qemu-nbd: fix regression with qemu-nbd --fork run over ssh

Commit e6df58a
    Author: Hanna Reitz <hreitz@redhat.com>
    Date:   Wed May 8 23:18:18 2019 +0200
    qemu-nbd: Do not close stderr

has introduced an interesting regression. Original behavior of
    ssh somehost qemu-nbd /home/den/tmp/file -f raw --fork
was the following:
 * qemu-nbd was started as a daemon
 * the command execution is done and ssh exited with success

The patch has changed this behavior and 'ssh' command now hangs forever.

According to the normal specification of the daemon() call, we should
endup with STDERR pointing to /dev/null. That should be done at the
very end of the successful startup sequence when the pipe to the
bootstrap process (used for diagnostics) is no longer needed.

This could be achived in the same way as done for 'qemu-nbd -c' case.
That was commit 0eaf453, also fixing up e6df58a. STDOUT copying to
STDERR does the trick.

This also leads to proper 'ssh' connection closing which fixes my
original problem.

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: <qemu-stable@nongnu.org>
Message-ID: <20230717145544.194786-3-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 5c56dd2)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

qemu-nbd.c

@@ -274,6 +274,7 @@ static void *show_parts(void *arg)
 
 struct NbdClientOpts {
     char *device;
+    bool fork_process;
 };
 
 static void *nbd_client_thread(void *arg)
@@ -317,7 +318,7 @@ static void *nbd_client_thread(void *arg)
     /* update partition table */
     pthread_create(&show_parts_thread, NULL, show_parts, opts->device);
 
-    if (verbose) {
+    if (verbose && !opts->fork_process) {
         fprintf(stderr, "NBD device %s is now connected to %s\n",
                 opts->device, srcpath);
     } else {
@@ -579,7 +580,6 @@ int main(int argc, char **argv)
     bool writethrough = false; /* Client will flush as needed. */
     bool fork_process = false;
     bool list = false;
-    int old_stderr = -1;
     unsigned socket_activation;
     const char *pid_file_name = NULL;
     const char *selinux_label = NULL;
@@ -934,11 +934,6 @@ int main(int argc, char **argv)
         } else if (pid == 0) {
             close(stderr_fd[0]);
 
-            /* Remember parent's stderr if we will be restoring it. */
-            if (fork_process) {
-                old_stderr = dup(STDERR_FILENO);
-            }
-
             ret = qemu_daemon(1, 0);
 
             /* Temporarily redirect stderr to the parent's pipe...  */
@@ -1127,6 +1122,7 @@ int main(int argc, char **argv)
         int ret;
         struct NbdClientOpts opts = {
             .device = device,
+            .fork_process = fork_process,
         };
 
         ret = pthread_create(&client_thread, NULL, nbd_client_thread, &opts);
@@ -1155,8 +1151,7 @@ int main(int argc, char **argv)
     }
 
     if (fork_process) {
-        dup2(old_stderr, STDERR_FILENO);
-        close(old_stderr);
+        dup2(STDOUT_FILENO, STDERR_FILENO);
     }
 
     state = RUNNING;