Re: [Qemu-devel] [PATCH v2] qapi: Document optional arguments' backwards compatibility

[Eric Blake]

On 05/05/2014 08:05 PM, Fam Zheng wrote:
> Signed-off-by: Fam Zheng <address@hidden>
> 
> ---
> v2: Employ the text suggested by Eric. (Thanks!)

Since much of it is my wording, it's probably better to credit me as an
author, by adding:

Signed-off-by: Eric Blake <address@hidden>

> 
> Signed-off-by: Fam Zheng <address@hidden>
> ---
>  docs/qapi-code-gen.txt | 26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
> 
> +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.

Maybe worth adding:

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.


-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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