Re: [PATCH 1/5] qapi: allow for g_autoptr(Error) usage

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> While common error propagation practice does not require manually
> free'ing of local 'Error' objects, there are some cases where this
> is needed. One example is where the 'Error' object is only used
> for providing info to a trace event probe. Supporting g_autoptr
> avoids the need to manually call 'error_free'.
>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>  include/qapi/error.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/include/qapi/error.h b/include/qapi/error.h
> index 71f8fb2c50..6e429809d8 100644
> --- a/include/qapi/error.h
> +++ b/include/qapi/error.h
> @@ -437,6 +437,8 @@ Error *error_copy(const Error *err);
>   */
>  void error_free(Error *err);
>  
> +G_DEFINE_AUTOPTR_CLEANUP_FUNC(Error, error_free);
> +
>  /*
>   * Convenience function to assert that *@errp is set, then silently free it.
>   */

The Error interface is designed for a certain way of using it: an Error
object flows from the spot detecting the error to a spot handling it.
Failure to handle the error is a memory leak.  Our tooling can help with
tracking these down.

The interface tries to make the intended use easy: functions that report
an error consume the Error object.  Explicit error_free() should only
needed when you handle an error in some other way.

When such an explicit error_free() is needed on all paths to return,
then replacing it with auto-freeing is nice.  But what if it isn't?

Say we add a new error path and use error_report_err(err) there.  This
has always been just fine.  No more: if @err is auto-freed, this is a
double-free.  We have to also add err = NULL.  Feels like a trap for
developers to me.

Your use of auto-freeing is in the next patch.  It's this pattern:

    g_autoptr(Error) err = NULL;

    if (!frobnicate(args, &err)) {
        trace_frobnicate_err(..., error_get_pretty(err));
    }

You want to report the error to a trace point.  That's perfectly
legitimate.  The problem is that this kind of error reporting function
does not free, unlike the ones provided by qapi/error.h.

We could extend tracing to accept Error values, so that

        trace_frobnicate_err(..., err);

does free.  Doesn't seem worthwhile unless we find quite a few more uses
for it.

If we conclude we want to provide auto-free as an option, we at least
need to point out the trap in a comment.  A bit of a pain to write, and
whether people will read, understand, and remember it is uncertain.

My gut feeling right now: stick to the design, and free manually.  If
you think my gut is wrong, tell me.