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Has the thread cancellation problem evolved ?
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Date: 2007-08-25 (13:58)
From: Daniel_Bünzli <daniel.buenzli@e...>
Subject: Has the thread cancellation problem evolved ?
Hello everybody,

I would like to raise again the problem of portable thread  
cancellation. The potential coming (?) of native dynamic linking  
brings the stunning perspective of type-safe, native, plugins.  
However to be able to use this in conjunction with user interfaces  
(ui) or in a malicious environment it becomes even more acute to be  
able to monitor/sandbox the execution of functions to avoid  
application hangs.

Thread cancellation allows to invoke an arbitrary function f on a  
thread and stop its execution before completion, without introducing  
checks for conditions in f -- rewriting every function to do that  
goes against separation of concerns and malicious code could just  
ignore such conditions.

Xavier Leroy mentioned many time [1,2,3] that "Thread.kill" is not an  
option. The argument is that this function is "unsafe" (not in the  
typical sense) because the victim may be holding ressources and it  
should be given the opportunity to release them before dying. As such  
this function is not implemented in the native thread library and  
will never be (btw. it is still not deprecated in 3.10). Other  
solutions have been proposed [4,5]. [5] by Gerd Stolpmann is  
portable. Note however that neither solution will work with  
uncooperative threads, i.e. those that never perform any system call  
or trigger the gc [7].

In this [6] message Xavier Leroy acknowledges that having a function  
"Thread.raise_in : exn -> Thread.t -> unit" to raise an exception in  
the given thread is a reasonable alternative to "Thread.kill".  
However the same message points out that this would be difficult to  
implement for native threads that are blocked on system calls and  
concludes :

"So, the only easily implementable behavior is that the target thread  
of a "raise_to" operation can delay the processing of the exception  
until it returns from a system call.  But this behavior is nearly  

While it wouldn't (or would it ?) solve the problem of uncooperative  
threads and hence be useless in a malicious environment, the "easy  
implementation" would be useful to monitor cooperative computations  
in general -- well those that do not block on system calls, but in a  
ui there may be many of these.

I would appreciate having "Thread.raise_in" implemented, with the  
system call caveat properly documented, instead of having to resort  
to [5]. There is no feature request about this in the bug tracker.  
Before filling a potentially useless one I would like to know what  
you think about these problems and if there are things I have missed.



P.S. If your computations need to interact with the ui, forking is  
out of question.

[7] The difference between cooperative and uncooperative threads can  
be seen by compiling the following two programs to _native_ code. The  
first one loops forever while the other doesn't (because it allocates).

let () =
   ignore (Thread.create (fun () -> while true do ignore (1 + 2)  
done) ());
   Thread.delay 1.
let () =
   ignore (Thread.create (fun () -> while true do ignore (1. +. 2.)  
done) ());
   Thread.delay 1.