Re: [Libguestfs] libnbd: When are callbacks freed

> 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.