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authorEric Blake <eblake@redhat.com>2016-04-28 15:45:31 -0600
committerMarkus Armbruster <armbru@redhat.com>2016-05-12 09:47:55 +0200
commitd9f62dde1303286b24ac8ce88be27e2b9b9c5f46 (patch)
tree3205f18c58a69af31f50c14c0131d1d81416844c /include/qapi
parent74f24cb6306d065045d0e2215a7d10533fa59c57 (diff)
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qapi: Simplify semantics of visit_next_list()
The semantics of the list visit are somewhat baroque, with the following pseudocode when FooList is used: start() for (prev = head; cur = next(prev); prev = &cur) { visit(&cur->value) } Note that these semantics (advance before visit) requires that the first call to next() return the list head, while all other calls return the next element of the list; that is, every visitor implementation is required to track extra state to decide whether to return the input as-is, or to advance. It also requires an argument of 'GenericList **' to next(), solely because the first iteration might need to modify the caller's GenericList head, so that all other calls have to do a layer of dereferencing. Thankfully, we only have two uses of list visits in the entire code base: one in spapr_drc (which completely avoids visit_next_list(), feeding in integers from a different source than uint8List), and one in qapi-visit.py. That is, all other list visitors are generated in qapi-visit.c, and share the same paradigm based on a qapi FooList type, so we can refactor how lists are laid out with minimal churn among clients. We can greatly simplify things by hoisting the special case into the start() routine, and flipping the order in the loop to visit before advance: start(head) for (tail = *head; tail; tail = next(tail)) { visit(&tail->value) } With the simpler semantics, visitors have less state to track, the argument to next() is reduced to 'GenericList *', and it also becomes obvious whether an input visitor is allocating a FooList during visit_start_list() (rather than the old way of not knowing if an allocation happened until the first visit_next_list()). As a minor drawback, we now allocate in two functions instead of one, and have to pass the size to both functions (unless we were to tweak the input visitors to cache the size to start_list for reuse during next_list, but that defeats the goal of less visitor state). The signature of visit_start_list() is chosen to match visit_start_struct(), with the new parameters after 'name'. The spapr_drc case is a virtual visit, done by passing NULL for list, similarly to how NULL is passed to visit_start_struct() when a qapi type is not used in those visits. It was easy to provide these semantics for qmp-output and dealloc visitors, and a bit harder for qmp-input (several prerequisite patches refactored things to make this patch straightforward). But it turned out that the string and opts visitors munge enough other state during visit_next_list() to make it easier to just document and require a GenericList visit for now; an assertion will remind us to adjust things if we need the semantics in the future. Several pre-requisite cleanup patches made the reshuffling of the various visitors easier; particularly the qmp input visitor. Signed-off-by: Eric Blake <eblake@redhat.com> Message-Id: <1461879932-9020-24-git-send-email-eblake@redhat.com> Signed-off-by: Markus Armbruster <armbru@redhat.com>
Diffstat (limited to 'include/qapi')
-rw-r--r--include/qapi/opts-visitor.h3
-rw-r--r--include/qapi/string-input-visitor.h3
-rw-r--r--include/qapi/string-output-visitor.h3
-rw-r--r--include/qapi/visitor-impl.h8
-rw-r--r--include/qapi/visitor.h49
5 files changed, 37 insertions, 29 deletions
diff --git a/include/qapi/opts-visitor.h b/include/qapi/opts-visitor.h
index fe37ed9..ae1bf7c 100644
--- a/include/qapi/opts-visitor.h
+++ b/include/qapi/opts-visitor.h
@@ -32,7 +32,8 @@ typedef struct OptsVisitor OptsVisitor;
*
* The Opts input visitor does not implement support for visiting QAPI
* alternates, numbers (other than integers), null, or arbitrary
- * QTypes.
+ * QTypes. It also requires a non-null list argument to
+ * visit_start_list().
*/
OptsVisitor *opts_visitor_new(const QemuOpts *opts);
void opts_visitor_cleanup(OptsVisitor *nv);
diff --git a/include/qapi/string-input-visitor.h b/include/qapi/string-input-visitor.h
index a8d8f67..7b76c2b 100644
--- a/include/qapi/string-input-visitor.h
+++ b/include/qapi/string-input-visitor.h
@@ -19,7 +19,8 @@ typedef struct StringInputVisitor StringInputVisitor;
/*
* The string input visitor does not implement support for visiting
- * QAPI structs, alternates, null, or arbitrary QTypes.
+ * QAPI structs, alternates, null, or arbitrary QTypes. It also
+ * requires a non-null list argument to visit_start_list().
*/
StringInputVisitor *string_input_visitor_new(const char *str);
void string_input_visitor_cleanup(StringInputVisitor *v);
diff --git a/include/qapi/string-output-visitor.h b/include/qapi/string-output-visitor.h
index 89b7e4b..e10522a 100644
--- a/include/qapi/string-output-visitor.h
+++ b/include/qapi/string-output-visitor.h
@@ -19,7 +19,8 @@ typedef struct StringOutputVisitor StringOutputVisitor;
/*
* The string output visitor does not implement support for visiting
- * QAPI structs, alternates, null, or arbitrary QTypes.
+ * QAPI structs, alternates, null, or arbitrary QTypes. It also
+ * requires a non-null list argument to visit_start_list().
*/
StringOutputVisitor *string_output_visitor_new(bool human);
void string_output_visitor_cleanup(StringOutputVisitor *v);
diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
index b20a922..145afd0 100644
--- a/include/qapi/visitor-impl.h
+++ b/include/qapi/visitor-impl.h
@@ -49,11 +49,13 @@ struct Visitor
/* Must be set to visit structs */
void (*end_struct)(Visitor *v);
- /* Must be set */
- void (*start_list)(Visitor *v, const char *name, Error **errp);
+ /* Must be set; implementations may require @list to be non-null,
+ * but must document it. */
+ void (*start_list)(Visitor *v, const char *name, GenericList **list,
+ size_t size, Error **errp);
/* Must be set */
- GenericList *(*next_list)(Visitor *v, GenericList **list, size_t size);
+ GenericList *(*next_list)(Visitor *v, GenericList *tail, size_t size);
/* Must be set */
void (*end_list)(Visitor *v);
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index fe268fe..8de6b43 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -176,7 +176,7 @@
* if (err) {
* goto out;
* }
- * visit_start_list(v, "list", &err);
+ * visit_start_list(v, "list", NULL, 0, &err);
* if (err) {
* goto outobj;
* }
@@ -281,19 +281,27 @@ void visit_end_struct(Visitor *v);
* @name expresses the relationship of this list to its parent
* container; see the general description of @name above.
*
+ * @list must be non-NULL for a real walk, in which case @size
+ * determines how much memory an input visitor will allocate into
+ * *@list (at least sizeof(GenericList)). Some visitors also allow
+ * @list to be NULL for a virtual walk, in which case @size is
+ * ignored.
+ *
* @errp obeys typical error usage, and reports failures such as a
- * member @name is not present, or present but not a list.
+ * member @name is not present, or present but not a list. On error,
+ * input visitors set *@list to NULL.
*
* After visit_start_list() succeeds, the caller may visit its members
- * one after the other. A real visit uses visit_next_list() for
- * traversing the linked list, while a virtual visit uses other means.
- * For each list element, call the appropriate visit_type_FOO() with
- * name set to NULL and obj set to the address of the value member of
- * the list element. Finally, visit_end_list() needs to be called to
- * clean up, even if intermediate visits fail. See the examples
- * above.
+ * one after the other. A real visit (where @obj is non-NULL) uses
+ * visit_next_list() for traversing the linked list, while a virtual
+ * visit (where @obj is NULL) uses other means. For each list
+ * element, call the appropriate visit_type_FOO() with name set to
+ * NULL and obj set to the address of the value member of the list
+ * element. Finally, visit_end_list() needs to be called to clean up,
+ * even if intermediate visits fail. See the examples above.
*/
-void visit_start_list(Visitor *v, const char *name, Error **errp);
+void visit_start_list(Visitor *v, const char *name, GenericList **list,
+ size_t size, Error **errp);
/*
* Iterate over a GenericList during a non-virtual list visit.
@@ -301,20 +309,15 @@ void visit_start_list(Visitor *v, const char *name, Error **errp);
* @size represents the size of a linked list node (at least
* sizeof(GenericList)).
*
- * @list must not be NULL; on the first call, @list contains the
- * address of the list head, and on subsequent calls *@list must be
- * the previously returned value. Should be called in a loop until a
- * NULL return or error occurs; for each non-NULL return, the caller
- * then calls the appropriate visit_type_*() for the element type
- * of the list, with that function's name parameter set to NULL and
- * obj set to the address of (*@list)->value.
- *
- * FIXME: This interface is awkward; it requires all callbacks to
- * track whether it is the first or a subsequent call. A better
- * interface would pass the head of the list through
- * visit_start_list().
+ * @tail must not be NULL; on the first call, @tail is the value of
+ * *list after visit_start_list(), and on subsequent calls @tail must
+ * be the previously returned value. Should be called in a loop until
+ * a NULL return or error occurs; for each non-NULL return, the caller
+ * then calls the appropriate visit_type_*() for the element type of
+ * the list, with that function's name parameter set to NULL and obj
+ * set to the address of @tail->value.
*/
-GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size);
+GenericList *visit_next_list(Visitor *v, GenericList *tail, size_t size);
/*
* Complete a list visit started earlier.