> It seems my original concerns [1] about thread safety are somewhat justified
> now, according to helgrind:
> 
> development-branch: 236 errors from 8 contexts
> This branch: 1998 errors from 9 contexts
> 
> Tested with:
>     valgrind --tool=helgrind bin/mir_unit_tests
> --gtest_filter="SwitchingBundle*"
> 
> [1] https://code.launchpad.net/~alan-griffiths/mir/refactoring-so-
> SwitchingBundle-can-control-completion-of-client_acquire/+merge/204244
> 
> I can't immediately see what the new context is, but the significant increase
> in errors warrants some attention.

With the following suppression file we get a clean run. (Please check you're happy with these suppressions.)

AFAICS the only possibly contentious one is for client_acquire_blocking() in test_swapping_swappers.cpp

###############################################################################
# Part 1: Suppress spurious races in std library functions

{
   std::atomic::load
   Helgrind:Race
   fun:_ZNKSt13__atomic_baseIbE4loadESt12memory_order
}
{
   std::atomic::store
   Helgrind:Race
   fun:_ZNSt13__atomic_baseIbE5storeEbSt12memory_order
}

{
   std::mutex::unlock()
   Helgrind:Race
   fun:_ZNSt5mutex6unlockEv
   obj:*
}

{
   std::mutex::lock()
   Helgrind:Race
   fun:_ZNSt5mutex4lockEv
   obj:*
}

{
   std::thread::join
   Helgrind:Race
   obj:/usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so
   fun:_ZNSt6thread4joinEv
}

{
   std::unique_lock::~unique_lock()
   Helgrind:Race
   fun:_ZNSt11unique_lockISt5mutexED1Ev
   obj:*
}

{
   std::unique_lock::unique_lock(std::mutex&)
   Helgrind:Race
   fun:_ZNSt11unique_lockISt5mutexEC1ERS0_
}

{
   std::shared_ptr<mir::graphics::Buffer, (__gnu_cxx::_Lock_policy)2>::get() const
   Helgrind:Race
   fun:_ZNKSt12__shared_ptrIN3mir8graphics6BufferELN9__gnu_cxx12_Lock_policyE2EE3getEv
   obj:*
}

{
   std::chrono::duration_cast
   Helgrind:Race
   fun:_ZNSt6chrono20__duration_cast_implINS_8durationIlSt5ratioILl1ELl1EEEES2_ILl1ELl1000EElLb1ELb0EE6__castIlS5_EES4_RKNS1_IT_T0_EE
}

{
   std::thread::_Impl_base::~_Impl_base()
   Helgrind:Race
   fun:_ZNSt6thread10_Impl_baseD1Ev
}

{
   std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count()
   Helgrind:Race
   fun:_ZNSt14__shared_countILN9__gnu_cxx12_Lock_policyE2EED1Ev
}

###############################################################################
# Part 2: Suppress spurious races in std library template instantiations

{
   std::thread::_Impl<std::_Bind_simple<void (*(std::reference_wrapper<mir::compositor::SwitchingBundle>, std::reference_wrapper<unsigned long>, std::reference_wrapper<mir::graphics::BufferID>))(mir::compositor::SwitchingBundle&, unsigned long&, mir::graphics::BufferID&)> >::~_Impl()
   Helgrind:Race
   fun:_ZNSt6thread5_ImplISt12_Bind_simpleIFPFvRN3mir10compositor15SwitchingBundleERmRNS2_8graphics8BufferIDEESt17reference_wrapperIS4_ESC_ImESC_IS8_EEEED1Ev
}

{
   std::thread::_Impl<std::_Bind_simple<void (*(std::reference_wrapper<mir::compositor::SwitchingBundle>, int))(mir::compositor::SwitchingBundle&, int)> >::~_Impl()
   Helgrind:Race
   fun:_ZNSt6thread5_ImplISt12_Bind_simpleIFPFvRN3mir10compositor15SwitchingBundleEiESt17reference_wrapperIS4_EiEEED1Ev
}


###############################################################################
# Part 3: Suppress spurious races in Mir test code that I've reviewed carefully

{
   (anonymous namespace)::client_acquire_blocking(mir::compositor::SwitchingBundle&)
   Helgrind:Race
   fun:_ZN12_GLOBAL__N_123client_acquire_blockingERN3mir10compositor15SwitchingBundleE
}