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Re: [Libguestfs] [libnbd PATCH] api: Add way to avoid structured replies

On Wed, Sep 04, 2019 at 01:28:08PM -0500, Eric Blake wrote:
> We want to default to requesting structured replies, whether or not
> that request will be honored (it's essential for efficient sparse file
> reads and the DF flag for structured pread, as well as for meta
> context support even if we do not request a default meta context).
> However, for integration testing, it can be nice to easily request a
> client that does not request structured replies.
> We can test this by reusing our eflags test.  Note that nbdkit does
> not provide a 'can_df' callback in the sh script (so our key=value
> override is silently ignored), rather, we control whether nbdkit
> advertises df based on whether we request structured replies.
> ---
> I'm open to renaming the API to the shorter 'nbd_set_request_sr' if
> the existing name choice seems too long.

There's not really a limit on the length of API names, and in this
case the longer name explains what the option does more clearly.

Anyway this looks fine to me.


I have one comment unrelated to the patch:

> +  "set_request_structured_replies", {
> +    default_call with
> +    args = [Bool "request"]; ret = RErr;
> +    permitted_states = [ Created ];
> +    first_version = (1, 2);

I just know that we're going to end up adding new APIs and forgetting
to set the first_version field.  There are various things we could do
to prevent this:

(1) In ‘default_call’ set first_version = (0, 0).  Update all existing
calls with first_version = (1, 0).  Then add a check that
first_version <> (0, 0).  There's still a danger of copy and paste
from an existing API, but we should be able to catch that in review.

(2) Store first_version in a separate table.  Add checks to ensure the
new table exhaustively covers all APIs.  It should be obvious when
submitting a new API that the first_version table must be updated and
what to add here:

  let first_version = [
    "set_debug", (1, 0);
    "set_request_structured_replies", (1, 2);
    "get_request_structured_replies", (1, 2);

Not sure which is better.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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