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, 2 insertions, 2 deletions

diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index c30f4f2..08e673a 100644
--- a/hw/9pfs/9p-local.c
+++ b/hw/9pfs/9p-local.c
@@ -1509,8 +1509,8 @@ static int local_parse_opts(QemuOpts *opts, FsDriverEntry *fse, Error **errp)
 
     fsdev_throttle_parse_opts(opts, &fse->fst, &local_err);
     if (local_err) {
-        error_propagate(errp, local_err);
-        error_prepend(errp, "invalid throttle configuration: ");
+        error_propagate_prepend(errp, local_err,
+                                "invalid throttle configuration: ");
         return -1;
     }