qemu-nbd: properly report error on error in dup2() after qemu_daemon()

We are trying to temporarily redirect stderr of daemonized process to
a pipe to report a error and get failed. In that case we could not
use error_report() helper, but should write the message directly into
the problematic pipe.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20230717145544.194786-4-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: rearrange patch series, fix typo]
Signed-off-by: Eric Blake <eblake@redhat.com>

qemu-nbd.c

@@ -940,7 +940,20 @@ int main(int argc, char **argv)
             saved_errno = errno;    /* dup2 will overwrite error below */
 
             /* Temporarily redirect stderr to the parent's pipe...  */
-            dup2(stderr_fd[1], STDERR_FILENO);
+            if (dup2(stderr_fd[1], STDERR_FILENO) < 0) {
+                char str[256];
+                snprintf(str, sizeof(str),
+                         "%s: Failed to link stderr to the pipe: %s\n",
+                         g_get_prgname(), strerror(errno));
+                /*
+                 * We are unable to use error_report() here as we need to get
+                 * stderr pointed to the parent's pipe. Write to that pipe
+                 * manually.
+                 */
+                ret = write(stderr_fd[1], str, strlen(str));
+                exit(EXIT_FAILURE);
+            }
+
             if (ret < 0) {
                 error_report("Failed to daemonize: %s", strerror(saved_errno));
                 exit(EXIT_FAILURE);