qapi: Change visit_start_implicit_struct to visit_start_alternate

After recent changes, the only remaining use of
visit_start_implicit_struct() is for allocating the space needed
when visiting an alternate.  Since the term 'implicit struct' is
hard to explain, rename the function to its current usage.  While
at it, we can merge the functionality of visit_get_next_type()
into the same function, making it more like visit_start_struct().

Generated code is now slightly smaller:

| {
|     Error *err = NULL;
|
|-    visit_start_implicit_struct(v, (void**) obj, sizeof(BlockdevRef), &err);
|+    visit_start_alternate(v, name, (GenericAlternate **)obj, sizeof(**obj),
|+                          true, &err);
|     if (err) {
|         goto out;
|     }
|-    visit_get_next_type(v, name, &(*obj)->type, true, &err);
|-    if (err) {
|-        goto out_obj;
|-    }
|     switch ((*obj)->type) {
|     case QTYPE_QDICT:
|         visit_start_struct(v, name, NULL, 0, &err);
...
|     }
|-out_obj:
|-    visit_end_implicit_struct(v);
|+    visit_end_alternate(v);
| out:
|     error_propagate(errp, err);
| }

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1455778109-6278-16-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

2 files changed, 47 insertions, 19 deletions

diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
index c4af3e0..6a1ddfb 100644
--- a/include/qapi/visitor-impl.h
+++ b/include/qapi/visitor-impl.h
@@ -22,22 +22,23 @@ struct Visitor
                          size_t size, Error **errp);
     void (*end_struct)(Visitor *v, Error **errp);
 
-    void (*start_implicit_struct)(Visitor *v, void **obj, size_t size,
-                                  Error **errp);
-    /* May be NULL */
-    void (*end_implicit_struct)(Visitor *v);
-
     void (*start_list)(Visitor *v, const char *name, Error **errp);
     /* Must be set */
     GenericList *(*next_list)(Visitor *v, GenericList **list, size_t size);
     /* Must be set */
     void (*end_list)(Visitor *v);
 
+    /* Optional, needed for input and dealloc visitors.  */
+    void (*start_alternate)(Visitor *v, const char *name,
+                            GenericAlternate **obj, size_t size,
+                            bool promote_int, Error **errp);
+
+    /* Optional, needed for dealloc visitor.  */
+    void (*end_alternate)(Visitor *v);
+
+    /* Must be set. */
     void (*type_enum)(Visitor *v, const char *name, int *obj,
                       const char *const strings[], Error **errp);
-    /* May be NULL; only needed for input visitors. */
-    void (*get_next_type)(Visitor *v, const char *name, QType *type,
-                          bool promote_int, Error **errp);
 
     /* Must be set. */
     void (*type_int64)(Visitor *v, const char *name, int64_t *obj,
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index a6678b1..83cad74 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -27,17 +27,52 @@ typedef struct GenericList {
     char padding[];
 } GenericList;
 
+/* This struct is layout-compatible with all Alternate types
+ * created by the qapi generator. */
+typedef struct GenericAlternate {
+    QType type;
+    char padding[];
+} GenericAlternate;
+
 void visit_start_struct(Visitor *v, const char *name, void **obj,
                         size_t size, Error **errp);
 void visit_end_struct(Visitor *v, Error **errp);
-void visit_start_implicit_struct(Visitor *v, void **obj, size_t size,
-                                 Error **errp);
-void visit_end_implicit_struct(Visitor *v);
 
 void visit_start_list(Visitor *v, const char *name, Error **errp);
 GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size);
 void visit_end_list(Visitor *v);
 
+/*
+ * Start the visit of an alternate @obj with the given @size.
+ *
+ * @name specifies the relationship to the containing struct (ignored
+ * for a top level visit, the name of the key if this alternate is
+ * part of an object, or NULL if this alternate is part of a list).
+ *
+ * @obj must not be NULL. Input visitors will allocate @obj and
+ * determine the qtype of the next thing to be visited, stored in
+ * (*@obj)->type.  Other visitors will leave @obj unchanged.
+ *
+ * If @promote_int, treat integers as QTYPE_FLOAT.
+ *
+ * If successful, this must be paired with visit_end_alternate(), even
+ * if visiting the contents of the alternate fails.
+ */
+void visit_start_alternate(Visitor *v, const char *name,
+                           GenericAlternate **obj, size_t size,
+                           bool promote_int, Error **errp);
+
+/*
+ * Finish visiting an alternate type.
+ *
+ * Must be called after a successful visit_start_alternate(), even if
+ * an error occurred in the meantime.
+ *
+ * TODO: Should all the visit_end_* interfaces take obj parameter, so
+ * that dealloc visitor need not track what was passed in visit_start?
+ */
+void visit_end_alternate(Visitor *v);
+
 /**
  * Check if an optional member @name of an object needs visiting.
  * For input visitors, set *@present according to whether the
@@ -46,14 +81,6 @@ void visit_end_list(Visitor *v);
  */
 bool visit_optional(Visitor *v, const char *name, bool *present);
 
-/**
- * Determine the qtype of the item @name in the current object visit.
- * For input visitors, set *@type to the correct qtype of a qapi
- * alternate type; for other visitors, leave *@type unchanged.
- * If @promote_int, treat integers as QTYPE_FLOAT.
- */
-void visit_get_next_type(Visitor *v, const char *name, QType *type,
-                         bool promote_int, Error **errp);
 void visit_type_enum(Visitor *v, const char *name, int *obj,
                      const char *const strings[], Error **errp);
 void visit_type_int(Visitor *v, const char *name, int64_t *obj, Error **errp);