qga/vss-win32: Document the DLL requires non-null errp

requester.cpp uses this pattern to receive an error and pass it on to
the caller (err_is_set() macro peeled off for clarity):

    ... code that may set errset->errp ...
    if (errset->errp && *errset->errp) {
        ... handle error ...
    }

This breaks when errset->errp is null.  As far as I can tell, it
currently isn't, so this is merely fragile, not actually broken.

The robust way to do this is to receive the error in a local variable,
then propagate it up, like this:

    Error *err = NULL;

    ... code that may set err ...
    if (err)
        ... handle error ...
        error_propagate(errset->errp, err);
    }

See also commit 5e54769, 0f230bf, a903f40.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/qga/vss-win32/requester.cpp b/qga/vss-win32/requester.cpp
index b130fee934..aae0d5f2bf 100644
--- a/qga/vss-win32/requester.cpp
+++ b/qga/vss-win32/requester.cpp
@@ -25,8 +25,9 @@
 
 #define err_set(e, err, fmt, ...) \
     ((e)->error_setg_win32((e)->errp, err, fmt, ## __VA_ARGS__))
+/* Bad idea, works only when (e)->errp != NULL: */
 #define err_is_set(e) ((e)->errp && *(e)->errp)
-
+/* To lift this restriction, error_propagate(), like we do in QEMU code */
 
 /* Handle to VSSAPI.DLL */
 static HMODULE hLib;