Re: [Libguestfs] libnbd: When are callbacks freed

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.

Rich.