[M3devel] RuntimeError doesn't work quite as advertised

[Tony Hosking]

It's probably not generally safe to use RAISE inside a signal  
handler.  There are esoteric rules as to what signal handlers can  
handle...

I think we need to think harder on how best to reflect NIL dereference  
as a signal.  Perhaps we need explicit NIL checks after all...

On 27 Apr 2009, at 18:31, Mika Nystrom wrote:

> Ok so I tried the "obvious thing", namely, I changed RTSignal.SegV to
> do
>
>    RAISE RuntimeError.E(RuntimeError.T.BadMemoryReference);
>
> Somewhat to my surprise, this does the right thing for a null method.
> But somewhat less surprisingly, it loops for NilJump and NilDeref.
> Is the machine jumping back and re-executing the same instructions
> after the signal handler returns?
>
> Would all hell break loose if one were to special-case EXCEPT
> RuntimeError.E to store a thread-local jmp_buf that one does a
> C-style longjmp to?  (With a chain back to any higher RuntimeError.E
> handlers, I suppose...)  Or is there a way to get the exception to
> work out of the signal handler with the normal exception mechanism?
>
>    Mika
>
> Mika Nystrom writes:
>>
>> Maybe someone else knows more about this than me...
>>
>> The following program:
>>
>> MODULE Main;
>> IMPORT RuntimeError, IO;
>>
>> PROCEDURE Range() = BEGIN EVAL VAL(-1,CARDINAL) END Range;
>>
>> PROCEDURE Assert() = BEGIN <*ASSERT FALSE*> END Assert;
>>
>> PROCEDURE NilJump() = VAR x : PROCEDURE() := NIL; BEGIN x() END  
>> NilJump;
>>
>> PROCEDURE NilDeref() =
>> VAR x : REF INTEGER := NIL; b : INTEGER;
>> BEGIN b := x^ END NilDeref;
>>
>> PROCEDURE NilMethod() =
>> TYPE T = OBJECT METHODS m() END;
>> VAR t : T; BEGIN t.m() END NilMethod;
>>
>> BEGIN
>> WITH ps = ARRAY OF PROCEDURE() { Range, Assert, NilMethod, NilJump,  
>> NilDeref } DO
>>   FOR i := FIRST(ps) TO LAST(ps) DO
>>     TRY
>>       ps[i]()
>>     EXCEPT
>>       RuntimeError.E(e) => IO.Put("RuntimeError: " &  
>> RuntimeError.Tag(e) & " \n")
>>     END
>>   END
>> END
>> END Main.
>>
>> yields the following result:
>>
>> (1053)trs80:~/test/src>../FreeBSD4/prog
>> RuntimeError: An enumeration or subrange value was out of range.
>> RuntimeError: <*ASSERT*> failed.
>>
>>
>> ***
>> *** runtime error:
>> ***    Segmentation violation - possible attempt to dereference NIL
>> ***    pc = 0x8048a71 = NilMethod + 0x10 in ../src/Main.m3
>> ***
>>
>> Abort
>>
>> Seems to me I ought to see a few more exceptions and not a crash.
>>
>> I'm happy to investigate a bit more, but does anyone have a clue  
>> where
>> to begin?
>>
>>    Mika