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From: Markus Armbruster <armbru@redhat.com>
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Date: Mon, 13 Jun 2016 13:42:15 +0200
In-Reply-To: <1465589538-24998-3-git-send-email-ehabkost@redhat.com> (Eduardo
	Habkost's message of "Fri, 10 Jun 2016 17:12:17 -0300")
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Subject: Re: [Qemu-devel] [PATCH v2 2/3] error: Remove unnecessary local_err
 variables
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To: Eduardo Habkost <ehabkost@redhat.com>
Cc: qemu-devel@nongnu.org, kwolf@redhat.com, qemu-block@nongnu.org, mreitz@redhat.com, borntraeger@de.ibm.com, cornelia.huck@de.ibm.com

Eduardo Habkost <ehabkost@redhat.com> writes:

> This patch simplifies code that uses a local_err variable just to
> immediately use it for an error_propagate() call.
>
> Coccinelle patch used to perform the changes added to
> scripts/coccinelle/remove_local_err.cocci.
>
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
[...]
> diff --git a/scripts/coccinelle/remove_local_err.cocci b/scripts/coccinelle/remove_local_err.cocci
> new file mode 100644
> index 0000000..5f0b6c0
> --- /dev/null
> +++ b/scripts/coccinelle/remove_local_err.cocci
> @@ -0,0 +1,27 @@
> +// Replace unnecessary usage of local_err variable with
> +// direct usage of errp argument
> +
> +@@
> +expression list ARGS;
> +expression F2;
> +identifier LOCAL_ERR;
> +expression ERRP;
> +idexpression V;
> +typedef Error;
> +expression I;

I isn't used.

> +@@
> + {
> +     ...
> +-    Error *LOCAL_ERR;
> +     ... when != LOCAL_ERR
> +(
> +-    F2(ARGS, &LOCAL_ERR);
> +-    error_propagate(ERRP, LOCAL_ERR);
> ++    F2(ARGS, ERRP);
> +|
> +-    V = F2(ARGS, &LOCAL_ERR);
> +-    error_propagate(ERRP, LOCAL_ERR);
> ++    V = F2(ARGS, ERRP);
> +)
> +     ... when != LOCAL_ERR
> + }

There is an (ugly) difference between

    error_setg(&local_err, ...);
    error_propagate(errp, &local_err);

and

    error_setg(errp, ...);

The latter aborts when @errp already contains an error, the former does
nothing.

Your transformation has the error_setg() or similar hidden in F2().  It
can add aborts.

I think it can be salvaged: we know that @errp must not contain an error
on function entry.  If @errp doesn't occur elsewhere in this function,
it cannot pick up an error on the way to the transformed spot.  Can you
add that to your when constraints?

[...]