[Libguestfs] libnbd: When are callbacks freed

Thu Jul 13 16:50:41 UTC 2023

On Thu, Jul 13, 2023 at 05:46:56PM +0100, Richard W.M. Jones wrote:
> 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. *)

ocaml/tests/test_505_aio_pread_structured_callback.ml has an example
of how this is used.  The errno is translated twice, once from
OCaml -> C inside the callback.  And a second time from C -> OCaml
when the error is returned by the NBD.* call.

Rich.

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