• Markus Armbruster's avatar
    qapi: Replace uncommon use of the error API by the common one · 297a3646
    Markus Armbruster authored
    We commonly use the error API like this:
    
        err = NULL;
        foo(..., &err);
        if (err) {
            goto out;
        }
        bar(..., &err);
    
    Every error source is checked separately.  The second function is only
    called when the first one succeeds.  Both functions are free to pass
    their argument to error_set().  Because error_set() asserts no error
    has been set, this effectively means they must not be called with an
    error set.
    
    The qapi-generated code uses the error API differently:
    
        // *errp was initialized to NULL somewhere up the call chain
        frob(..., errp);
        gnat(..., errp);
    
    Errors accumulate in *errp: first error wins, subsequent errors get
    dropped.  To make this work, the second function does nothing when
    called with an error set.  Requires non-null errp, or else the second
    function can't see the first one fail.
    
    This usage has also bled into visitor tests, and two device model
    object property getters rtc_get_date() and balloon_stats_get_all().
    
    With the "accumulate" technique, you need fewer error checks in
    callers, and buy that with an error check in every callee.  Can be
    nice.
    
    However, mixing the two techniques is confusing.  You can't use the
    "accumulate" technique with functions designed for the "check
    separately" technique.  You can use the "check separately" technique
    with functions designed for the "accumulate" technique, but then
    error_set() can't catch you setting an error more than once.
    
    Standardize on the "check separately" technique for now, because it's
    overwhelmingly prevalent.
    Signed-off-by: default avatarMarkus Armbruster <armbru@redhat.com>
    Reviewed-by: default avatarEric Blake <eblake@redhat.com>
    Signed-off-by: default avatarLuiz Capitulino <lcapitulino@redhat.com>
    297a3646
test-qmp-input-visitor.c 23.2 KB