Daniel P. Berrangé <berra...@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 <berra...@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.
