Re: [PATCH v2 3/4] qsd: Add --daemonize

[Eric Blake <eblake@redhat.com>]

On Thu, Mar 03, 2022 at 05:48:13PM +0100, Hanna Reitz wrote:
> To implement this, we reuse the existing daemonizing functions from the
> system emulator, which mainly do the following:
> - Fork off a child process, and set up a pipe between parent and child
> - The parent process waits until the child sends a status byte over the
>   pipe (0 means that the child was set up successfully; anything else
>   (including errors or EOF) means that the child was not set up
>   successfully), and then exits with an appropriate exit status
> - The child process enters a new session (forking off again), changes
>   the umask, and will ignore terminal signals from then on
> - Once set-up is complete, the child will chdir to /, redirect all
>   standard I/O streams to /dev/null, and tell the parent that set-up has
>   been completed successfully
> 
> In contrast to qemu-nbd's --fork implementation, during the set up
> phase, error messages are not piped through the parent process.
> qemu-nbd mainly does this to detect errors, though (while os_daemonize()
> has the child explicitly signal success after set up); because we do not
> redirect stderr after forking, error messages continue to appear on
> whatever the parent's stderr was (until set up is complete).
> 
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
>  docs/tools/qemu-storage-daemon.rst   |  7 +++++++
>  storage-daemon/qemu-storage-daemon.c | 15 +++++++++++++++
>  2 files changed, 22 insertions(+)
>

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org