qapi: Fix crash on missing alternate member of QAPI struct

If a QAPI struct has a mandatory alternate member which is not
present on input, the input visitor reports an error for the
missing alternate without setting the discriminator, but the
cleanup code for the struct still tries to use the dealloc
visitor to clean up the alternate.

Commit dbf11922 changed visit_start_alternate to set *obj to NULL
when an error occurs, where it was previously left untouched.
Thus, before the patch, the dealloc visitor is blindly trying to
cleanup whatever branch corresponds to (*obj)->type == 0 (that is,
QTYPE_NONE, because *obj still pointed to zeroed memory), which
selects the default branch of the switch and sets an error, but
this second error is ignored by the way the dealloc visitor is
used; but after the patch, the attempt to switch dereferences NULL.

When cleaning up after a partial object parse, we specifically
check for !*obj after visit_start_struct() (see gen_visit_object());
doing the same for alternates fixes the crash. Enhance the testsuite
to give coverage for both missing struct and missing alternate
members.

Also add an abort - we expect visit_start_alternate() to either set an
error or to set (*obj)->type to a valid QType that corresponds to
actual user input, and QTYPE_NONE should never be reachable from valid
input.  Had the abort() been in place earlier, we might have noticed
the dealloc visitor dereferencing bogus zeroed memory prior to when
commit dbf11922 forced our hand by setting *obj to NULL and causing a
fault.

Test case:

{'execute':'blockdev-add', 'arguments':{'options':{'driver':'raw'}}}

The choice of 'driver':'raw' selects a BlockdevOptionsGenericFormat
struct, which has a mandatory 'file':'BlockdevRef' in QAPI.  Since
'file' is missing as a sibling of 'driver', this should report a
graceful error rather than fault.  After this patch, we are back to:

{"error": {"class": "GenericError", "desc": "Parameter 'file' is missing"}}

Generated code in qapi-visit.c changes as:

|@@ -2444,6 +2444,9 @@ void visit_type_BlockdevRef(Visitor *v,
|     if (err) {
|         goto out;
|     }
|+    if (!*obj) {
|+        goto out_obj;
|+    }
|     switch ((*obj)->type) {
|     case QTYPE_QDICT:
|         visit_start_struct(v, name, NULL, 0, &err);
|@@ -2459,10 +2462,13 @@ void visit_type_BlockdevRef(Visitor *v,
|     case QTYPE_QSTRING:
|         visit_type_str(v, name, &(*obj)->u.reference, &err);
|         break;
|+    case QTYPE_NONE:
|+        abort();
|     default:
|         error_setg(&err, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
|                    "BlockdevRef");
|     }
|+out_obj:
|     visit_end_alternate(v);

Reported by Kashyap Chamarthy <kchamart@redhat.com>
CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1466012271-5204-1-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
