Re: [Libguestfs] libnbd: When are callbacks freed

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.

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?