Kill a NIF's process

[Sverker Eriksson]

If enif_monitor_process(_, Rsrc, Proc, _) returns 0 (success) then the monitor 
is active and the down callback is guaranteed to be called when process Proc 
terminates, unless

*	the resource Rsrc was deallocated before. The destructor callback will 
always be the last callback of a resource object.
*	the monitor was cancelled before with enif_demitor_process().



If the target process has already been killed then enif_monitor_process() is 
guaranteed to fail. There is no in between race, as with all other monitor and 
link creation.



Side note:

A dirty NIF can check the liveness of its process with 
enif_is_current_process_alive() without the need of self monitoring.



/Sverker



From: Robert Harris <robert.harris@REDACTED>
Sent: den 22 april 2021 09:05
To: Sverker Eriksson <sverker.eriksson@REDACTED>
Cc: erlang-questions@REDACTED
Subject: Re: Kill a NIF's process



Hi Sverker,

> On 21 Apr 2021, at 17:37, Sverker Eriksson <sverker.eriksson@REDACTED 
> <mailto:sverker.eriksson@REDACTED> > wrote:
>
> At process termination the deallocation of the heap is delayed until a 
> running dirty NIF has returned. This to make sure the dirty NIF can safely 
> continue read its arguments.

That makes things somewhat simpler; thank you.

> The firing of links and monitors (including NIF monitor down calls), on the 
> other hand, are not delayed by a running dirty NIF.

Is enif_monitor_process() serialised with respect to the delivery of
an exit signal by a concurrent thread? I.e. if the function returns zero
and the caller is later killed then is it guaranteed that its down
callback will be invoked? I am wondering if (absent any documentation
to the contrary) there might exist a race condition in which process A
might kill process B at exactly the time that the latter calls
enif_monitor_process(), e.g. in the (untested) code below.

Regards,

Robert



/* resource object */
typedef struct foo {
int abort;
...
} foo_t;

void
down(ErlNifEnv *env, void *obj, ErlNifPid *pid, ErlNifMonitor *mon)
{
((foo_t *)obj)->abort = 1;
}

ERL_NIF_TERM
some_dirty_nif(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[])
{
foo_t *foop;
ErlNifPid pid;
ErlNifMonitor monitor;

enif_get_resource(env, argv[0], foo_resource, (void **)&foop));
foop->abort = 0;

enif_self(env, &pid);
if (enif_monitor_process(env, foop, pid, &monitor) > 0)
/* process already dead; abort */

/* lost a signal here? */

while (/* some very long loop */) {
if (foop->abort) {
/* process waiting to die; abort */
...
}
/* do stuff */
}

/* no longer care about being directed to abort */
enif_demonitor_process(env, foop, &monitor);
/* return */
...
}
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