qemu-nbd: Add --fork option

Using the --fork option, one can make qemu-nbd fork the worker process.
The original process will exit on error of the worker or once the worker
enters the main loop.

Suggested-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-nbd.c

@@ -48,6 +48,7 @@
 #define QEMU_NBD_OPT_OBJECT        260
 #define QEMU_NBD_OPT_TLSCREDS      261
 #define QEMU_NBD_OPT_IMAGE_OPTS    262
+#define QEMU_NBD_OPT_FORK          263
 
 #define MBR_SIZE 512
 
@@ -92,6 +93,8 @@ static void usage(const char *name)
 "                            passwords and/or encryption keys\n"
 "  -T, --trace [[enable=]<pattern>][,events=<file>][,file=<file>]\n"
 "                            specify tracing options\n"
+"  --fork                    fork off the server process and exit the parent\n"
+"                            once the server is running\n"
 #ifdef __linux__
 "Kernel NBD client support:\n"
 "  -c, --connect=DEV         connect FILE to the local NBD device DEV\n"
@@ -503,6 +506,7 @@ int main(int argc, char **argv)
         { "tls-creds", required_argument, NULL, QEMU_NBD_OPT_TLSCREDS },
         { "image-opts", no_argument, NULL, QEMU_NBD_OPT_IMAGE_OPTS },
         { "trace", required_argument, NULL, 'T' },
+        { "fork", no_argument, NULL, QEMU_NBD_OPT_FORK },
         { NULL, 0, NULL, 0 }
     };
     int ch;
@@ -524,6 +528,8 @@ int main(int argc, char **argv)
     bool imageOpts = false;
     bool writethrough = true;
     char *trace_file = NULL;
+    bool fork_process = false;
+    int old_stderr = -1;
 
     /* The client thread uses SIGTERM to interrupt the server.  A signal
      * handler ensures that "qemu-nbd -v -c" exits with a nice status code.
@@ -715,6 +721,9 @@ int main(int argc, char **argv)
             g_free(trace_file);
             trace_file = trace_opt_parse(optarg);
             break;
+        case QEMU_NBD_OPT_FORK:
+            fork_process = true;
+            break;
         }
     }
 
@@ -774,7 +783,7 @@ int main(int argc, char **argv)
         return 0;
     }
 
-    if (device && !verbose) {
+    if ((device && !verbose) || fork_process) {
         int stderr_fd[2];
         pid_t pid;
         int ret;
@@ -797,6 +806,7 @@ int main(int argc, char **argv)
             ret = qemu_daemon(1, 0);
 
             /* Temporarily redirect stderr to the parent's pipe...  */
+            old_stderr = dup(STDERR_FILENO);
             dup2(stderr_fd[1], STDERR_FILENO);
             if (ret < 0) {
                 error_report("Failed to daemonize: %s", strerror(errno));
@@ -960,6 +970,11 @@ int main(int argc, char **argv)
         exit(EXIT_FAILURE);
     }
 
+    if (fork_process) {
+        dup2(old_stderr, STDERR_FILENO);
+        close(old_stderr);
+    }
+
     state = RUNNING;
     do {
         main_loop_wait(false);

qemu-nbd.texi

@@ -86,6 +86,8 @@ the new style NBD protocol negotiation
 Enable mandatory TLS encryption for the server by setting the ID
 of the TLS credentials object previously created with the --object
 option.
+@item --fork
+Fork off the server process and exit the parent once the server is running.
 @item -v, --verbose
 Display extra debugging information
 @item -h, --help