Mir

Merge lp://qastaging/~afrantzis/mir/mir-client-ensure-global-symbol-resolution into lp://qastaging/mir

mir-client-ensure-global-symbol-resolution
Merge into development-branch

Proposed by Alexandros Frantzis on 2013-11-21

Status:	Work in progress
Proposed branch:	lp://qastaging/~afrantzis/mir/mir-client-ensure-global-symbol-resolution
Merge into:	lp://qastaging/mir
Diff against target:	219 lines (+151/-1) 7 files modified debian/rules (+2/-1) include/client/mir_toolkit/mir_client_ensure_global_symbol_resolution.h (+40/-0) src/client/CMakeLists.txt (+2/-0) src/client/mir_client_ensure_global_symbol_resolution.cpp (+28/-0) tests/CMakeLists.txt (+5/-0) tests/special-linkage-tests/CMakeLists.txt (+24/-0) tests/special-linkage-tests/test_client_symbol_resolution.cpp (+50/-0)
To merge this branch:	bzr merge lp://qastaging/~afrantzis/mir/mir-client-ensure-global-symbol-resolution
Related bugs:	Link a bug report

Reviewer	Review Type	Date Requested	Status
Daniel van Vugt			Needs Fixing on 2013-12-18
PS Jenkins bot (community)	continuous-integration		Needs Fixing on 2013-11-27
Alan Griffiths		2013-11-21	Approve on 2013-11-26
Review via email: mp+196110@code.qastaging.launchpad.net

Commit message

client: Provide a way to ensure libmirclient's symbols are available for symbol resolution in other shared objects

This is useful if libmirclient, or another library linking to libmirclient, is loaded
at runtime with dlopen() using RTLD_LOCAL (e.g., through a plugin mechanism)
and libmirclient's symbols need to be resolved at runtime in other shared objects
(like Mesa's libEGL).

Description of the change

client: Provide a way to ensure libmirclient's symbols are available for symbol resolution in other shared objects

The specific problem that we are trying to solve is that with Qt client applications on the desktop, EGL fails to recognize native MirConnections as valid EGLNativeDisplayType objects, falling back to X11 and crashing. The reason is that the symbols needed for the MirConnection validation and which are exported by libmirclient, are not available to libEGL for dynamic runtime resolution. The problem occurs because Qt loads the platform shared object at runtime with local symbol visibility (RTLD_LOCAL), which is reasonable in general, but not suitable for our use case.

(See reference diagram: http://people.ubuntu.com/~afrantzis/mir-egl-symbol-resolution.png for the following)
Concerning our options for using this:

Ideally we would want to deal with the issue in libEGL, but unfortunately we cannot, since it would defeat the purpose of the runtime resolution, which is to transparently support applications linking against either libmirserver or libmirclient or both (or neither) and doing the right thing.

Next choice would be the Qt platform plugin (libqubuntumirclient), but the plugin doesn't have any knowledge of libmirclient, it's all abstracted through platform-api. We could make it work by linking with libmirclient at either runtime or loadtime and call the mir_client_ensure_global_symbol_resolution() from here.

Moving down a step we reach the platform-api from where we do have direct access to libmirclient. It's easy to call mir_client_ensure_global_symbol_resolution() from here. The trade-off is that we are then going to force this behavior on all users of the platform-api's mirclient backend (which might be OK).

We could even make libmirclient itself force all of its symbols to become available for global resolution, but this seems too intrusive for now (but it is an option if we find that this problem is widespread).