[PATCH v2 01/44] error: Improve examples in error.h's big comment

[Markus Armbruster <armbru@redhat.com>]

Show errp instead of &err where &err is actually unusual.  Add a
missing declaration.  Add 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>
---
 include/qapi/error.h | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/include/qapi/error.h b/include/qapi/error.h
index ad5b6e896d..3e64324b7a 100644
--- a/include/qapi/error.h
+++ b/include/qapi/error.h
@@ -16,15 +16,15 @@
  * Error reporting system loosely patterned after Glib's GError.
  *
  * Create an error:
- *     error_setg(&err, "situation normal, all fouled up");
+ *     error_setg(errp, "situation normal, all fouled up");
  *
  * 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");
  *
  * Do *not* contract this to
- *     error_setg(&err, "invalid quark\n"
+ *     error_setg(errp, "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:
@@ -108,12 +108,23 @@
  *     }
  *
  * 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
-- 
2.26.2