[Toybox] Broken find [Was: Re: Towards find cleanup]

[Felix Janda]

Now I think that I know better how this toy works and see that my cleanup
has broken it.

> > >  	if (filter->op==OP_OR) {
> > > -		result = evaluate(filter->next, node, fnext);
> > > -		result2 = evaluate(*fnext, node, fnext);
> > > -		if (result) {
> > > -			return result;
> > > -		} else {
> > > -			if (result2) {
> > > -				return result2;
> > > -			} else {
> > > -				return 0;
> > > -			}
> > > -		}
> > > +		return evaluate(filter->next, node, fnext) || evaluate(*fnext, node, fnext);
> > This will have different side effects than the original code.
> > The original code evaluates both sides, and then returns the 'or' operation
> > on the results.  The new one will (of course) do short-circuit evaluation of
> > the '||',  meaning that sometimes the second node won't be evaluated.
> > I believe that at one time, I actually had it coded that way, but for some
> > reason or other I went back to forcing the evaluation of both nodes.
> > Unfortunately, I can't remember the reason why I went back.  I would have expected
> > regular find to do short-circuit evaluation of expressions with 'or' operations,
> > so I'd be happy if this were correct.  I just have a nagging feeling that
> > there was some reason I couldn't do it this way.  I could be completely wrong,
> > though.  So unless testing reveals the need for evaluating both sides of the
> > 'or', this should be fine.

The reason is that we want the side effects evaluate has on *fnext. If
we have a prefix expression like

OR AND CONDITION1 CONDITION2 CONDITION3

and CONDITION1 is false, the current version of evaluate will
1. see OR
2. descend to AND
3. find that CONDITION1 is false (side effect: *fnext=CONDITION2)
4. don't evaluate *fnext but go back to OR
5. OR now checks CONDITION2 instead of CONDITION3.

In the previous version of evaluate in step 4 CONDITION2 would have been
evaluated, ignored and as a side effect fnext would then point to
CONDITION3.

> > 
> > Do you know if regular find does short-circuit evaluation?
> > What would regular find do with
> >   'find . -exec test1_with_side_effects {} \; -o -exec test2_with_side_effects {} \;'
> > ?
> 
> POSIX says that these short-circuit evaluations shall be applied. (The
> same applies for -a.)

Now we have the problem that we want to have some side effects of
evaluate but not all of them.

Felix

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