error: Fix examples in error.h's big comment

Mark a bad example more clearly.  Fix the error_propagate_prepend()
example.  Add a missing declaration and a second error pileup example.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <20200707160613.848843-2-armbru@redhat.com>

1 files changed, 14 insertions, 2 deletions

diff --git a/include/qapi/error.h b/include/qapi/error.h
index ad5b6e896d..e8960eaad5 100644
--- a/include/qapi/error.h
+++ b/include/qapi/error.h
@@ -24,7 +24,7 @@
  *                       "charm, top, bottom.\n");
  *
  * Do *not* contract this to
- *     error_setg(&err, "invalid quark\n"
+ *     error_setg(&err, "invalid quark\n" // WRONG!
  *                "Valid quarks are up, down, strange, charm, top, bottom.");
  *
  * Report an error to the current monitor if we have one, else stderr:
@@ -52,7 +52,8 @@
  * where Error **errp is a parameter, by convention the last one.
  *
  * Pass an existing error to the caller with the message modified:
- *     error_propagate_prepend(errp, err);
+ *     error_propagate_prepend(errp, err,
+ *                             "Could not frobnicate '%s': ", name);
  *
  * Avoid
  *     error_propagate(errp, err);
@@ -108,12 +109,23 @@
  *     }
  *
  * Do *not* "optimize" this to
+ *     Error *err = NULL;
  *     foo(arg, &err);
  *     bar(arg, &err); // WRONG!
  *     if (err) {
  *         handle the error...
  *     }
  * because this may pass a non-null err to bar().
+ *
+ * Likewise, do *not*
+ *     Error *err = NULL;
+ *     if (cond1) {
+ *         error_setg(&err, ...);
+ *     }
+ *     if (cond2) {
+ *         error_setg(&err, ...); // WRONG!
+ *     }
+ * because this may pass a non-null err to error_setg().
  */
 
 #ifndef ERROR_H