qapi: Don't box struct branch of alternate

There's no reason to do two malloc's for an alternate type visiting
a QAPI struct; let's just inline the struct directly as the C union
branch of the struct.

Surprisingly, no clients were actually using the struct member prior
to this patch outside of the testsuite; an earlier patch in the series
added some testsuite coverage to make the effect of this patch more
obvious.

In qapi.py, c_type() gains a new is_unboxed flag to control when we
are emitting a C struct unboxed within the context of an outer
struct (different from our other two modes of usage with no flags
for normal local variable declarations, and with is_param for adding
'const' in a parameter list).  I don't know if there is any more
pythonic way of collapsing the two flags into a single parameter,
as we never have a caller setting both flags at once.

Ultimately, we want to also unbox branches for QAPI unions, but as
that touches a lot more client code, it is better as separate
patches.  But since unions and alternates share gen_variants(), I
had to hack in a way to test if we are visiting an alternate type
for setting the is_unboxed flag: look for a non-object branch.
This works because alternates have at least two branches, with at
most one object branch, while unions have only object branches.
The hack will go away in a later patch.

The generated code difference to qapi-types.h is relatively small:

| struct BlockdevRef {
|     QType type;
|     union { /* union tag is @type */
|         void *data;
|-        BlockdevOptions *definition;
|+        BlockdevOptions definition;
|         char *reference;
|     } u;
| };

The corresponding spot in qapi-visit.c calls visit_type_FOO(), which
first calls visit_start_struct() to allocate or deallocate the member
and handle a layer of {} from the JSON stream, then visits the
members.  To peel off the indirection and the memory management that
comes with it, we inline this call, then suppress allocation /
deallocation by passing NULL to visit_start_struct(), and adjust the
member visit:

|     switch ((*obj)->type) {
|     case QTYPE_QDICT:
|-        visit_type_BlockdevOptions(v, name, &(*obj)->u.definition, &err);
|+        visit_start_struct(v, name, NULL, 0, &err);
|+        if (err) {
|+            break;
|+        }
|+        visit_type_BlockdevOptions_fields(v, &(*obj)->u.definition, &err);
|+        error_propagate(errp, err);
|+        err = NULL;
|+        visit_end_struct(v, &err);
|         break;
|     case QTYPE_QSTRING:
|         visit_type_str(v, name, &(*obj)->u.reference, &err);

The visit of non-object fields is unchanged.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1455778109-6278-13-git-send-email-eblake@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 5 insertions, 6 deletions

diff --git a/tests/test-qmp-output-visitor.c b/tests/test-qmp-output-visitor.c
index 2b0f7e9c53..fe2f1a1326 100644
--- a/tests/test-qmp-output-visitor.c
+++ b/tests/test-qmp-output-visitor.c
@@ -457,12 +457,11 @@ static void test_visitor_out_alternate(TestOutputVisitorData *data,
 
     tmp = g_new0(UserDefAlternate, 1);
     tmp->type = QTYPE_QDICT;
-    tmp->u.udfu = g_new0(UserDefFlatUnion, 1);
-    tmp->u.udfu->integer = 1;
-    tmp->u.udfu->string = g_strdup("str");
-    tmp->u.udfu->enum1 = ENUM_ONE_VALUE1;
-    tmp->u.udfu->u.value1 = g_new0(UserDefA, 1);
-    tmp->u.udfu->u.value1->boolean = true;
+    tmp->u.udfu.integer = 1;
+    tmp->u.udfu.string = g_strdup("str");
+    tmp->u.udfu.enum1 = ENUM_ONE_VALUE1;
+    tmp->u.udfu.u.value1 = g_new0(UserDefA, 1);
+    tmp->u.udfu.u.value1->boolean = true;
 
     visit_type_UserDefAlternate(data->ov, NULL, &tmp, &error_abort);
     arg = qmp_output_get_qobject(data->qov);