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

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) {