diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/devel/qapi-code-gen.txt | 99 |
1 files changed, 64 insertions, 35 deletions
diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt index 663ef10a56..ac36ed34e3 100644 --- a/docs/devel/qapi-code-gen.txt +++ b/docs/devel/qapi-code-gen.txt @@ -178,14 +178,11 @@ While the C code starts numbering at 0, it is better to use explicit comparisons to enum values than implicit comparisons to 0; the C code will also include a generated enum member ending in _MAX for tracking the size of the enum, useful when using common functions for -converting between strings and enum values. Since the wire format -always passes by name, it is acceptable to reorder or add new -enumeration members in any location without breaking clients of Client -JSON Protocol; however, removing enum values would break -compatibility. For any struct that has a member that will only contain -a finite set of string values, using an enum type for that member is -better than open-coding the member to be type 'str'. +converting between strings and enum values. +For any struct that has a member that will only contain a finite set +of string values, using an enum type for that member is better than +open-coding the member to be type 'str'. === Struct types === @@ -203,34 +200,6 @@ name. An example of a struct is: The use of '*' as a prefix to the name means the member is optional in the corresponding JSON protocol usage. -The default initialization value of an optional argument should not be changed -between versions of QEMU unless the new default maintains backward -compatibility to the user-visible behavior of the old default. - -With proper documentation, this policy still allows some flexibility; for -example, documenting that a default of 0 picks an optimal buffer size allows -one release to declare the optimal size at 512 while another release declares -the optimal size at 4096 - the user-visible behavior is not the bytes used by -the buffer, but the fact that the buffer was optimal size. - -On input structures (only mentioned in the 'data' side of a command), changing -from mandatory to optional is safe (older clients will supply the option, and -newer clients can benefit from the default); changing from optional to -mandatory is backwards incompatible (older clients may be omitting the option, -and must continue to work). - -On output structures (only mentioned in the 'returns' side of a command), -changing from mandatory to optional is in general unsafe (older clients may be -expecting the member, and could crash if it is missing), although it -can be done if the only way that the optional argument will be omitted -is when it is triggered by the presence of a new input flag to the -command that older clients don't know to send. Changing from optional -to mandatory is safe. - -A structure that is used in both input and output of various commands -must consider the backwards compatibility constraints of both directions -of use. - A struct definition can specify another struct as its base. In this case, the members of the base type are included as top-level members of the new struct's dictionary in the Client JSON Protocol wire @@ -1037,6 +1006,66 @@ the names of built-in types. Clients should examine member "json-type" instead of hard-coding names of built-in types. +== Compatibility considerations == + +Maintaining backward compatibility at the Client JSON Protocol level +while evolving the schema requires some care. This section is about +syntactic compatibility, which is necessary, but not sufficient, for +actual compatibility. + +Clients send commands with argument data, and receive command +responses with return data and events with event data. + +Adding opt-in functionality to the send direction is backwards +compatible: adding commands, optional arguments, enumeration values, +union and alternate branches; turning an argument type into an +alternate of that type; making mandatory arguments optional. Clients +oblivious of the new functionality continue to work. + +Incompatible changes include removing commands, command arguments, +enumeration values, union and alternate branches, adding mandatory +command arguments, and making optional arguments mandatory. + +The specified behavior of an absent optional argument should remain +the same. With proper documentation, this policy still allows some +flexibility; for example, when an optional 'buffer-size' argument is +specified to default to a sensible buffer size, the actual default +value can still be changed. The specified default behavior is not the +exact size of the buffer, only that the default size is sensible. + +Adding functionality to the receive direction is generally backwards +compatible: adding events, adding return and event data members. +Clients are expected to ignore the ones they don't know. + +Removing "unreachable" stuff like events that can't be triggered +anymore, optional return or event data members that can't be sent +anymore, and return or event data member (enumeration) values that +can't be sent anymore makes no difference to clients, except for +introspection. The latter can conceivably confuse clients, so tread +carefully. + +Incompatible changes include removing return and event data members. + +Any change to a command definition's 'data' or one of the types used +there (recursively) needs to consider send direction compatibility. + +Any change to a command definition's 'return', an event definition's +'data', or one of the types used there (recursively) needs to consider +receive direction compatibility. + +Any change to types used in both contexts need to consider both. + +Members of enumeration types, complex types and alternate types may be +reordered freely. For enumerations and alternate types, this doesn't +affect the wire encoding. For complex types, this might make the +implementation emit JSON object members in a different order, which +the Client JSON Protocol permits. + +Since type names are not visible in the Client JSON Protocol, types +may be freely renamed. Even certain refactorings are invisible, such +as splitting members from one type into a common base type. + + == Code generation == The QAPI code generator qapi-gen.py generates code and documentation |