[Libguestfs] [libnbd PATCH v2 11/12] api: Add nbd_[aio_]opt_set_meta_context

[Laszlo Ersek]

On 08/31/22 20:28, Eric Blake wrote:
> On Wed, Aug 31, 2022 at 09:39:27AM -0500, Eric Blake wrote:
>> Make it possible for the client to choose when to manually request
>> meta contexts when using nbd_set_opt_mode().  By itself, this patch
>> allows us to test how a server reacts to NBD_OPT_SET_META_CONTEXT
>> without a previous negotiation of structured headers; then an upcoming
>> patch will also add APIs for choosing when to negotiate OPT_STARTTLS
>> and OPT_STRUCTURED_REPLY for even more fine-grained testing control.
>> Most users won't need this API (the automatic defaults are good
>> enough), but when writing a server, this level of integration testing
>> can be useful.
>>
>> Overhaul the comment on OPT_META_CONTEXT.START to explain 5 different
>> scenarios that this state group can be reached in, to better document
>> which conditionals are in use later in the state machine:
>>   - h->current_opt == opt is a witness of whether we are doing a
>> mandatory manual LIST/SET request with an expected callback and
>> transition to NEGOTIATIONG, or a conditional SET request dependent on
>> nbd_set_request_meta_context() and prior negotiation and transition to
>> OPT_GO
>>  - h->current_opt == NBD_OPT_LIST_META_CONTEXT is a witness of whether
>>  we are stateless or modifying h->meta_contexts
>>
>> As this is a new API, the unit test in C is included in this patch;
>> there is no point in splitting it as reordering the patches would not
>> compile.  However, porting the tests to other languages will be done
>> in the next commit.
>>
>> ---
> 
> Also intended to be squashed into this patch:
> 
> diff --git a/generator/states-newstyle-opt-meta-context.c b/generator/states-newstyle-opt-meta-context.c
> index 03b63c6..49e08bc 100644
> --- a/generator/states-newstyle-opt-meta-context.c
> +++ b/generator/states-newstyle-opt-meta-context.c
> @@ -23,9 +23,24 @@ STATE_MACHINE {
>    size_t i;
>    uint32_t len, opt;
> 
> -  /* If the server doesn't support SRs then we must skip this group.
> -   * Also we skip the group if the client didn't request any metadata
> -   * contexts, when doing SET (but an empty LIST is okay).
> +  /* This state group is reached from:
> +   * h->opt_mode == false (h->current_opt == 0):
> +   *   nbd_connect_*()
> +   *     -> conditionally use SET, next state OPT_GO
> +   * h->opt_mode == true (h->current_opt matches calling API):
> +   *   nbd_opt_info(), nbd_opt_go()
> +   *     -> conditionally use SET, next state OPT_GO
> +   *   nbd_opt_list_meta_context()
> +   *     -> unconditionally use LIST, next state NEGOTIATING
> +   *   nbd_opt_set_meta_context()
> +   *     -> unconditionally use SET, next state NEGOTIATING
> +   *
> +   * If the next state is OPT_GO, we want to clear h->exportsize and friends.
> +   * If SET is conditional, we skip it if h->request_meta is false, if
> +   * structured replies were not negotiated, or if no contexts to request.
> +   * There is a callback if and only if the command is unconditional.
> +   * If we use SET, we clear h->meta_contexts and rebuild it.
> +   * If we use LIST, we are stateless.
>     */
>    assert (h->gflags & LIBNBD_HANDSHAKE_FLAG_FIXED_NEWSTYLE);
>    if (h->opt_current == NBD_OPT_LIST_META_CONTEXT) {
> 
> 

I'd like to review this one in its final v3 state.

Thanks
Laszlo