                    ${CMAKE_CURRENT_BINARY_DIR}/staticconfigure.h)
@@ -102,6 +104,16 @@ else()
     endif()
 endif()
 
+# If the option is enabled, the setdlopenflags() code in __init__.py is used,
+# otherwise it's commented out
+if(MAGNUM_BUILD_STATIC AND UNIX AND MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL)
+    set(_MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL "")
+else()
+    set(_MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL "## ")
+endif()
+configure_file(${CMAKE_CURRENT_SOURCE_DIR}/__init__.py.in
+               ${CMAKE_CURRENT_BINARY_DIR}/__init__.py)
+
 pybind11_add_module(corrade ${pybind11_add_module_SYSTEM} ${corrade_SRCS})
 target_include_directories(corrade PRIVATE
     ${PROJECT_SOURCE_DIR}/src
@@ -113,7 +125,7 @@ set_target_properties(corrade PROPERTIES
     LIBRARY_OUTPUT_DIRECTORY ${output_dir})
 
 file(GENERATE OUTPUT ${output_dir}/corrade/__init__.py
-    INPUT ${CMAKE_CURRENT_SOURCE_DIR}/__init__.py)
+    INPUT ${CMAKE_CURRENT_BINARY_DIR}/__init__.py)
 
 if(MAGNUM_BUILD_TESTS)
     add_subdirectory(test)

src/python/corrade/__init__.py → src/python/corrade/__init__.py.in

@@ -25,6 +25,14 @@
 
 """Root Corrade module"""
 
+# If MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL is enabled in CMake, the lines
+# below are included, otherwise they are commented out. What it does is a
+# futile attempt to make Corrade (and Magnum) built as static libraries work
+# together when built into multiple dynamic Python modules.
+${_MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL}import sys
+${_MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL}import ctypes
+${_MAGNUM_BUILD_PYTHON_BINDINGS_RTLD_GLOBAL}sys.setdlopenflags(sys.getdlopenflags()|ctypes.RTLD_GLOBAL)
+
 # On Windows, if a known directory layout is detected, add paths containing
 # binaries to the DLL search path
 import platform
@@ -59,3 +67,5 @@
     # to globals and this would likely cause conflicts (magnum also defines
     # BUILD_*)
 ]
+
+# kate: hl python