Re: [Libguestfs] [PATCH nbdkit v2] ssh: Allow the remote file to be created

On Sun, Apr 17, 2022 at 12:08:36PM +0200, Laszlo Ersek wrote: > On 04/17/22 11:09, Richard W.M. Jones wrote: >> On Sun, Apr 17, 2022 at 09:32:03AM +0200, Laszlo Ersek wrote: >>> Two comments: >>> >>> (1) If the file already exists, then O_CREAT will have no effect; >>> however, we'll still resize the file. I think that's not ideal. >>> >>> I think the usual approach for this is: >>> >>> (a) attempt O_CREAT | O_EXCL; if it succeeds, perform the truncate as well >>> >>> (b) if O_CREAT | O_EXCL fails, then just attempt the open without >>> *either* flag. If that succeeds, do not perform the truncate. >> >> I wonder here if what we feel about: >> >> nbdkit ssh ... create=true create-size=100M >> >> that ends up working but the size isn't 100M. >> >> Should we fail if the remote exists already? Maybe this is something >> we should let the user choose. >> >> [Also I wonder what qemu blockdev-create does - need to investigate.] > > "create-mode" is ignored if the file exists; ignoring "create-size" in > that case would be consistent. "Use this image if it exists; if it does > not, create it with these permissons and this size". This is one way it could work, but from a usability point of view, how does the nbdkit user know if the remote file exists or not? With the current implementation we leave an zero-sized file lying around if setstat fails. We should fix that bug, but I doubt an implementation can be 100% reliable (eg: if sftp_setstat fails, is the connection dead? will sftp_unlink work?) Ignoring create-size if the file exists could result in a zero-sized file that's impossible to recover from except by manual intervention. I don't think there's a good answer here. Possibly we want to let the user decide how they want it to work ...