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Re: [RFC v5 024/126] error: auto propagated local_err

From: Eric Blake
Subject: Re: [RFC v5 024/126] error: auto propagated local_err
Date: Fri, 8 Nov 2019 16:45:12 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1

On 11/8/19 3:10 PM, Marc-André Lureau wrote:

+ *
+ * This macro is created to be the first line of a function with Error **errp
+ * OUT parameter. It's needed only in cases where we want to use error_prepend,
+ * error_append_hint or dereference *errp. It's still safe (but useless) in
+ * other cases.
+ *
+ * If errp is NULL or points to error_fatal, it is rewritten to point to a
+ * local Error object, which will be automatically propagated to the original
+ * errp on function exit (see error_propagator_cleanup).
+ *
+ * After invocation of this macro it is always safe to dereference errp
+ * (as it's not NULL anymore) and to add information (by error_prepend or
+ * error_append_hint)
+ * (as, if it was error_fatal, we swapped it with a local_error to be
+ * propagated on cleanup).

Nice improvements. Minor drawback, the abort()/exit() will now take
place when going out of scope and running the cleanup instead of error
location. Not a big problem I guess.

Your assessment is not quite right:

Any abort() will happen at the leaf node (because we are no longer wrapping thing into a local err and skipping error_propagate altogether for &error_abort).

You are correct that any exit() will now happen during cleanup, but that is an undetectable change (there is no stack trace present for &error_fatal, so calling error_propagate at a later point in time does not affect the observable end behavior).

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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