The current output of the qapi code generator includes some chained
error handling, which looks like:
| visit_start_struct(v, (void **)obj, "foo", name, sizeof(FOO), &err);
| if (!err) {
| if (*obj) {
| visit_type_FOO_fields(v, obj, errp);
| }
| visit_end_struct(v, &err);
| }
| error_propagate(errp, err);
Although there are plans to revisit that code for qemu 2.6, it is
still a useful idiom to mention. It is safe because error_propagate()
is different than most functions in that you can pass in an
already-set errp, and it still does the right thing.
Also, describe an alternative form of chained error handling that was
proposed during the qapi work, and which may be a bit more obvious
to a reader what is happening:
| visit_start_implicit_struct(v, (void **)obj, sizeof(FOO), &err);
| if (!err) {
| visit_type_FOO_fields(v, obj, &err);
| visit_end_implicit_struct(v, err ? NULL : &err);
| }
| error_propagate(errp, err);
Signed-off-by: Eric Blake <ebl...@redhat.com>
---
based on feedback of my qapi series v5 7/46; doc only, so might
be worth having in 2.5
---
include/qapi/error.h | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/include/qapi/error.h b/include/qapi/error.h
index 4d42cdc..4310195 100644
--- a/include/qapi/error.h
+++ b/include/qapi/error.h
@@ -76,6 +76,21 @@
* But when all you do with the error is pass it on, please use
* foo(arg, errp);
* for readability.
+ *
+ * In a situation where cleanup must happen even if a first step fails,
+ * but the cleanup may also set an error, the first error to occur will
+ * take priority when combined by:
+ * Error *err = NULL;
+ * action1(arg, errp);
+ * action2(arg, &err);
+ * error_propagate(errp, err);
+ * or by:
+ * Error *err = NULL;
+ * action1(arg, &err);
+ * action2(arg, err ? NULL : &err);
+ * error_propagate(errp, err);
+ * although the second form is required if any further error handling
+ * will inspect err to see if all earlier locations succeeded.
*/
#ifndef ERROR_H
--
2.4.3
