[boost] [interprocess] interprocess_condition blocks on notify_all if other process crashes

Subject: [boost] [interprocess] interprocess_condition blocks on notify_all if other process crashes
From: Remi Chateauneu (remi.chateauneu_at_[hidden])
Date: 2016-10-20 19:27:51

When two processes share memory, and if one of them dies leaving a mutex 
in locked state, then the other process blocks on a notify using a 
interprocess_condition, stored in shared memory.

This is described here:
http://stackoverflow.com/questions/18240263/boost-interprocess-condition-blocking-on-notify-all

The reason is that this condition uses in general, a 
ipcdetail::spin_condition. Here is what it does in 
boost/interprocess/sync/interprocess_condition.hpp and 
boost/interprocess/sync/spin/condition.hpp :

inline void spin_condition::notify(boost::uint32_t command)
{
    //This mutex guarantees that no other thread can enter to the
    //do_timed_wait method logic, so that thread count will be
    //constant until the function writes a NOTIFY_ALL command.
    //It also guarantees that no other notification can be signaled
    //on this spin_condition before this one ends
    m_enter_mut.lock();

It locks an internal mutex, and if it is already locked and not unlocked 
by the crashed process, we are blocked.

A fix is to replace lock() by:

if( ! m_enter_mut.timed_lock(delay) )
         throw an_exception();

A cleaner fix would be to derive a subclass from spin_condition to 
implement something like:

bool derived_spin_condition::timed_notify(boost::uint32_t, ptime delay)

What do you think ? Thanks.