error: Fix use of error_prepend() with &error_fatal, &error_abort

From include/qapi/error.h:

  * Pass an existing error to the caller with the message modified:
  *     error_propagate(errp, err);
  *     error_prepend(errp, "Could not frobnicate '%s': ", name);

Fei Li pointed out that doing error_propagate() first doesn't work
well when @errp is &error_fatal or &error_abort: the error_prepend()
is never reached.

Since I doubt fixing the documentation will stop people from getting
it wrong, introduce error_propagate_prepend(), in the hope that it
lures people away from using its constituents in the wrong order.
Update the instructions in error.h accordingly.

Convert existing error_prepend() next to error_propagate to
error_propagate_prepend().  If any of these get reached with
&error_fatal or &error_abort, the error messages improve.  I didn't
check whether that's the case anywhere.

Cc: Fei Li <fli@suse.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181017082702.5581-2-armbru@redhat.com>

1 files changed, 6 insertions, 6 deletions

diff --git a/migration/migration.c b/migration/migration.c
index d6ae879dc8..f0f299a773 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1546,9 +1546,9 @@ static GSList *migration_blockers;
 int migrate_add_blocker(Error *reason, Error **errp)
 {
     if (migrate_get_current()->only_migratable) {
-        error_propagate(errp, error_copy(reason));
-        error_prepend(errp, "disallowing migration blocker "
-                          "(--only_migratable) for: ");
+        error_propagate_prepend(errp, error_copy(reason),
+                                "disallowing migration blocker "
+                                "(--only_migratable) for: ");
         return -EACCES;
     }
 
@@ -1557,9 +1557,9 @@ int migrate_add_blocker(Error *reason, Error **errp)
         return 0;
     }
 
-    error_propagate(errp, error_copy(reason));
-    error_prepend(errp, "disallowing migration blocker (migration in "
-                      "progress) for: ");
+    error_propagate_prepend(errp, error_copy(reason),
+                            "disallowing migration blocker "
+                            "(migration in progress) for: ");
     return -EBUSY;
 }