| Message ID | 20200703090816.3295-2-vsementsov@virtuozzo.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | error: auto propagated local_err part I | expand |
Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> writes: > 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> > --- > > Cc: Eric Blake <eblake@redhat.com> > Cc: Kevin Wolf <kwolf@redhat.com> > Cc: Max Reitz <mreitz@redhat.com> > Cc: Greg Kurz <groug@kaod.org> > Cc: Christian Schoenebeck <qemu_oss@crudebyte.com> > Cc: Stefan Hajnoczi <stefanha@redhat.com> > Cc: Stefano Stabellini <sstabellini@kernel.org> > Cc: Anthony Perard <anthony.perard@citrix.com> > Cc: Paul Durrant <paul@xen.org> > Cc: "Philippe Mathieu-Daudé" <philmd@redhat.com> > Cc: Laszlo Ersek <lersek@redhat.com> > Cc: Gerd Hoffmann <kraxel@redhat.com> > Cc: Markus Armbruster <armbru@redhat.com> > Cc: Michael Roth <mdroth@linux.vnet.ibm.com> > Cc: qemu-devel@nongnu.org > Cc: qemu-block@nongnu.org > Cc: xen-devel@lists.xenproject.org > > include/qapi/error.h | 205 ++++++++++++++++++++++++++++++++++++------- > 1 file changed, 172 insertions(+), 33 deletions(-) > > diff --git a/include/qapi/error.h b/include/qapi/error.h > index 5ceb3ace06..b54aedbfd7 100644 > --- a/include/qapi/error.h > +++ b/include/qapi/error.h > @@ -39,7 +39,7 @@ > * • pointer-valued functions return non-null / null pointer, and > * • integer-valued functions return non-negative / negative. > * > - * How to: > + * = Deal with Error object = > * > * Create an error: > * error_setg(errp, "situation normal, all fouled up"); > @@ -73,28 +73,91 @@ > * 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: > + * > + * 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 rebase onto my "error: Document Error API usage rules" rendered this this partly redundant. I'll try my hand at a proper merge, then ask you to check it. Should I fail to complete this in time for the soft freeze, we can merge the thing as is. Comment improvements are fair game until -rc1 or so. > * > - * Create a new error and pass it to the caller: > + * 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 Also when a function returns something, but there is no distinct error value. Example: object_property_get_int(). I shouldn't criticize comments without suggesting improvements. But since I'm going to mess with this text anyway to merge your work with my prior work, I take it easy and only note what I think needs work. I'll then try to address all that in or on top of my merge. > + * to check success is by consulting errp; doing this safely requires the use > + * of the ERRP_AUTO_PROPAGATE macro, like this: "Requires" is inaccurate. Using a local variable with error_propagate() also works (there's even an example right below). We prefer ERRP_AUTO_PROPAGATE(), because it's more readable and improves the debugging experience. > + * > + * int our_func(..., Error **errp) { A function's opening brace goes on its own line. More of the same below. > + * 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). I'd prefer not to shout DEPRECATED. > + * > + * 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. "Must" should be reserved for situations where failure to adhere is categorically wrong. While we *want* people to use ERRP_AUTO_PROPAGATE() with error_append_hint() and error_prepend(), failure to do so need not be wrong. Apropos error_append_hint(), the "Show errp instead of &err where &err is actually unusual" part of my "error: Improve examples in error.h's big comment" now feels premature to me. E.g. * Create an error and add additional explanation: - * error_setg(&err, "invalid quark"); - * error_append_hint(&err, "Valid quarks are up, down, strange, " + * error_setg(errp, "invalid quark"); + * error_append_hint(errp, "Valid quarks are up, down, strange, " * "charm, top, bottom.\n"); is actually bad advice until ERRP_AUTO_PROPAGATE() turns it into good advice. I think I'll drop it from my commit, then see how I like the comment with yours applied. > + * > + * In rare cases, we need to pass existing Error object to the caller by hand: > + * error_propagate(errp, err); Out of curiosity: can you describe such a case? > + * > + * Pass an existing error to the caller with the message modified: > + * error_propagate_prepend(errp, err); > + * > + * > * Call a function ignoring errors: > * foo(arg, NULL); > * > @@ -104,26 +167,6 @@ > * 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); > @@ -151,6 +194,61 @@ > * error_setg(&err, ...); // WRONG! > * } > * because this may pass a non-null err to error_setg(). > + * > + * 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; > + * } > + * ... > + * } This is another spot where we need to merge your work with mine properly. When foo() returns a distinct error value, then checking that is even better: void fn(..., Error **errp) { if (!foo(arg, errp)) { handle the error... return; } ... } > */ > > #ifndef ERROR_H > @@ -359,6 +457,47 @@ void error_set_internal(Error **errp, > ErrorClass err_class, const char *fmt, ...) > GCC_FMT_ATTR(6, 7); > The ERRP_AUTO_PROPAGATE stuff starts rather abruptly. I'm afraid an unprepared reader will get what's going on only at G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(), or even at #define ERRP_AUTO_PROPAGATE(). Let's move the typedef, helper function and macro invocation behind the definition of ERRP_AUTO_PROPAGATE(), similar to how we declare each error_FOO_internal() helper function right after the macro that needs it. > +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 > + * No other definition comment in this file repeats the name being defined. Let's keep the comment style locally consistent. > + * 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) > + Let's align the backslashes for consistency with nearby macros: #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.
diff --git a/include/qapi/error.h b/include/qapi/error.h index 5ceb3ace06..b54aedbfd7 100644 --- a/include/qapi/error.h +++ b/include/qapi/error.h @@ -39,7 +39,7 @@ * • pointer-valued functions return non-null / null pointer, and * • integer-valued functions return non-negative / negative. * - * How to: + * = Deal with Error object = * * Create an error: * error_setg(errp, "situation normal, all fouled up"); @@ -73,28 +73,91 @@ * 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: + * + * 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. * - * Create a new error and pass it to the caller: + * 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); * @@ -104,26 +167,6 @@ * 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); @@ -151,6 +194,61 @@ * error_setg(&err, ...); // WRONG! * } * because this may pass a non-null err to error_setg(). + * + * 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 @@ -359,6 +457,47 @@ 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.