[Libguestfs] libnbd: When are callbacks freed

[Richard W.M. Jones]

On Thu, Jul 13, 2023 at 04:18:03PM +0000, Tage Johansson wrote:
> 
> On 7/13/2023 5:37 PM, Richard W.M. Jones wrote:
> >On Thu, Jul 13, 2023 at 03:05:30PM +0000, Tage Johansson wrote:
> >>On 7/13/2023 4:36 PM, Eric Blake wrote:
> >>>On Thu, Jul 13, 2023 at 01:37:49PM +0000, Tage Johansson wrote:
> >>>>On 7/13/2023 3:26 PM, Richard W.M. Jones wrote:
> >>>>>On Thu, Jul 13, 2023 at 08:01:09AM -0500, Eric Blake wrote:
> >>>>>>On Thu, Jul 13, 2023 at 07:13:37AM -0500, Eric Blake wrote:
> >>>>>>>>>I have replaced a call to `nbd_opt_info()` with a call to
> >>>>>>>>>`nbd_aio_opt_info()` and passed in a completion callback which just
> >>>>>>>>>calls `exit(EXIT_FAILURE)`. So if the completion callback is called
> >>>>>>>>>the test should fail, which it doesn't, at least not on my machine.
> >>>>>>>>Isn't that OK?  Only .free is required to be called.
> >>>>>>>For the context callback (for opt_set_meta), .callback may be called
> >>>>>>>zero, one, or multiple times, but .free should be called exactly once.
> >>>>>>>But for the completion callback, I agree that the docs state that both
> >>>>>>>.callback and .free should each be called exactly once ("On the other
> >>>>>>>hand, if a completion callback is supplied (only possible with
> >>>>>>>asynchronous commands), it will always be reached exactly once, and
> >>>>>>>the completion callback must not ignore the value pointed to by
> >>>>>>>C<error>."); we are indeed missing the call to .callback.  I'll work
> >>>>>>>on a patch.
> >>>>>>Eww, the bug is bigger than just nbd_aio_opt* not always calling
> >>>>>>completion.callback exactly once.  With just this diff (to be folded
> >>>>>>into the larger patch I'm working on), I'm getting an assertion
> >>>>>>failure that we fail to call completion.callback for
> >>>>>>nbd_aio_pread_structured when calling nbd_close() prior to the command
> >>>>>>running to completion, so I'll have to fix that too.
> >>>>>Just to be clear about this, are we really sure the completion
> >>>>>callback should always be called once?  I'm not clear why that should
> >>>>>be the case.  (The .free callback however should be.)
> >>>>>
> >>>>>For example, if I call a function with bogus invalid parameters so
> >>>>>that it fails very early on (or when the handle is in an incorrect
> >>>>>state), should I expect the completion callback to be called?  I would
> >>>>>expect not.
> >>>>>
> >>>>>Rich.
> >>>>The user needs a way to know if an error occurred. So the completion
> >>>>callback must be called if the asynchronous function did not fail (returned
> >>>>0). If the completion callback should be called, with the error parameter
> >>>>set, even when the asynchronous function immediately failed with a non-zero
> >>>>return value is another question. I see two possibilities: Either the
> >>>>completion callback should always be called. Or it should be called iff the
> >>>>asynchronous function returned 0 (did not fail).
> >>>Good point.
> >>>
> >>>Our documentation currently states (docs/libnbd.pod) that the
> >>>completion callback is ALWAYS called, but this is inconsistent with
> >>>current code - you are correct that at present, the completion
> >>>callback is NOT called if the aio command returns non-zero (easy test:
> >>>call nbd_aio_block_status before nbd_connect*).  I'm game to updating
> >>>that part of the documentation to match existing practice (changing
> >>>our guarantee to the completion callback will be called once iff the
> >>>aio command reported success), since our testsuite wasn't covering it,
> >>>and it is probably an easier fix than munging the generator to call
> >>>completion.callback even for aio failures.  Meanwhile, the .free
> >>>callback MUST be called unconditionally, and I think our testsuite is
> >>>already covering that.
> >>>
> >>>Life-cycle wise, I see the following sequence as being something we
> >>>could usefully rely on (although we don't yet have enough testsuite
> >>>coverage to prove that we already have it).  Note that it is not only
> >>>important how many times things are called, but I would like it if we
> >>>can guarantee the specific ordering between them (neither .free should
> >>>be called until all .callback opportunities are complete finished, to
> >>>avoid any use-after-free issues regardless of which of the two .free
> >>>slots the programmer actually uses):
> >>>
> >>>- if aio command fails:
> >>>mid-command .callback: 0 calls
> >>>completion .callback: 0 calls
> >>>mid-command .free: 1 call
> >>>completion .free: 1 call
> >>>
> >>>- if aio command succeeds:
> >>>mid-command .callback: 0, 1, or multiple times
> >>>completion .callback: 1 call
> >>>mid-command .free: 1 call
> >>>completion .free: 1 call
> >>>
> >>>What I'm not sure of yet (without more code inspection) is whether we
> >>>can sometimes call completion.callback after mid-command.free.
> >>>
> >>>>By the way, if the error parameter is set in the completion callback, how
> >>>>can the user retrieve the error text? Is it possible to call
> >>>>`get_nbd_error(3)` from inside the completion callback?
> >>>You mean nbd_get_error().  We currently document that it is not safe
> >>>to call any nbd_* from within a callback.  Maybe we could relax that
> >>>to carve out exceptions for nbd_get_error/errno as special cases,
> >>>since they only inspect thread-local state and can be used even when
> >>>there is no current nbd_handle.  The problem is that I'm not sure if
> >>>there is a reliable string at that point in time: part of the reason
> >>>the completion callback has an errnum parameter is that the point of
> >>>reporting the completion failure may be in a different thread than
> >>>when the failure was first detected, and we only preserved a numeric
> >>>value in cmd->error rather than also preserving a string.
> >>>
> >>>Put another way, there is no way to guarantee that nbd_get_errno()
> >>>will return the same value as the errnum parameter to the callback;
> >>>and if that is true, then even if calling nbd_get_error() doesn't
> >>>crash or deadlock from within a callback, it is likewise probable that
> >>>any such string returned is not consistent with the errnum parameter
> >>>passed to the callback.
> >>
> >>So is there any safe way to get some description of the error from a
> >>completion callback apart from a non-zero number? It isn't too
> >>helpful to report to the user that the read operation faild with -1.
> >As I recall, from the callback, no.
> >
> >The error is not lost however, it will be returned by the API call
> >itself.  eg. If you're in nbd_aio_opt_list -> callback (error) then
> >nbd_aio_opt_list will return -1 and at that point you can use
> >nbd_get_error to report the error.
> 
> 
> I don't understand. If I call `nbd_aio_opt_list()` with a completion
> callback. `nbd_aio_opt_list()` will return immediately, maybe with a
> successful result. But the command is not complete until
> `nbd_aio_is_connecting()` returns `false`, so the completion
> callback may be invoked with an error after `nbd_aio_opt_list()` has
> returned.

It's returned by some later call, such as nbd_aio_notify_read.

Basically libnbd doesn't create threads, so the only place that
callbacks can be called is when you call into libnbd.  That function
that you call is what will return an error.

> Also, does the value of `err` (passed into a caller) has any meaning
> like a known errno value or something else that can be converted to
> a description? Or is it just an arbitrary non zero integer?

It's a C errno number.  Setting it will change what nbd_get_errno
returns.

In the OCaml bindings we actually provide a function to translate
OCaml <-> C errnos for this purpose:

  val errno_of_unix_error : Unix.error -> int
  (** Return the raw C errno corresponding to a {!Unix.error}.  This
      can be used in callbacks to update the [int ref] parameter. *)

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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