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, 4 deletions

diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
index c2271e6ed4..58afa46204 100644
--- a/hw/ppc/spapr_pci.c
+++ b/hw/ppc/spapr_pci.c
@@ -1724,16 +1724,15 @@ static void spapr_phb_realize(DeviceState *dev, Error **errp)
         if (smc->legacy_irq_allocation) {
             irq = spapr_irq_findone(spapr, &local_err);
             if (local_err) {
-                error_propagate(errp, local_err);
-                error_prepend(errp, "can't allocate LSIs: ");
+                error_propagate_prepend(errp, local_err,
+                                        "can't allocate LSIs: ");
                 return;
             }
         }
 
         spapr_irq_claim(spapr, irq, true, &local_err);
         if (local_err) {
-            error_propagate(errp, local_err);
-            error_prepend(errp, "can't allocate LSIs: ");
+            error_propagate_prepend(errp, local_err, "can't allocate LSIs: ");
             return;
         }