On Thu, Jun 11, 2020 at 03:40:18PM +0200, Greg Kurz wrote: > From: Vladimir Sementsov-Ogievskiy > > 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 > Reviewed-by: Paul Durrant > Reviewed-by: Greg Kurz > Reviewed-by: Eric Blake > Signed-off-by: Greg Kurz > Reviewed-by: Laurent Vivier 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