qapi: Document visitor interfaces, add assertions

The visitor interface for mapping between QObject/QemuOpts/string
and QAPI is scandalously under-documented, making changes to visitor
core, individual visitors, and users of visitors difficult to
coordinate.  Among other questions: when is it safe to pass NULL,
vs. when a string must be provided; which visitors implement which
callbacks; the difference between concrete and virtual visits.

Correct this by retrofitting proper contracts, and document where some
of the interface warts remain (for example, we may want to modify
visit_end_* to require the same 'obj' as the visit_start counterpart,
so the dealloc visitor can be simplified).  Later patches in this
series will tackle some, but not all, of these warts.

Add assertions to (partially) enforce the contract.  Some of these
were only made possible by recent cleanup commits.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-13-git-send-email-eblake@redhat.com>
[Doc fix from Eric squashed in]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 427 insertions, 20 deletions

diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 690589f37d..221163742c 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -16,8 +16,196 @@
 
 #include "qapi/qmp/qobject.h"
 
+/*
+ * The QAPI schema defines both a set of C data types, and a QMP wire
+ * format.  QAPI objects can contain references to other QAPI objects,
+ * resulting in a directed acyclic graph.  QAPI also generates visitor
+ * functions to walk these graphs.  This file represents the interface
+ * for doing work at each node of a QAPI graph; it can also be used
+ * for a virtual walk, where there is no actual QAPI C struct.
+ *
+ * There are three kinds of visitor classes: input visitors (QMP,
+ * string, and QemuOpts) parse an external representation and build
+ * the corresponding QAPI graph, output visitors (QMP and string) take
+ * a completed QAPI graph and generate an external representation, and
+ * the dealloc visitor can take a QAPI graph (possibly partially
+ * constructed) and recursively free its resources.  While the dealloc
+ * and QMP input/output visitors are general, the string and QemuOpts
+ * visitors have some implementation limitations; see the
+ * documentation for each visitor for more details on what it
+ * supports.  Also, see visitor-impl.h for the callback contracts
+ * implemented by each visitor, and docs/qapi-code-gen.txt for more
+ * about the QAPI code generator.
+ *
+ * All QAPI types have a corresponding function with a signature
+ * roughly compatible with this:
+ *
+ * void visit_type_FOO(Visitor *v, const char *name, T obj, Error **errp);
+ *
+ * where T is FOO for scalar types, and FOO * otherwise.  The scalar
+ * visitors are declared here; the remaining visitors are generated in
+ * qapi-visit.h.
+ *
+ * The @name parameter of visit_type_FOO() describes the relation
+ * between this QAPI value and its parent container.  When visiting
+ * the root of a tree, @name is ignored; when visiting a member of an
+ * object, @name is the key associated with the value; and when
+ * visiting a member of a list, @name is NULL.
+ *
+ * FIXME: Clients must pass NULL for @name when visiting a member of a
+ * list, but this leads to poor error messages; it might be nicer to
+ * require a non-NULL name such as "key.0" for '{ "key": [ "value" ]
+ * }' if an error is encountered on "value" (or to have the visitor
+ * core auto-generate the nicer name).
+ *
+ * The visit_type_FOO() functions expect a non-null @obj argument;
+ * they allocate *@obj during input visits, leave it unchanged on
+ * output visits, and recursively free any resources during a dealloc
+ * visit.  Each function also takes the customary @errp argument (see
+ * qapi/error.h for details), for reporting any errors (such as if a
+ * member @name is not present, or is present but not the specified
+ * type).
+ *
+ * FIXME: At present, visit_type_FOO() is an awkward interface: input
+ * visitors may allocate an incomplete *@obj even when reporting an
+ * error, but using an output visitor with an incomplete object has
+ * undefined behavior.  To avoid a memory leak, callers must use
+ * qapi_free_FOO() even on error (this uses the dealloc visitor, and
+ * safely handles an incomplete object).
+ *
+ * For the QAPI object types (structs, unions, and alternates), there
+ * is an additional generated function in qapi-visit.h compatible
+ * with:
+ *
+ * void visit_type_FOO_members(Visitor *v, FOO *obj, Error **errp);
+ *
+ * for visiting the members of a type without also allocating the QAPI
+ * struct.
+ *
+ * Additionally, in qapi-types.h, all QAPI pointer types (structs,
+ * unions, alternates, and lists) have a generated function compatible
+ * with:
+ *
+ * void qapi_free_FOO(FOO *obj);
+ *
+ * which behaves like free() in that @obj may be NULL.  Because of
+ * these functions, the dealloc visitor is seldom used directly
+ * outside of generated code.  QAPI types can also inherit from a base
+ * class; when this happens, a function is generated for easily going
+ * from the derived type to the base type:
+ *
+ * BASE *qapi_CHILD_base(CHILD *obj);
+ *
+ * For a real QAPI struct, typical input usage involves:
+ *
+ * <example>
+ *  Foo *f;
+ *  Error *err = NULL;
+ *  Visitor *v;
+ *
+ *  v = ...obtain input visitor...
+ *  visit_type_Foo(v, NULL, &f, &err);
+ *  if (err) {
+ *      qapi_free_Foo(f);
+ *      ...handle error...
+ *  } else {
+ *      ...use f...
+ *  }
+ *  ...clean up v...
+ *  qapi_free_Foo(f);
+ * </example>
+ *
+ * For a list, it is:
+ * <example>
+ *  FooList *l;
+ *  Error *err = NULL;
+ *  Visitor *v;
+ *
+ *  v = ...obtain input visitor...
+ *  visit_type_FooList(v, NULL, &l, &err);
+ *  if (err) {
+ *      qapi_free_FooList(l);
+ *      ...handle error...
+ *  } else {
+ *      for ( ; l; l = l->next) {
+ *          ...use l->value...
+ *      }
+ *  }
+ *  ...clean up v...
+ *  qapi_free_FooList(l);
+ * </example>
+ *
+ * Similarly, typical output usage is:
+ *
+ * <example>
+ *  Foo *f = ...obtain populated object...
+ *  Error *err = NULL;
+ *  Visitor *v;
+ *
+ *  v = ...obtain output visitor...
+ *  visit_type_Foo(v, NULL, &f, &err);
+ *  if (err) {
+ *      ...handle error...
+ *  }
+ *  ...clean up v...
+ * </example>
+ *
+ * When visiting a real QAPI struct, this file provides several
+ * helpers that rely on in-tree information to control the walk:
+ * visit_optional() for the 'has_member' field associated with
+ * optional 'member' in the C struct; and visit_next_list() for
+ * advancing through a FooList linked list.  Only the generated
+ * visit_type functions need to use these helpers.
+ *
+ * It is also possible to use the visitors to do a virtual walk, where
+ * no actual QAPI struct is present.  In this situation, decisions
+ * about what needs to be walked are made by the calling code, and
+ * structured visits are split between pairs of start and end methods
+ * (where the end method must be called if the start function
+ * succeeded, even if an intermediate visit encounters an error).
+ * Thus, a virtual walk corresponding to '{ "list": [1, 2] }' looks
+ * like:
+ *
+ * <example>
+ *  Visitor *v;
+ *  Error *err = NULL;
+ *  int value;
+ *
+ *  v = ...obtain visitor...
+ *  visit_start_struct(v, NULL, NULL, 0, &err);
+ *  if (err) {
+ *      goto out;
+ *  }
+ *  visit_start_list(v, "list", &err);
+ *  if (err) {
+ *      goto outobj;
+ *  }
+ *  value = 1;
+ *  visit_type_int(v, NULL, &value, &err);
+ *  if (err) {
+ *      goto outlist;
+ *  }
+ *  value = 2;
+ *  visit_type_int(v, NULL, &value, &err);
+ *  if (err) {
+ *      goto outlist;
+ *  }
+ * outlist:
+ *  visit_end_list(v);
+ * outobj:
+ *  error_propagate(errp, err);
+ *  err = NULL;
+ *  visit_end_struct(v, &err);
+ * out:
+ *  error_propagate(errp, err);
+ *  ...clean up v...
+ * </example>
+ */
+
+/*** Useful types ***/
+
 /* This struct is layout-compatible with all other *List structs
- * created by the qapi generator.  It is used as a typical
+ * created by the QAPI generator.  It is used as a typical
  * singly-linked list. */
 typedef struct GenericList {
     struct GenericList *next;
@@ -25,35 +213,126 @@ typedef struct GenericList {
 } GenericList;
 
 /* This struct is layout-compatible with all Alternate types
- * created by the qapi generator. */
+ * created by the QAPI generator. */
 typedef struct GenericAlternate {
     QType type;
     char padding[];
 } GenericAlternate;
 
+/*** Visiting structures ***/
+
+/*
+ * Start visiting an object @obj (struct or union).
+ *
+ * @name expresses the relationship of this object to its parent
+ * container; see the general description of @name above.
+ *
+ * @obj must be non-NULL for a real walk, in which case @size
+ * determines how much memory an input visitor will allocate into
+ * *@obj.  @obj may also 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 an object.  On
+ * error, input visitors set *@obj to NULL.
+ *
+ * After visit_start_struct() succeeds, the caller may visit its
+ * members one after the other, passing the member's name and address
+ * within the struct.  Finally, visit_end_struct() needs to be called
+ * to clean up, even if intermediate visits fail.  See the examples
+ * above.
+ *
+ * FIXME Should this be named visit_start_object, since it is also
+ * used for QAPI unions, and maps to JSON objects?
+ */
 void visit_start_struct(Visitor *v, const char *name, void **obj,
                         size_t size, Error **errp);
+
+/*
+ * Complete an object visit started earlier.
+ *
+ * @errp obeys typical error usage, and reports failures such as
+ * unparsed keys remaining in the input stream.
+ *
+ * Must be called after any successful use of visit_start_struct(),
+ * even if intermediate processing was skipped due to errors, to allow
+ * the backend to release any resources.  Destroying the visitor early
+ * behaves as if this was implicitly called.
+ */
 void visit_end_struct(Visitor *v, Error **errp);
 
+
+/*** Visiting lists ***/
+
+/*
+ * Start visiting a list.
+ *
+ * @name expresses the relationship of this list to its parent
+ * container; see the general description of @name above.
+ *
+ * @errp obeys typical error usage, and reports failures such as a
+ * member @name is not present, or present but not a list.
+ *
+ * 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.
+ */
 void visit_start_list(Visitor *v, const char *name, Error **errp);
+
+/*
+ * Iterate over a GenericList during a non-virtual list visit.
+ *
+ * @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().
+ */
 GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size);
+
+/*
+ * Complete a list visit started earlier.
+ *
+ * Must be called after any successful use of visit_start_list(), even
+ * if intermediate processing was skipped due to errors, to allow the
+ * backend to release any resources.  Destroying the visitor early
+ * behaves as if this was implicitly called.
+ */
 void visit_end_list(Visitor *v);
 
+
+/*** Visiting alternates ***/
+
 /*
- * Start the visit of an alternate @obj with the given @size.
+ * Start the visit of an alternate @obj.
  *
- * @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).
+ * @name expresses the relationship of this alternate to its parent
+ * container; see the general description of @name above.
  *
- * @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.
+ * @obj must not be NULL. Input visitors use @size to determine how
+ * much memory to allocate into *@obj, then 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.
+ * If successful, this must be paired with visit_end_alternate() to
+ * clean up, even if visiting the contents of the alternate fails.
  */
 void visit_start_alternate(Visitor *v, const char *name,
                            GenericAlternate **obj, size_t size,
@@ -62,27 +341,48 @@ void visit_start_alternate(Visitor *v, const char *name,
 /*
  * Finish visiting an alternate type.
  *
- * Must be called after a successful visit_start_alternate(), even if
- * an error occurred in the meantime.
+ * Must be called after any successful use of visit_start_alternate(),
+ * even if intermediate processing was skipped due to errors, to allow
+ * the backend to release any resources.  Destroying the visitor early
+ * behaves as if this was implicitly called.
  *
  * 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
- * corresponding visit_type_*() needs calling; for other visitors,
- * leave *@present unchanged.  Return *@present for convenience.
+
+/*** Other helpers ***/
+
+/*
+ * Does optional struct member @name need visiting?
+ *
+ * @name must not be NULL.  This function is only useful between
+ * visit_start_struct() and visit_end_struct(), since only objects
+ * have optional keys.
+ *
+ * @present points to the address of the optional member's has_ flag.
+ *
+ * Input visitors set *@present according to input; other visitors
+ * leave it unchanged.  In either case, return *@present for
+ * convenience.
  */
 bool visit_optional(Visitor *v, const char *name, bool *present);
 
 /*
  * Visit an enum value.
  *
- * @strings expresses the mapping between C enum values and QAPI enum
- * names; it should be the ENUM_lookup array from visit-types.h.
+ * @name expresses the relationship of this enum to its parent
+ * container; see the general description of @name above.
+ *
+ * @obj must be non-NULL.  Input visitors parse input and set *@obj to
+ * the enumeration value, leaving @obj unchanged on error; other
+ * visitors use *@obj but leave it unchanged.
+ *
+ * Currently, all input visitors parse text input, and all output
+ * visitors produce text output.  The mapping between enumeration
+ * values and strings is done by the visitor core, using @strings; it
+ * should be the ENUM_lookup array from visit-types.h.
  *
  * May call visit_type_str() under the hood, and the enum visit may
  * fail even if the corresponding string visit succeeded; this implies
@@ -91,28 +391,135 @@ bool visit_optional(Visitor *v, const char *name, bool *present);
 void visit_type_enum(Visitor *v, const char *name, int *obj,
                      const char *const strings[], Error **errp);
 
+/*** Visiting built-in types ***/
+
+/*
+ * Visit an integer value.
+ *
+ * @name expresses the relationship of this integer to its parent
+ * container; see the general description of @name above.
+ *
+ * @obj must be non-NULL.  Input visitors set *@obj to the value;
+ * other visitors will leave *@obj unchanged.
+ */
 void visit_type_int(Visitor *v, const char *name, int64_t *obj, Error **errp);
+
+/*
+ * Visit a uint8_t value.
+ * Like visit_type_int(), except clamps the value to uint8_t range.
+ */
 void visit_type_uint8(Visitor *v, const char *name, uint8_t *obj,
                       Error **errp);
+
+/*
+ * Visit a uint16_t value.
+ * Like visit_type_int(), except clamps the value to uint16_t range.
+ */
 void visit_type_uint16(Visitor *v, const char *name, uint16_t *obj,
                        Error **errp);
+
+/*
+ * Visit a uint32_t value.
+ * Like visit_type_int(), except clamps the value to uint32_t range.
+ */
 void visit_type_uint32(Visitor *v, const char *name, uint32_t *obj,
                        Error **errp);
+
+/*
+ * Visit a uint64_t value.
+ * Like visit_type_int(), except clamps the value to uint64_t range,
+ * that is, ensures it is unsigned.
+ */
 void visit_type_uint64(Visitor *v, const char *name, uint64_t *obj,
                        Error **errp);
+
+/*
+ * Visit an int8_t value.
+ * Like visit_type_int(), except clamps the value to int8_t range.
+ */
 void visit_type_int8(Visitor *v, const char *name, int8_t *obj, Error **errp);
+
+/*
+ * Visit an int16_t value.
+ * Like visit_type_int(), except clamps the value to int16_t range.
+ */
 void visit_type_int16(Visitor *v, const char *name, int16_t *obj,
                       Error **errp);
+
+/*
+ * Visit an int32_t value.
+ * Like visit_type_int(), except clamps the value to int32_t range.
+ */
 void visit_type_int32(Visitor *v, const char *name, int32_t *obj,
                       Error **errp);
+
+/*
+ * Visit an int64_t value.
+ * Identical to visit_type_int().
+ */
 void visit_type_int64(Visitor *v, const char *name, int64_t *obj,
                       Error **errp);
+
+/*
+ * Visit a uint64_t value.
+ * Like visit_type_uint64(), except that some visitors may choose to
+ * recognize additional syntax, such as suffixes for easily scaling
+ * values.
+ */
 void visit_type_size(Visitor *v, const char *name, uint64_t *obj,
                      Error **errp);
+
+/*
+ * Visit a boolean value.
+ *
+ * @name expresses the relationship of this boolean to its parent
+ * container; see the general description of @name above.
+ *
+ * @obj must be non-NULL.  Input visitors set *@obj to the value;
+ * other visitors will leave *@obj unchanged.
+ */
 void visit_type_bool(Visitor *v, const char *name, bool *obj, Error **errp);
+
+/*
+ * Visit a string value.
+ *
+ * @name expresses the relationship of this string to its parent
+ * container; see the general description of @name above.
+ *
+ * @obj must be non-NULL.  Input visitors set *@obj to the value
+ * (never NULL).  Other visitors leave *@obj unchanged, and commonly
+ * treat NULL like "".
+ *
+ * It is safe to cast away const when preparing a (const char *) value
+ * into @obj for use by an output visitor.
+ *
+ * FIXME: Callers that try to output NULL *obj should not be allowed.
+ */
 void visit_type_str(Visitor *v, const char *name, char **obj, Error **errp);
+
+/*
+ * Visit a number (i.e. double) value.
+ *
+ * @name expresses the relationship of this number to its parent
+ * container; see the general description of @name above.
+ *
+ * @obj must be non-NULL.  Input visitors set *@obj to the value;
+ * other visitors will leave *@obj unchanged.  Visitors should
+ * document if infinity or NaN are not permitted.
+ */
 void visit_type_number(Visitor *v, const char *name, double *obj,
                        Error **errp);
+
+/*
+ * Visit an arbitrary value.
+ *
+ * @name expresses the relationship of this value to its parent
+ * container; see the general description of @name above.
+ *
+ * @obj must be non-NULL.  Input visitors set *@obj to the value;
+ * other visitors will leave *@obj unchanged.  *@obj must be non-NULL
+ * for output visitors.
+ */
 void visit_type_any(Visitor *v, const char *name, QObject **obj, Error **errp);
 
 #endif