qemu-nbd: Add --pid-file option

--fork is a bit boring if there is no way to get the child's PID.  This
option helps.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20190508211820.17851-2-mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

qemu-nbd.c

@@ -61,6 +61,7 @@
 #define QEMU_NBD_OPT_IMAGE_OPTS    262
 #define QEMU_NBD_OPT_FORK          263
 #define QEMU_NBD_OPT_TLSAUTHZ      264
+#define QEMU_NBD_OPT_PID_FILE      265
 
 #define MBR_SIZE 512
 
@@ -113,6 +114,7 @@ static void usage(const char *name)
 "                            specify tracing options\n"
 "  --fork                    fork off the server process and exit the parent\n"
 "                            once the server is running\n"
+"  --pid-file=PATH           store the server's process ID in the given file\n"
 #if HAVE_NBD_DEVICE
 "\n"
 "Kernel NBD client support:\n"
@@ -641,6 +643,7 @@ int main(int argc, char **argv)
         { "image-opts", no_argument, NULL, QEMU_NBD_OPT_IMAGE_OPTS },
         { "trace", required_argument, NULL, 'T' },
         { "fork", no_argument, NULL, QEMU_NBD_OPT_FORK },
+        { "pid-file", required_argument, NULL, QEMU_NBD_OPT_PID_FILE },
         { NULL, 0, NULL, 0 }
     };
     int ch;
@@ -667,6 +670,7 @@ int main(int argc, char **argv)
     bool list = false;
     int old_stderr = -1;
     unsigned socket_activation;
+    const char *pid_file_name = NULL;
 
     /* The client thread uses SIGTERM to interrupt the server.  A signal
      * handler ensures that "qemu-nbd -v -c" exits with a nice status code.
@@ -866,6 +870,9 @@ int main(int argc, char **argv)
         case 'L':
             list = true;
             break;
+        case QEMU_NBD_OPT_PID_FILE:
+            pid_file_name = optarg;
+            break;
         }
     }
 
@@ -1186,6 +1193,10 @@ int main(int argc, char **argv)
 
     nbd_update_server_watch();
 
+    if (pid_file_name) {
+        qemu_write_pidfile(pid_file_name, &error_fatal);
+    }
+
     /* now when the initialization is (almost) complete, chdir("/")
      * to free any busy filesystems */
     if (chdir("/") < 0) {

qemu-nbd.texi

@@ -117,6 +117,8 @@ option; or provide the credentials needed for connecting as a client
 in list mode.
 @item --fork
 Fork off the server process and exit the parent once the server is running.
+@item --pid-file=PATH
+Store the server's process ID in the given file.
 @item --tls-authz=ID
 Specify the ID of a qauthz object previously created with the
 --object option. This will be used to authorize connecting users