[M3devel] idioms for tracking initialization state and raising errors?

[Rodney M. Bates]

On 07/31/2016 04:07 AM, Jay K wrote:
> Is this a correct idiom?
> If so, I think it is fairly reasonable.
>
>
> That is:
>   be very willing to "early return"
>   put all cleanup in finally
>   raise errors in finally
>
>
> In particular, I don't want to repeat the cleanup.
> I don't want to raise before leaving critical sections.
>
>
> I don't intend this to raise within a raise, but only
> from the "normal" exit of the finally block.
>
>
> I also don't want extra local booleans, i.e. raise: boolean to indicate failure.
> Just the one to indicate success.
>
>
> PROCEDURE InitMutex (mutex: Mutex) =
>    VAR
>      lock: PCRITICAL_SECTION := NIL;
>      locked: PCRITICAL_SECTION := NIL;
>
>    BEGIN
>      IF mutex.initialized THEN RETURN END;
>
>      TRY
>
>        lock := NewCriticalSection();
>        IF lock = NIL THEN RETURN; END;
>

          ^I suggest moving the two statements above inside the critical
           section on initLock, after the inner check on mutex.initialized.
           That way, if we lose a race, it avoids executing creation then
           deletion of an unneeded CriticalSection, and further eliminates
           the cleanup action in the code.

BTW, I am assuming initLock is a single global CriticalSection?


>        EnterCriticalSection(ADR(initLock));
>        locked := ADR(initLock);
>
>        IF mutex.initialized THEN RETURN END;
>
>        (* We won the race. *)
>        RTHeapRep.RegisterFinalCleanup (mutex, CleanMutex);
>        mutex.lock := lock;
>        lock := NIL;
>        mutex.initialized := TRUE;
>
>      FINALLY
>        IF locked # NIL THEN LeaveCriticalSection(locked); END;
>        DelCriticalSection(lock);
>        IF NOT mutex.initialized THEN (* Raise after leaving critical section. *)
>          RuntimeError.Raise (RuntimeError.T.OutOfMemory);
>        END;
>      END;
>
>    END InitMutex;
>
>
> Thank you,
>   - Jay
>
>
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-- 
Rodney Bates
rodney.m.bates at acm.org