[Xen-devel] [RFC v2 1/9] error: auto propagated local_err

[Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx>]

Here is introduced ERRP_FUNCTION_BEGIN macro, to be used at start of
any function with errp parameter.

It has three goals:

1. Fix issue with error_fatal & error_append_hint: user can't see these
hints, because exit() happens in error_setg earlier than hint is
appended. [Reported by Greg Kurz]

2. Fix issue with error_abort & error_propagate: when we wrap
error_abort by local_err+error_propagate, resulting coredump will
refer to error_propagate and not to the place where error happened.
(the macro itself don't fix the issue, but it allows to [3.] drop all
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 should be merely updated to
return int error code).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx>
---
 include/qapi/error.h | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/include/qapi/error.h b/include/qapi/error.h
index 3f95141a01..f6f4fa0fac 100644
--- a/include/qapi/error.h
+++ b/include/qapi/error.h
@@ -322,6 +322,43 @@ 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_FUNCTION_BEGIN
+ *
+ * This macro MUST be the first line of EACH function with Error **errp
+ * parameter.
+ *
+ * If errp is NULL or points to error_fatal, it is rewritten to point to
+ * local Error object, which will be automatically propagated to 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 append hint (by error_append_hint)
+ * (as, if it was error_fatal, we swapped it by local_error to be
+ * propagated on cleanup).
+ *
+ * Note: we don't wrap error_abort case, as we want resulting coredump
+ * to point to the place where the error happened, not to error_propagate.
+ */
+#define ERRP_FUNCTION_BEGIN() \
+g_auto(ErrorPropagator) __auto_errp_prop = {.errp = errp}; \
+Error **__local_errp_unused __attribute__ ((unused)) = \
+    (errp = (errp == NULL || *errp == error_fatal ? \
+             &__auto_errp_prop.local_err : errp))
+
+
 /*
  * Special error destination to abort on error.
  * See error_setg() and error_propagate() for details.
-- 
2.21.0


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