Re: [PATCH v3 2/4] error: auto propagated local_err

[David Gibson]

On Thu, Jun 11, 2020 at 03:40:18PM +0200, Greg Kurz wrote:
> From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> 
> Introduce a new ERRP_AUTO_PROPAGATE macro, to be used at start of
> functions with an errp OUT parameter.
> 
> It has three goals:
> 
> 1. Fix issue with error_fatal and error_prepend/error_append_hint: user
> can't see this additional information, because exit() happens in
> error_setg earlier than information is added. [Reported by Greg Kurz]
> 
> 2. Fix issue with error_abort and error_propagate: when we wrap
> error_abort by local_err+error_propagate, the resulting coredump will
> refer to error_propagate and not to the place where error happened.
> (the macro itself doesn't fix the issue, but it allows us to [3.] drop
> the local_err+error_propagate pattern, which will definitely fix the
> issue) [Reported by Kevin Wolf]
> 
> 3. Drop local_err+error_propagate pattern, which is used to workaround
> void functions with errp parameter, when caller wants to know resulting
> status. (Note: actually these functions could be merely updated to
> return int error code).
> 
> To achieve these goals, later patches will add invocations
> of this macro at the start of functions with either use
> error_prepend/error_append_hint (solving 1) or which use
> local_err+error_propagate to check errors, switching those
> functions to use *errp instead (solving 2 and 3).
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> Reviewed-by: Paul Durrant <paul@xen.org>
> Reviewed-by: Greg Kurz <groug@kaod.org>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> Signed-off-by: Greg Kurz <groug@kaod.org>
> Reviewed-by: Laurent Vivier <lvivier@redhat.com>

I don't feel terribly qualified to comment on this generic change to
the errors mechanism.  I can take it through my tree if necessary, but
I'd want an ack from Markus.

> ---
>  include/qapi/error.h |  205 
> ++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 173 insertions(+), 32 deletions(-)
> 
> diff --git a/include/qapi/error.h b/include/qapi/error.h
> index ad5b6e896ded..30140d9bfea9 100644
> --- a/include/qapi/error.h
> +++ b/include/qapi/error.h
> @@ -15,6 +15,8 @@
>  /*
>   * Error reporting system loosely patterned after Glib's GError.
>   *
> + * = Deal with Error object =
> + *
>   * Create an error:
>   *     error_setg(&err, "situation normal, all fouled up");
>   *
> @@ -47,28 +49,91 @@
>   * reporting it (primarily useful in testsuites):
>   *     error_free_or_abort(&err);
>   *
> - * Pass an existing error to the caller:
> - *     error_propagate(errp, err);
> - * where Error **errp is a parameter, by convention the last one.
> + * = Deal with Error ** function parameter =
>   *
> - * Pass an existing error to the caller with the message modified:
> - *     error_propagate_prepend(errp, err);
> + * A function may use the error system to return errors. In this case, the
> + * function defines an Error **errp parameter, by convention the last one 
> (with
> + * exceptions for functions using ... or va_list).
>   *
> - * Avoid
> - *     error_propagate(errp, err);
> - *     error_prepend(errp, "Could not frobnicate '%s': ", name);
> - * because this fails to prepend when @errp is &error_fatal.
> + * The caller may then pass in the following errp values:
>   *
> - * Create a new error and pass it to the caller:
> + * 1. &error_abort
> + *    Any error will result in abort().
> + * 2. &error_fatal
> + *    Any error will result in exit() with a non-zero status.
> + * 3. NULL
> + *    No error reporting through errp parameter.
> + * 4. The address of a NULL-initialized Error *err
> + *    Any error will populate errp with an error object.
> + *
> + * The following rules then implement the correct semantics desired by the
> + * caller.
> + *
> + * Create a new error to pass to the caller:
>   *     error_setg(errp, "situation normal, all fouled up");
>   *
> - * Call a function and receive an error from it:
> + * Calling another errp-based function:
> + *     f(..., errp);
> + *
> + * == Checking success of subcall ==
> + *
> + * If a function returns a value indicating an error in addition to setting
> + * errp (which is recommended), then you don't need any additional code, just
> + * do:
> + *
> + *     int ret = f(..., errp);
> + *     if (ret < 0) {
> + *         ... handle error ...
> + *         return ret;
> + *     }
> + *
> + * If a function returns nothing (not recommended for new code), the only way
> + * to check success is by consulting errp; doing this safely requires the use
> + * of the ERRP_AUTO_PROPAGATE macro, like this:
> + *
> + *     int our_func(..., Error **errp) {
> + *         ERRP_AUTO_PROPAGATE();
> + *         ...
> + *         subcall(..., errp);
> + *         if (*errp) {
> + *             ...
> + *             return -EINVAL;
> + *         }
> + *         ...
> + *     }
> + *
> + * ERRP_AUTO_PROPAGATE takes care of wrapping the original errp as needed, so
> + * that the rest of the function can directly use errp (including
> + * dereferencing), where any errors will then be propagated on to the 
> original
> + * errp when leaving the function.
> + *
> + * In some cases, we need to check result of subcall, but do not want to
> + * propagate the Error object to our caller. In such cases we don't need
> + * ERRP_AUTO_PROPAGATE, but just a local Error object:
> + *
> + * Receive an error and not pass it:
>   *     Error *err = NULL;
> - *     foo(arg, &err);
> + *     subcall(arg, &err);
>   *     if (err) {
>   *         handle the error...
> + *         error_free(err);
>   *     }
>   *
> + * Note that older code that did not use ERRP_AUTO_PROPAGATE would instead 
> need
> + * a local Error * variable and the use of error_propagate() to properly 
> handle
> + * all possible caller values of errp. Now this is DEPRECATED* (see below).
> + *
> + * Note that any function that wants to modify an error object, such as by
> + * calling error_append_hint or error_prepend, must use ERRP_AUTO_PROPAGATE, 
> in
> + * order for a caller's use of &error_fatal to see the additional 
> information.
> + *
> + * In rare cases, we need to pass existing Error object to the caller by 
> hand:
> + *     error_propagate(errp, err);
> + *
> + * Pass an existing error to the caller with the message modified:
> + *     error_propagate_prepend(errp, err);
> + *
> + *
>   * Call a function ignoring errors:
>   *     foo(arg, NULL);
>   *
> @@ -78,26 +143,6 @@
>   * Call a function treating errors as fatal:
>   *     foo(arg, &error_fatal);
>   *
> - * Receive an error and pass it on to the caller:
> - *     Error *err = NULL;
> - *     foo(arg, &err);
> - *     if (err) {
> - *         handle the error...
> - *         error_propagate(errp, err);
> - *     }
> - * where Error **errp is a parameter, by convention the last one.
> - *
> - * Do *not* "optimize" this to
> - *     foo(arg, errp);
> - *     if (*errp) { // WRONG!
> - *         handle the error...
> - *     }
> - * because errp may be NULL!
> - *
> - * But when all you do with the error is pass it on, please use
> - *     foo(arg, errp);
> - * for readability.
> - *
>   * Receive and accumulate multiple errors (first one wins):
>   *     Error *err = NULL, *local_err = NULL;
>   *     foo(arg, &err);
> @@ -114,6 +159,61 @@
>   *         handle the error...
>   *     }
>   * because this may pass a non-null err to bar().
> + *
> + * DEPRECATED*
> + *
> + * The following pattern of receiving, checking, and then forwarding an error
> + * to the caller by hand is now deprecated:
> + *
> + *     Error *err = NULL;
> + *     foo(arg, &err);
> + *     if (err) {
> + *         handle the error...
> + *         error_propagate(errp, err);
> + *     }
> + *
> + * Instead, use ERRP_AUTO_PROPAGATE macro.
> + *
> + * The old pattern is deprecated because of two things:
> + *
> + * 1. Issue with error_abort and error_propagate: when we wrap error_abort by
> + * local_err+error_propagate, the resulting coredump will refer to
> + * error_propagate and not to the place where error happened.
> + *
> + * 2. A lot of extra code of the same pattern
> + *
> + * How to update old code to use ERRP_AUTO_PROPAGATE?
> + *
> + * All you need is to add ERRP_AUTO_PROPAGATE() invocation at function start,
> + * than you may safely dereference errp to check errors and do not need any
> + * additional local Error variables or calls to error_propagate().
> + *
> + * Example:
> + *
> + * old code
> + *
> + *     void fn(..., Error **errp) {
> + *         Error *err = NULL;
> + *         foo(arg, &err);
> + *         if (err) {
> + *             handle the error...
> + *             error_propagate(errp, err);
> + *             return;
> + *         }
> + *         ...
> + *     }
> + *
> + * updated code
> + *
> + *     void fn(..., Error **errp) {
> + *         ERRP_AUTO_PROPAGATE();
> + *         foo(arg, errp);
> + *         if (*errp) {
> + *             handle the error...
> + *             return;
> + *         }
> + *         ...
> + *     }
>   */
>  
>  #ifndef ERROR_H
> @@ -322,6 +422,47 @@ void error_set_internal(Error **errp,
>                          ErrorClass err_class, const char *fmt, ...)
>      GCC_FMT_ATTR(6, 7);
>  
> +typedef struct ErrorPropagator {
> +    Error *local_err;
> +    Error **errp;
> +} ErrorPropagator;
> +
> +static inline void error_propagator_cleanup(ErrorPropagator *prop)
> +{
> +    error_propagate(prop->errp, prop->local_err);
> +}
> +
> +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(ErrorPropagator, error_propagator_cleanup);
> +
> +/*
> + * ERRP_AUTO_PROPAGATE
> + *
> + * This macro exists to assist with proper error handling in a function which
> + * uses an Error **errp parameter.  It must be used as the first line of a
> + * function which modifies an error (with error_prepend, error_append_hint, 
> or
> + * similar) or which wants to dereference *errp.  It is still safe (but
> + * useless) to use in other functions.
> + *
> + * 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).
> + *
> + * Note: we don't wrap the error_abort case, as we want resulting coredump
> + * to point to the place where the error happened, not to error_propagate.
> + */
> +#define ERRP_AUTO_PROPAGATE() \
> +    g_auto(ErrorPropagator) _auto_errp_prop = {.errp = errp}; \
> +    do { \
> +        if (!errp || errp == &error_fatal) { \
> +            errp = &_auto_errp_prop.local_err; \
> +        } \
> +    } while (0)
> +
>  /*
>   * Special error destination to abort on error.
>   * See error_setg() and error_propagate() for details.
> 
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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