diff options
Diffstat (limited to 'include/qapi/visitor.h')
| -rw-r--r-- | include/qapi/visitor.h | 447 |
1 files changed, 427 insertions, 20 deletions
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h index 690589f..2211637 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 |