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Entry  20 Feb 2019, Konstantin Olchanski, Info, odb needs protection against ctrl-c 
    Reply  20 Feb 2019, Stefan Ritt, Info, odb needs protection against ctrl-c 
       Reply  20 Feb 2019, Konstantin Olchanski, Info, odb needs protection against ctrl-c 
          Reply  20 Feb 2019, Stefan Ritt, Info, odb needs protection against ctrl-c 
    Reply  20 Feb 2019, Konstantin Olchanski, Info, odb needs protection against ctrl-c 
Message ID: 1451     Entry time: 20 Feb 2019     In reply to: 1449     Reply to this: 1452
Author: Konstantin Olchanski 
Topic: Info 
Subject: odb needs protection against ctrl-c 
> Not sure if you realized, but there is a two-stage Ctrl-C handling inside midas.

Hmm... I am looking at the ctrl-c handler inside odbedit.

Yes, and the original bug report is against odbedit - press of ctrl-c in odbedit corrupts odb,
see stack trace in

So maybe only the odbedit ctrl-c handler is defective...

I will take a look at what the other ctrl-c handler does.

Safest is probably to call exit() without calling cm_disconnect_experiment().

From the ctrl-c handler, if we call cm_disconnect_experiment() -
- if we hold odb locked, we deadlock (after I remove the recursive mutex) or corrupt odb (if we run form inside db_create or db_set_data).
- if we run from inside db_lock/unlock, we abort() (with my newly added protection) or explode (if we run from inside mprotect(), like in the stack strace in the bug report)

I would say, from a signal handler, only safe things are - set a flag, or abort()/exit().

exit() is not super safe because the user may have attached some code to it that may access odb. (our
default atexit() handler just prints an error message).


> The first time you hit ctrl-c, the handler just sets a flag for the main event loop, so that the program can gracefully exit without trouble. This is 
> done inside cm_ctrlc_handler(), which sets _ctrlc_pressed true if called. Then cm_yield() tests this flag and returns RPC_SHUTDOWN if so. I agree not very obvious, maybe we should return a more appropriate status. So the 
> main loop must check the return status of cm_yield() and break if it's RPC_SHUTDOWN. The frontend framework mfe.c does this for example in
>  while (status != RPC_SHUTDOWN && status != SS_ABORT);
> Any use-written program should do the same (well, probably this is nowhere documented).
> Now when the program does not exit (e.g. if it's in an infinite loop), then the second ctrl-c creates a hard abort and terminates the program non-gracefully, which as you noticed can lead to undesired results. All the 
> semaphores (at least when I implemented it) had a SEM_UNDO flag when obtaining ownership. This means that if the semaphore is locked and the process who owns it terminates (even with a hard kill), then the semaphore 
> is released by the OS. This way a crashed program cannot keep the ODB locked for example. Not sure that with all your modifications in the semaphore calls this functionality is still guaranteed.
> Stefan
> > Even with the cm_watchdog signal removed, some trouble from UNIX signals remains.
> > 
> > This time, when one presses Ctrl-C at the wrong time, the Ctrl-C signal handler will run at the wrong time
> > and strange things will happen (including odb corruption).
> > 
> > In the captured stack trace, I pressed Ctrl-C right when odbedit was inside db_lock_database(). I had to make special
> > arrangements to make it happen, but I have seen it happen in normal use when running experiments.
> > 
> > K.O.
> > 
> > (lldb) bt
> > * thread #1, queue = '', stop reason = signal SIGABRT
> >   * frame #0: 0x00007fff6c2ceb66 libsystem_kernel.dylib`__pthread_kill + 10
> >     frame #1: 0x00007fff6c499080 libsystem_pthread.dylib`pthread_kill + 333
> >     frame #2: 0x00007fff6c22a1ae libsystem_c.dylib`abort + 127
> >     frame #3: 0x00000001057ccf95 odbedit`db_lock_database(hDB=<unavailable>) at odb.c:2048 [opt]
> >     frame #4: 0x00000001057aed9d odbedit`cm_delete_client_info(hDB=1, pid=46856) at midas.c:1702 [opt]
> >     frame #5: 0x00000001057b08fe odbedit`cm_disconnect_experiment at midas.c:2704 [opt]
> >     frame #6: 0x00000001057a8231 odbedit`ctrlc_odbedit(i=<unavailable>) at odbedit.cxx:2863 [opt]
> >     frame #7: 0x00007fff6c48cf5a libsystem_platform.dylib`_sigtramp + 26
> >     frame #8: 0x00007fff6c2ced83 libsystem_kernel.dylib`__semwait_signal + 11
> >     frame #9: 0x00007fff6c249724 libsystem_c.dylib`nanosleep + 199
> >     frame #10: 0x00007fff6c249586 libsystem_c.dylib`sleep + 41
> >     frame #11: 0x00000001057cce6b odbedit`db_lock_database(hDB=<unavailable>) at odb.c:2057 [opt]
> >     frame #12: 0x00000001057e129a odbedit`db_get_record_size(hDB=1, hKey=141848, align=8, buf_size=0x00007ffeea44c14c) at odb.c:10232 [opt]
> >     frame #13: 0x00000001057e1b58 odbedit`db_get_record1(hDB=1, hKey=141848, data=0x00007ffeea44c320, buf_size=0x00007ffeea44c2a8, align=0, rec_str="[.]\nWrite system message = BOOL : 
> > y\nWrite Elog message = BOOL : n\nSystem message interval = INT : 60\nSystem message last = DWORD : 0\nExecute command = STRING : [256] \nExecute interval = INT : 0\nExecute last = DWORD : 
> > 0\nStop run = BOOL : n\nDisplay BGColor = STRING : [32] red\nDisplay FGColor = STRING : [32] black\n\n") at odb.c:10390 [opt]
> >     frame #14: 0x00000001057f1de3 odbedit`al_trigger_class(alarm_class="Warning", alarm_message="This is an example alarm", first=YES) at alarm.c:389 [opt]
> >     frame #15: 0x00000001057f19e8 odbedit`al_trigger_alarm(alarm_name="Example alarm", alarm_message="This is an example alarm", default_class="Warning", cond_str="", type=<unavailable>) at 
> > alarm.c:310 [opt]
> >     frame #16: 0x00000001057f2c4e odbedit`al_check at alarm.c:655 [opt]
> >     frame #17: 0x00000001057b9f88 odbedit`cm_periodic_tasks at midas.c:5066 [opt]
> >     frame #18: 0x00000001057ba26d odbedit`cm_yield(millisec=100) at midas.c:5137 [opt]
> >     frame #19: 0x00000001057a30b8 odbedit`cmd_idle() at odbedit.cxx:1238 [opt]
> >     frame #20: 0x00000001057a92df odbedit`cmd_edit(prompt="[local:javascript1:S]/>", cmd=<unavailable>, dir=(odbedit`cmd_dir(char*, int*) at odbedit.cxx:705), idle=(odbedit`cmd_idle() at 
> > odbedit.cxx:1233))(char*, int*), int (*)()) at cmdedit.cxx:235 [opt]
> >     frame #21: 0x00000001057a3863 odbedit`command_loop(host_name="", exp_name="javascript1", cmd="", start_dir=<unavailable>) at odbedit.cxx:1435 [opt]
> >     frame #22: 0x00000001057a8664 odbedit`main(argc=1, argv=<unavailable>) at odbedit.cxx:2997 [opt]
> >     frame #23: 0x00007fff6c17e015 libdyld.dylib`start + 1
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