[PATCH] qemu-storage-daemon: add --pidfile option

[Stefan Hajnoczi <stefanha@redhat.com>]

Daemons often have a --pidfile option where the pid is written to a file
so that scripts can stop the daemon by sending a signal.

The pid file also acts as a lock to prevent multiple instances of the
daemon from launching for a given pid file.

QEMU, qemu-nbd, qemu-ga, virtiofsd, and qemu-pr-helper all support the
--pidfile option. Add it to qemu-storage-daemon too.

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 docs/tools/qemu-storage-daemon.rst   | 10 ++++++++++
 storage-daemon/qemu-storage-daemon.c | 29 ++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/docs/tools/qemu-storage-daemon.rst b/docs/tools/qemu-storage-daemon.rst
index f63627eaf6..8f4ab16ffc 100644
--- a/docs/tools/qemu-storage-daemon.rst
+++ b/docs/tools/qemu-storage-daemon.rst
@@ -118,6 +118,16 @@ Standard options:
   List object properties with ``<type>,help``. See the :manpage:`qemu(1)`
   manual page for a description of the object properties.
 
+.. option:: --pidfile PATH
+
+  is the path to a file where the daemon writes its pid. This allows scripts to
+  stop the daemon by sending a signal::
+
+    $ kill -SIGTERM $(<path/to/qsd.pid)
+
+  A file lock is applied to the file so only one instance of the daemon can run
+  with a given pid file path. The daemon unlinks its pid file when terminating.
+
 Examples
 --------
 Launch the daemon with QMP monitor socket ``qmp.sock`` so clients can execute
diff --git a/storage-daemon/qemu-storage-daemon.c b/storage-daemon/qemu-storage-daemon.c
index 9021a46b3a..011ae49ac3 100644
--- a/storage-daemon/qemu-storage-daemon.c
+++ b/storage-daemon/qemu-storage-daemon.c
@@ -59,6 +59,7 @@
 #include "sysemu/runstate.h"
 #include "trace/control.h"
 
+static const char *pid_file;
 static volatile bool exit_requested = false;
 
 void qemu_system_killed(int signal, pid_t pid)
@@ -126,6 +127,7 @@ enum {
     OPTION_MONITOR,
     OPTION_NBD_SERVER,
     OPTION_OBJECT,
+    OPTION_PIDFILE,
 };
 
 extern QemuOptsList qemu_chardev_opts;
@@ -164,6 +166,7 @@ static void process_options(int argc, char *argv[])
         {"monitor", required_argument, NULL, OPTION_MONITOR},
         {"nbd-server", required_argument, NULL, OPTION_NBD_SERVER},
         {"object", required_argument, NULL, OPTION_OBJECT},
+        {"pidfile", required_argument, NULL, OPTION_PIDFILE},
         {"trace", required_argument, NULL, 'T'},
         {"version", no_argument, NULL, 'V'},
         {0, 0, 0, 0}
@@ -275,6 +278,9 @@ static void process_options(int argc, char *argv[])
                 qobject_unref(args);
                 break;
             }
+        case OPTION_PIDFILE:
+            pid_file = optarg;
+            break;
         default:
             g_assert_not_reached();
         }
@@ -285,6 +291,27 @@ static void process_options(int argc, char *argv[])
     }
 }
 
+static void pid_file_cleanup(void)
+{
+    unlink(pid_file);
+}
+
+static void pid_file_init(void)
+{
+    Error *err = NULL;
+
+    if (!pid_file) {
+        return;
+    }
+
+    if (!qemu_write_pidfile(pid_file, &err)) {
+        error_reportf_err(err, "cannot create PID file: ");
+        exit(EXIT_FAILURE);
+    }
+
+    atexit(pid_file_cleanup);
+}
+
 int main(int argc, char *argv[])
 {
 #ifdef CONFIG_POSIX
@@ -312,6 +339,8 @@ int main(int argc, char *argv[])
     qemu_init_main_loop(&error_fatal);
     process_options(argc, argv);
 
+    pid_file_init();
+
     while (!exit_requested) {
         main_loop_wait(false);
     }
-- 
2.29.2