diff options
| -rw-r--r-- | docs/qapi-code-gen.txt | 32 |
1 files changed, 28 insertions, 4 deletions
diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt index 051d109..26312d8 100644 --- a/docs/qapi-code-gen.txt +++ b/docs/qapi-code-gen.txt @@ -60,10 +60,34 @@ example of a complex type is: { 'type': 'MyType', 'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } } -The use of '*' as a prefix to the name means the member is optional. Optional -members should always be added to the end of the dictionary to preserve -backwards compatibility. - +The use of '*' as a prefix to the name means the member is optional. + +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 field, 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 complex type definition can specify another complex type as its base. In this case, the fields of the base type are included as top-level fields |
