Re: [pbs-devel] [PATCH proxmox-backup v2] client: mount: flush output before exiting

[Christian Ebner <c.ebner@proxmox.com>]

On 6/18/24 14:29, Gabriel Goller wrote:
> 
> Hmm I think I know what happens...
> This patch is flaky for me as well, sometimes it works, sometimes it
> doesn't. The problem is that we move the `pw` file descriptor to
> `mount_do`, which means that after it returns, the file_descriptor gets
> dropped.
> 
>      let (pr, pw) = proxmox_sys::pipe()?;
>      let pr: OwnedFd = pr.into(); // until next sys bump
>      let pw: OwnedFd = pw.into();
>      match unsafe { fork() } {
>          Ok(ForkResult::Parent { .. }) => {
>              drop(pw);
>              // Blocks the parent process until we are ready to go in 
> the child
>              let _res = nix::unistd::read(pr.as_raw_fd(), &mut 
> [0]).unwrap();
>              Ok(Value::Null)
>          }
>          Ok(ForkResult::Child) => {
>              drop(pr);
>              nix::unistd::setsid().unwrap();
>              let result = proxmox_async::runtime::main(mount_do(param, 
> Some(pw)));
>                                                                             ^^^
> pw gets dropped here, which means the fd is closed.
> 
>              io::stdout().flush()?;
>              io::stderr().flush()?;
>              result
>          }
>          Err(_) => bail!("failed to daemonize process"),
>      }
> 
> 
> In the parent process we read from the fd—which is then closed—meaning
> that the parent exits at the same time as we do the stderr/stdout flush.
> 
> And actually this patch was stupid to begin with, because we don't even
> print the error in the mound_do function, but later in the api-handler
> :(.

That is actually true, no point in flushing here. So my suggestion for 
the fix was wrong, as I was primed on the error location :(

> 
> I think we need to do something like this:
> 
>      if let Err(e) = nix::unistd::read(pr.as_raw_fd(), &mut [0]) {
>          // if the fd is closed, or some other error happens, wait so
>          // that the child can return the error safely, then exit 
>          nix::sys::wait::wait().unwrap();
>      }
>      Ok(Value::Null)
> 
> 


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