Re: [PATCH vOther2 1/1] qemu-nbd: Restore "qemu-nbd -v --fork" output

[Eric Blake]

On Fri, Aug 25, 2023 at 10:08:38PM +0200, Denis V. Lunev wrote:
> Closing stderr earlier is good for daemonized qemu-nbd under ssh
> earlier, but breaks the case where -v is being used to track what is
> happening in the server, as in iotest 233.

Keeping the iotest output unchanged is a nice effect, even if it
requires a bit more code, so I'm leaning towards taking your patch.

> 
> When we know we are verbose, we should preserve original stderr and
> restore it once the setup stage is done. This commit restores the
> original behavior with -v option. In this case original output
> inside the test is kept intact.
> 
> Reported-by: Kevin Wolf <kwolf@redhat.com>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Eric Blake <eblake@redhat.com>
> CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> CC: Hanna Reitz <hreitz@redhat.com>
> CC: Mike Maslenkin <mike.maslenkin@gmail.com>
> Fixes: 5c56dd27a2 ("qemu-nbd: fix regression with qemu-nbd --fork run over 
> ssh")
> ---
> Changes from v1:
> * fixed compilation with undefined HAVE_NBD_DEVICE, thanks to Mike Maslenkin

> @@ -323,11 +324,14 @@ static void *nbd_client_thread(void *arg)
>                  opts->device, srcpath);
>      } else {
>          /* Close stderr so that the qemu-nbd process exits.  */
> -        if (dup2(STDOUT_FILENO, STDERR_FILENO) < 0) {
> +        if (dup2(opts->stderr, STDERR_FILENO) < 0) {
>              error_report("Could not set stderr to /dev/null: %s",

Both my patch and yours have a slight inaccuracy here: when -v is in
use, failure to dup2() is not a failure to set stderr to /dev/null.
Maybe we can reword it as "Could not release pipe to parent: %s", as
that is the other intentional side effect of the dup2()?

>                           strerror(errno));
>              exit(EXIT_FAILURE);
>          }
> +        if (opts->stderr != STDOUT_FILENO) {
> +            close(opts->stderr);

As long as we are checking dup2() for (unlikely) failure, we should
probably be doing the same for close().

> +        }
>      }
>  
>      if (nbd_client(fd) < 0) {
> @@ -589,9 +593,9 @@ int main(int argc, char **argv)
>      const char *pid_file_name = NULL;
>      const char *selinux_label = NULL;
>      BlockExportOptions *export_opts;
> -#if HAVE_NBD_DEVICE
> -    struct NbdClientOpts opts;
> -#endif
> +    struct NbdClientOpts opts = {
> +        .stderr = STDOUT_FILENO,
> +    };
>  
>  #ifdef CONFIG_POSIX
>      os_setup_early_signal_handling();
> @@ -944,6 +948,15 @@ int main(int argc, char **argv)
>  
>              close(stderr_fd[0]);
>  
> +            /* Remember parent's stderr if we will be restoring it. */
> +            if (verbose /* fork_process is set */) {
> +                opts.stderr = dup(STDERR_FILENO);
> +                if (opts.stderr < 0) {
> +                    error_report("Could not dup stdedd: %s", 
> strerror(errno));

s/stdedd/stderr/

> +                    exit(EXIT_FAILURE);
> +                }
> +            }
> +
>              ret = qemu_daemon(1, 0);
>              saved_errno = errno;    /* dup2 will overwrite error below */
>  
> @@ -1152,6 +1165,7 @@ int main(int argc, char **argv)
>              .device = device,
>              .fork_process = fork_process,
>              .verbose = verbose,
> +            .stderr = STDOUT_FILENO,

Huh. This looks redundant to pre-initializing .stderr above; but since
it is using a struct assignment, we do have to provide it again to
avoid the compiler setting unmentioned fields to zero-initialization.

If we are going to unconditionally have opts in scope, even when not
passing it to pthread_create(), maybe we could just directly assign to
opts.device and drop the local variable device (and so forth), instead
of first storing into device only to later copy it to opts.device.
But that would make this patch bigger, so I'm not sure it is worth it.

>          };
>  
>          ret = pthread_create(&client_thread, NULL, nbd_client_thread, &opts);
> @@ -1180,11 +1194,14 @@ int main(int argc, char **argv)
>      }
>  
>      if (fork_process) {
> -        if (dup2(STDOUT_FILENO, STDERR_FILENO) < 0) {
> +        if (dup2(opts.stderr, STDERR_FILENO) < 0) {
>              error_report("Could not set stderr to /dev/null: %s",
>                           strerror(errno));

Another spot where the error message is not entirely accurate,

>              exit(EXIT_FAILURE);
>          }
> +        if (opts.stderr != STDOUT_FILENO) {
> +            close(opts.stderr);

and another spot where we should be checking for close() failure.

> +        }
>      }
>  
>      state = RUNNING;
> -- 
> 2.34.1
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org