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, 14 insertions, 0 deletions

diff --git a/include/qapi/error.h b/include/qapi/error.h
index bcb86a79f5..51b63dd4b5 100644
--- a/include/qapi/error.h
+++ b/include/qapi/error.h
@@ -52,8 +52,12 @@
  * where Error **errp is a parameter, by convention the last one.
  *
  * Pass an existing error to the caller with the message modified:
+ *     error_propagate_prepend(errp, err);
+ *
+ * Avoid
  *     error_propagate(errp, err);
  *     error_prepend(errp, "Could not frobnicate '%s': ", name);
+ * because this fails to prepend when @errp is &error_fatal.
  *
  * Create a new error and pass it to the caller:
  *     error_setg(errp, "situation normal, all fouled up");
@@ -215,6 +219,16 @@ void error_setg_win32_internal(Error **errp,
  */
 void error_propagate(Error **dst_errp, Error *local_err);
 
+
+/*
+ * Propagate error object (if any) with some text prepended.
+ * Behaves like
+ *     error_prepend(&local_err, fmt, ...);
+ *     error_propagate(dst_errp, local_err);
+ */
+void error_propagate_prepend(Error **dst_errp, Error *local_err,
+                             const char *fmt, ...);
+
 /*
  * Prepend some text to @errp's human-readable error message.
  * The text is made by formatting @fmt, @ap like vprintf().