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, 3 insertions, 2 deletions

diff --git a/hw/usb/bus.c b/hw/usb/bus.c
index 11f7720..bf796d6 100644
--- a/hw/usb/bus.c
+++ b/hw/usb/bus.c
@@ -340,8 +340,9 @@ static USBDevice *usb_try_create_simple(USBBus *bus, const char *name,
     }
     object_property_set_bool(OBJECT(dev), true, "realized", &err);
     if (err) {
-        error_propagate(errp, err);
-        error_prepend(errp, "Failed to initialize USB device '%s': ", name);
+        error_propagate_prepend(errp, err,
+                                "Failed to initialize USB device '%s': ",
+                                name);
         return NULL;
     }
     return dev;