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, 13 insertions, 0 deletions

diff --git a/util/error.c b/util/error.c
index 3efdd69..b5ccbd8 100644
--- a/util/error.c
+++ b/util/error.c
@@ -292,3 +292,16 @@ void error_propagate(Error **dst_errp, Error *local_err)
         error_free(local_err);
     }
 }
+
+void error_propagate_prepend(Error **dst_errp, Error *err,
+                             const char *fmt, ...)
+{
+    va_list ap;
+
+    if (dst_errp && !*dst_errp) {
+        va_start(ap, fmt);
+        error_vprepend(&err, fmt, ap);
+        va_end(ap);
+    } /* else error is being ignored, don't bother with prepending */
+    error_propagate(dst_errp, err);
+}