[Libguestfs] [libnbd PATCH 1/2] api: Better use of nbd_internal_reset_size_and_flags

[Laszlo Ersek]

On 08/24/22 23:20, Eric Blake wrote:
> On Wed, Aug 24, 2022 at 01:56:03PM +0200, Laszlo Ersek wrote:
>> On 08/24/22 04:53, Eric Blake wrote:
>>> The documentation for nbd_internal_reset_size_and_flags() claims we
>>> should wipe all knowledge of previously-negotiated meta contexts when
>>> swapping export names.  However, we weren't actually doing that, which
>>> could lead to a user calling nbd_opt_info() for one export, then
>>> switching to another export name, then having nbd_get_size() still
>>> report the size of the old export.  At present, this is only mildly
>>> confusing, but once the next patch exposes nbd_opt_set_meta_context()
>>> for user control,
>>
>> (1) The "next patch" does not expose nbd_opt_set_meta_context(). In
>> fact, I can't find nbd_opt_set_meta_context() -- the function -- in the
>> codebase anywhere (@ 9e5bf05a2196).
>>
>> (I prefer writing "a subsequent patch in this series" rather than "next
>> patch", because that allows me to reshuffle patches without having to
>> rewrite commit messages, as long as inter-patch dependencies are still
>> satisfied.)
> 
> Yep, exactly what happened; where the subsequent patch is still not
> quite polished enough to push to the list (but hopefully later today).
> Will fix before committing.
> 
>>
>> (2) I see the NBD_OPT_SET_META_CONTEXT opcode, but I'm not sure what it
>> is used for. There seems to be documentation for
>> <https://libguestfs.org/nbd_opt_list_meta_context.3.html>, but not for
>> nbd_opt_set_meta_context(). So perhaps "introduces" is a better verb
>> than "exposes"?
>>
>> Is NBD_OPT_SET_META_CONTEXT internally used by nbd_add_meta_context()
>> and nbd_clear_meta_contexts()?
> 
> Right now, the code base does:
> 
> nbd_set_export_name() => update h->export_name; no server interaction
> 
> nbd_add_meta_context() => append to h->request_meta_contexts; no
> server interaction
> 
> nbd_clear_meta_contexts() => wipe all of h->request_meta_contexts; no
> server interaction
> 
> nbd_opt_list_meta_context() => Issue NBD_OPT_LIST_META_CONTEXT to the
> server using h->request_meta_contexts as its query list and
> h->export_name, but collect the results only in the user's callback -
> not stateful on the server, and only stateful in the client if the
> callback does something to preserve the server's answers
> 
> nbd_opt_info()/nbd_opt_go() => Issue NBD_OPT_SET_META_CONTEXT using
> h->request_meta_contexts as its query list and h->export_name, collect
> the results into h->meta_contexts, then issue NBD_OPT_INFO/GO to the
> server.  Stateful, because the server's replies map the users context
> names (strings) to the servers ids (uint32_t); and later
> NBD_CMD_BLOCK_STATUS refers only to the server's 32-bit id, even
> though the libnbd API does not directly expose those ids.
> 
> nbd_can_meta_context() => Query whether h->meta_contexts contains a
> server reply for an earlier call that triggered
> NBD_OPT_SET_META_CONTEXT and which has not been wiped by changing
> state via nbd_internal_reset_size_and_flags(); no server interaction.
> 
> with the additional caveat that all the nbd_opt_* functions are
> unreachable unless you do nbd_set_opt_mode(true) to enable manual
> option negotiation (the default is that nbd_connect_*() behaves as if
> it does nbd_opt_go() on your behalf).

Thanks for writing this up; it's a lot to digest. I think I understand
it now.

There's a whole lot of state here, which is something I don't really
like. (I don't know the design background however.) FWIW I've found this
stuff easier to understand by thinking about it in terms of
requests/responses, and by considering the client-side state only a
"lightweight state", solely existing for implementing the
request/response patterns. In other words, I kind of consider the
client-side state "ad hoc" (whereas the server-side state is
heavy-weight state).

The overlap between nbd_opt_list_meta_context() and nbd_opt_info() is
especially confusing. For the following reasons:

- both functions perform the same listing at the base level (build an
intersection between the client's desired meta context list and the
server's supported meta context list)

- the intersection is consumed differently on the client (the first
function reports results through a callback, the second one requires
subsequent, repeated calls to nbd_can_meta_context()).

- the first function is stateless (considering only the server-side
state "state" here), the second is stateful. While this is well
reflected by the opcode names LIST vs. SET, those opcodes are hidden
from the function names. In particular, the second function says "info"
in the name, which I would not expect to change state on the server.

- NBD_OPT_INFO in particular looks like a useless opcode.
NBD_OPT_SET_META_CONTEXT handles the filtering (intersection) and
mapping to numeric IDs. NBD_OPT_GO transitions the server to the "ready"
state (completes negotiation). So those two look useful. I don't
understand what the precise responsibility is that *only* NBD_OPT_INFO
can cover. The manual at <https://libguestfs.org/nbd_opt_info.3.html>
says that nbd_opt_info() enables export size querying (for example), but
that kind of querying is just as possible without nbd_opt_info(); only
nbd_set_opt_mode() is needed -- see the example code at
<https://libguestfs.org/nbd_set_opt_mode.3.html>.

> 
> The promised upcoming patch will add:
> 
> nbd_set_request_meta_context(false) => Skip the
> NBD_OPT_SET_META_CONTEXT portion in nbd_opt_info()/nbd_opt_go()

This makes sense for nbd_opt_go(), as it will then retain a single
responsibility: transitioning the server to the ready state.

However, with SET_META_CONTEXT skipped, nbd_opt_go() appears to be
reduced to sending the NBD_OPT_INFO opcode -- and that one looks
entirely useless to me (see above).

> 
> nbd_opt_set_meta_context() => Issue NBD_OPT_SET_META_CONTEXT to the
> server using h->request_meta_contexts as its query list and
> h->export_name, and collect the results into both the user's callback
> and h->meta_contexts at the same time.  Stateful - wipes out any ids
> returned by an earlier SET_META_CONTEXT, but the ids returned by the
> server can survive a number of other NBD_OPT_ commands prior to
> NBD_OPT_GO, provided that NBD_OPT_GO uses the same export name as
> NBD_OPT_SET_META_CONTEXT.

Right, so nbd_opt_set_meta_context() is the "first half" extracted from
the current behavior of nbd_opt_info()/nbd_opt_go(). (Again with my
personal confusion that the "second half" of nbd_opt_info(), i.e., the
sending of the NB_OPT_INFO opcode, looks useless.)

> 
> In that manner, it will become possible for manual option haggling to
> set the meta contexts at a different point in the negotiation phase,
> useful when trying to trigger various server corner cases.
> 
> The server wipes meta context state any time it gets
> NBD_OPT_SET_META_CONTEXT or NBD_OPT_GO with a different export name
> than before.  But since we don't pass in export name as a direct
> parameter to either nbd_opt_go() or the upcoming
> nbd_opt_set_meta_context(), we instead need to wipe the state when we
> change h->export_name (that is, at nbd_set_export_name()).  This does
> mean that if a client calls nbd_set_export_name(nbd, "a"),
> nbd_set_meta_context(...), nbd_set_export_name(nbd, "b"),
> nbd_set_export_name(nbd, "a"), nbd_opt_go(nbd), that the server will
> not observe the name change, and will actually support the
> previously-negotiated contexts, but libnbd will have wiped access to
> those, and trigger client-side failure on attempts to call
> nbd_block_status() on the grounds that it doesn't remember any
> negotiated contexts.  But a client actually doing this is unlikely.

So this is the problem with stateful protocols; we need to remember and
recognize the points at which the state goes stale and must be cleared.
I wonder why the NBD protocol was not designed to take as many
parameters as possible per-request (minimizing the state in both client
and server), and only turn the kind of information into actual "state"
that is required by the *bulk* operations (basically the block-level
operations on the device, such as read, write, zero/trim, etc) where the
parameter repetitions would have significant bandwidth and computation
(formatting/parsing) cost. The cost of the negotiation phase looks
trivial in comparison to the cost of the "ready state" work (IIRC the
"ready state"); all the cached information does not seem to buy us much.

> 
>>
>>> it would become actively dangerous: negotiating meta
>>> contexts for one export, then switching to a different export name
>>> before nbd_opt_go, can lead us into calling nbd_cmd_block_status() when
>>> the server is not expecting it.
>>
>> (3) What is nbd_cmd_block_status()? I can only find nbd_block_status().
>> Do you mean the NBD extent callback that is called once by metacontext?
> 
> Typo on my part.  The API nbd_block_status() issues
> NBD_CMD_BLOCK_STATUS to the server.
> 
>>
>> (4) The word "server" seems like a typo; libnbd is used by NBD clients.
> 
> Here, it is intended.  The NBD spec says a client should not issue
> NBD_OPT_BLOCK_STATUS to the server unless it successfully negotiated
> NBD_OPT_SET_META_CONTEXT with the same export name later passed to
> NBD_OPT_GO, without intervening commands that might wipe that state.
> The client code in nbd_block_status() tries to enforce that it won't
> confuse the server by checking whether h->meta_contexts is non-NULL
> before it will issue NBD_CMD_BLOCK_STATUS.  But once we allow manual
> control over setting meta contexts, rather than tightly coupling it
> with NBD_OPT_GO, we need to make sure we wipe h->meta_contexts in at
> least all places the server wipes state (even if we also happen to
> wipe it in more places, as in my "a"/set/"b"/"a"/go example).

OK. The wording of your commit message ("actively dangerous") confused
me, as I didn't expect the client to be able to break the server (for
any definition of "break"). However, if the spec uses the "SHOULD NOT"
term, then preventing that sequence of operations in the library makes
sense.

> 
>>
>>>
>>> While fixing this, code things to check whether we are actually
>>> swapping export names; an idempotent nbd_set_export_name() is not
>>> observable by the server, and therefore does not need to invalidate
>>> what has already been negotiated.
>>>
>>
>> OK.
>>
>>> Conversely, calling nbd_opt_list_meta_context() does not need to wipe
>>> the list - it does not touch h->meta_contexts, but relies on the
>>> user's callback instead.
>>
>> (5) I kind of wish this were a separate patch, but I see the point of
>> keeping it unified, too.
> 
> Maybe I can separate it; the test would be calling nbd_opt_info() to
> set h->meta_contexts, nbd_can_meta_context() to verify that
> "base:allocation" is supported by the server,
> nbd_opt_list_meta_context() to do a query, then nbd_can_meta_context()
> again to see whether "base:allocation" is still remembered or was
> accidentally wiped.

.... Again, anything around nbd_opt_info() keeps confusing me.

What we actually need here is the NBD_OPT_SET_META_CONTEXT. Currently,
we can do that in two ways, with nbd_opt_info() and nbd_opt_go(). The
latter is no good, as it kicks us out of the negotiation phase. The
former I *dislike*, because it does something *in addition* to
NBD_OPT_SET_META_CONTEXT that I don't understand -- namely the
NBD_OPT_INFO opcode.

Therefore, the new function nbd_opt_set_meta_context() would be *just
right* for this step, as it exposes NBD_OPT_SET_META_CONTEXT (and the
population of h->meta_contexts) in isilation. Perhaps introduce the new
function before updating the test case?

> 
>>
>> (6) I don't understand the "but". I see no conflict (or even: relevance)
>> with the user's callback. Listing the meta contexts does not touch
>> "h->meta_contexts", so we should not clear that list, period. I don't
>> understand what specifically we rely on the user's callback *for*, for
>> this particular discussion. If it's important, please elaborate;
>> otherwise I'm just getting confused! :)
> 
> Fair enough.  Basically, nbd_opt_list_meta_context() hands the
> server's answers to the user's callback instead of by modifications to
> h->meta_contexts, so it does not impact future nbd_can_meta_context().
> I'll see if I can reword that a bit.

Upon re-reading the commit message, after your explanation, I think the
"but" is fine, but benefits from a bit of expanding:

    Conversely, calling nbd_opt_list_meta_context() does not need to
    wipe the list - it does not touch h->meta_contexts, but relies on
    the user's callback instead, *for collecting the list of those
    metacontexts that the server acknowledges*.

How does it sound?

Laszlo

> 
>>
>> The code looks OK to my untrained eye, so I'm ready to R-b that.
>>
>> Thanks!
>> Laszlo
>>
>>> ---
>>>  generator/states-newstyle-opt-meta-context.c | 7 +++----
>>>  lib/handle.c                                 | 4 ++++
>>>  2 files changed, 7 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/generator/states-newstyle-opt-meta-context.c b/generator/states-newstyle-opt-meta-context.c
>>> index 30b9617..76f1032 100644
>>> --- a/generator/states-newstyle-opt-meta-context.c
>>> +++ b/generator/states-newstyle-opt-meta-context.c
>>> @@ -1,5 +1,5 @@
>>>  /* nbd client library in userspace: state machine
>>> - * Copyright (C) 2013-2020 Red Hat Inc.
>>> + * Copyright (C) 2013-2022 Red Hat Inc.
>>>   *
>>>   * This library is free software; you can redistribute it and/or
>>>   * modify it under the terms of the GNU Lesser General Public
>>> @@ -28,7 +28,6 @@ STATE_MACHINE {
>>>     * contexts, when doing SET (but an empty LIST is okay).
>>>     */
>>>    assert (h->gflags & LIBNBD_HANDSHAKE_FLAG_FIXED_NEWSTYLE);
>>> -  nbd_internal_reset_size_and_flags (h);
>>>    if (h->opt_current == NBD_OPT_LIST_META_CONTEXT) {
>>>      assert (h->opt_mode);
>>>      assert (h->structured_replies);
>>> @@ -37,6 +36,8 @@ STATE_MACHINE {
>>>    }
>>>    else {
>>>      assert (CALLBACK_IS_NULL (h->opt_cb.fn.context));
>>> +    nbd_internal_reset_size_and_flags (h);
>>> +    assert (h->meta_contexts == NULL);
>>>      opt = NBD_OPT_SET_META_CONTEXT;
>>>      if (!h->structured_replies || h->request_meta_contexts.len == 0) {
>>>        SET_NEXT_STATE (%^OPT_GO.START);
>>> @@ -44,8 +45,6 @@ STATE_MACHINE {
>>>      }
>>>    }
>>>
>>> -  assert (h->meta_contexts == NULL);
>>> -
> 
> This hunk fixes the nbd_opt_list_meta_context() accidentally wiping
> context when it doesn't have to,
> 
>>>    /* Calculate the length of the option request data. */
>>>    len = 4 /* exportname len */ + strlen (h->export_name) + 4 /* nr queries */;
>>>    for (i = 0; i < h->request_meta_contexts.len; ++i)
>>> diff --git a/lib/handle.c b/lib/handle.c
>>> index 8713e18..2aa89b9 100644
>>> --- a/lib/handle.c
>>> +++ b/lib/handle.c
>>> @@ -219,6 +219,10 @@ nbd_unlocked_set_export_name (struct nbd_handle *h, const char *export_name)
>>>      return -1;
>>>    }
>>>
>>> +  if (strcmp (export_name, h->export_name) == 0)
>>> +    return 0;
>>> +
>>> +  nbd_internal_reset_size_and_flags (h);
> 
> and this hunk fixes the missing wipe (the testsuite coverage so far
> only covered this case).
> 
>>>    new_name = strdup (export_name);
>>>    if (!new_name) {
>>>      set_error (errno, "strdup");
>>>
>>
>