Subject: Re: [PATCH] cocci: drop bogus xstrdup_or_null() rule

From: Derrick Stolee <[email-masked]>
Date: Mon, 2 May 2022 09:46:11 -0400

On 5/1/2022 1:00 AM, Junio C Hamano wrote:
> 13092a91 (cocci: refactor common patterns to use xstrdup_or_null(),
> 2016-10-12) introduced a rule to rewrite this conditional call to
> xstrdup(E) and an assignment to variable V:
> 
>     - if (E)
>     -    V = xstrdup(E);
> 
> into an unconditional call to xstrdup_or_null(E) and an assignment
> to variable V:
> 
>     + V = xstrdup_or_null(E);
> 
> which is utterly bogus.  The original code may already have an
> acceptable value in V and the conditional assignment may be to
> improve the value already in V with a copy of a better value E when
> (and only when) E is not NULL.

Yes, this makes sense.
 
> The rewritten construct unconditionally discards the existing value
> of V and replaces it with a copy of E, even when E is NULL, which
> changes the meaning of the program.
> 
> By the way, if it were
> 
> 	-if (E && !V)
> 	-	V = xstrdup(E);
> 	+V = xstrdup_or_null(E);

I think you mean if it were

	-if (E && !V)
	-	V = xstrdup(E);
	+if (!V)
	+	V = xstrdup_or_null(E);

or

	-if (E && !V)
	-	V = xstrdup(E);
	+free(V);
	+V = xstrdup_or_null(E);

But yes, there is no preimage matching this pattern, so it
doesn't matter.

Dropping the rule makes the most sense.

Thanks,
-Stolee