Re: [Boost-users] [boost] [interprocess] Semaphore cleanup after crash

From: Sachin Garg (schngrg_at_[hidden])
Date: 2008-07-29 14:03:12

On Tue, Jul 29, 2008 at 10:32 PM, Ion Gaztañaga <igaztanaga_at_[hidden]> wrote:
> Sachin Garg wrote:
>>
>> If a semaphore is not in-use (open) by any process, in this case (in
>> my application) I can safely 'remove' it and start afresh. Is there
>> some way to find out if any process is using a semaphore at a time so
>> that I can call 'remove'?
>
> Inteprocess is modeled after posix primitives, so there is no way to know if
> someone is attached. Think about this as if the semaphore was a file. What
> would you do if you are communicating two processes with a file and one
> process crashes? I think you should have some keepalive mechanism to detect
> that a process has died and recreate ipc mechanisms on failure.

Yep, I understand this is the posix way of removing everything, be it
semaphores or other stuff. By keepalive do you mean having an umbrella
process to take care of recovering from such crashes? Or is it some
other standard mechanism that I am not aware of?

>> When I just add a 'remove' on process start this works great on
>> windows (as remove just fails if another process has the semaphore
>> open), but on linux sem_unlink is used which has the behavior of
>> deleting it even if its in use.
>
> This same problem happens with std::remove(const char *filename) (windows
> version fails if the file is in use but unix version calls unlink and
> removes that file from the filesystem without failing while attached
> processes still write to that phantom file) but this is a difference I don't
> know how to solve.

Yep. I tried forcing use of interprocess' cywgin and windows
implementation of named_semaphore on linux (just for experimenting) as
these are done differently. Windows one fails to compile and cygwin
implementation fails as that uses shm_unlink which works same as
sem_unlink, the posix way :-)

>> What is the general practice when it comes to cleaning up semaphores
>> after process crashes? Maybe some way to ensure that 'post' and
>> 'close' are always called even when application has otherwise crashed?
>> Is there some way to use boost's windows style semaphores on linux
>> instead of native posix style?
>>
>> I tried looking and many have asked this question (in context of
>> recovering from posix semaphores, which are used by boost on linux),
>> but I couldn't find any answers. Lars had asked this here also, almost
>> an year ago but no answers in that thread either. This seems like a
>> basic issue but am totally lost on how to even approach it.
>
> In general I see no general solution. You can't register cleanup actions
> when a process crashes (well, the OS can, but not the user code). If anyone
> has any idea about this, I would be glad to hear it.

The method discussed with Bob (in same thread), does that makes sense?
To programatically do what he proposes using commands.

I am not aware of system calls for this but it seems possible (ipcs
does this 'somehow') to find which process last used a semaphore and
then it can be checked if that process id is still alive, and only
then we can call sem_unlink. All this can be abstracted with boost in
a smart_remove or a safe_remove. Idea being to sem_unlink only when no
other process is using it.

If it doesn't looks like something of too much general value (though I
think it would be) I would atleast like to do this in my code, so any
pointers to relevant system calls will be really really helpful.

Thanks for all the great work done in interprocess.

Sachin Garg