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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v8 01/10] error: auto propagated local_err
On Fri, 6 Mar 2020 08:15:27 +0300
Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx> wrote:
> Here is introduced 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@xxxxxxxxxxxxx>
> ---
>
Thanks for this impressive work Vladimir !
Reviewed-by: Greg Kurz <groug@xxxxxxxx>
> Cc: Eric Blake <eblake@xxxxxxxxxx>
> Cc: Kevin Wolf <kwolf@xxxxxxxxxx>
> Cc: Max Reitz <mreitz@xxxxxxxxxx>
> Cc: Greg Kurz <groug@xxxxxxxx>
> Cc: Christian Schoenebeck <qemu_oss@xxxxxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>
> Cc: Paul Durrant <paul@xxxxxxx>
> Cc: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> Cc: "Philippe Mathieu-Daudé" <philmd@xxxxxxxxxx>
> Cc: Laszlo Ersek <lersek@xxxxxxxxxx>
> Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> Cc: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> Cc: Markus Armbruster <armbru@xxxxxxxxxx>
> Cc: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
> Cc: qemu-block@xxxxxxxxxx
> Cc: qemu-devel@xxxxxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
>
> include/qapi/error.h | 203 ++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 170 insertions(+), 33 deletions(-)
>
> diff --git a/include/qapi/error.h b/include/qapi/error.h
> index ad5b6e896d..bb9bcf02fb 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 +49,88 @@
> * 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);
> + * Function may use error system to return errors. In this case function
> + * defines Error **errp parameter, which should be the last one (except for
> + * functions which varidic argument list), which has the following API:
> *
> - * Avoid
> - * error_propagate(errp, err);
> - * error_prepend(errp, "Could not frobnicate '%s': ", name);
> - * because this fails to prepend when @errp is &error_fatal.
> + * Caller may pass as errp:
> + * 1. &error_abort
> + * This means abort on any error
> + * 2. &error_fatal
> + * Exit with non-zero return code on error
> + * 3. NULL
> + * Ignore errors
> + * 4. Another value
> + * On error allocate error object and set errp
> *
> - * Create a new error and pass it to the caller:
> - * error_setg(errp, "situation normal, all fouled up");
> + * Error API functions with Error ** (like error_setg) argument supports
> these
> + * rules, so user functions just need to use them appropriately (read below).
> *
> - * Call a function and receive an error from it:
> + * Simple pass error to the caller:
> + * error_setg(errp, "Some error");
> + *
> + * Subcall of another errp-based function, passing the error to the caller
> + * f(..., errp);
> + *
> + * == Checking success of subcall ==
> + *
> + * If function returns error code in addition to errp (which is recommended),
> + * you don't need any additional code, just do:
> + * int ret = f(..., errp);
> + * if (ret < 0) {
> + * ... handle error ...
> + * return ret;
> + * }
> + *
> + * If function returns nothing (which is not recommended API) and the only
> way
> + * to check success is checking errp, we must care about cases [1-3] above.
> We
> + * need to use macro ERRP_AUTO_PROPAGATE (see below for details) like this:
> + *
> + * int our_func(..., Error **errp) {
> + * ERRP_AUTO_PROPAGATE();
> + * ...
> + * subcall(..., errp);
> + * if (*errp) {
> + * ...
> + * return -ERRNO;
> + * }
> + * ...
> + * }
> + *
> + * ERRP_AUTO_PROPAGATE cares about Error ** API, wraps original errp if
> needed,
> + * so that it can be safely used (including dereferencing), and
> auto-propagates
> + * error to original errp on function end.
> + *
> + * 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 before ERRP_AUTO_PROPAGATE introduction the pattern above (with
> + * error_propagate() instead of error_free()) was used to check and pass
> error
> + * to the caller. Now this is DEPRECATED* (see below).
> + *
> + * Note also, that if you want to use error_append_hint/error_prepend or
> their
> + * variants, you must use ERRP_AUTO_PROPAGATE too. Otherwise, in case of
> + * error_fatal, you'll miss the chance to insert your additional information
> + * into Error object.
> + *
> + * 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 +140,6 @@
> * 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 +156,61 @@
> * handle the error...
> * }
> * because this may pass a non-null err to bar().
> + *
> + * DEPRECATED*
> + *
> + * The following pattern of receiving checking and passing the caller of the
> + * error by hand is deprecated now:
> + *
> + * Error *err = NULL;
> + * foo(arg, &err);
> + * if (err) {
> + * handle the error...
> + * error_propagate(errp, err);
> + * }
> + *
> + * Instead, use ERRP_AUTO_PROPAGATE macro (defined below).
> + *
> + * 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 +419,46 @@ 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 is created to be the first line of a function which use
> + * Error **errp parameter to report error. 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).
> + *
> + * 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.
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