sd-bus: make "close+flush-on-exit" optional when using sd-event with sd-bus

This adds a new pair of API calls sd_bus_set_close_on_exit() and
sd_bus_get_close_on_exit(). They control whether an sd_bus object
attached to a an sd-event loop shall automatically be flushed/closed
when the event loop goes down. Usually that's a good thing, except for
very few cases where the bus connection is longer living than the event
loop it is attached on. Specifically, this is the case for nspawn, where
we run the event loop only while the container is up, but afterwards
still want to be able to use the bus connection.

src/libsystemd/libsystemd.sym

@@ -579,6 +579,9 @@ global:
 
         sd_bus_error_move;
 
+        sd_bus_set_close_on_exit;
+        sd_bus_get_close_on_exit;
+
         sd_device_ref;
         sd_device_unref;
 

src/libsystemd/sd-bus/bus-internal.h

@@ -211,6 +211,7 @@ struct sd_bus {
         bool accept_fd:1;
         bool attach_timestamp:1;
         bool connected_signal:1;
+        bool close_on_exit:1;
 
         int use_memfd;
 

src/libsystemd/sd-bus/sd-bus.c

@@ -232,18 +232,22 @@ _public_ int sd_bus_new(sd_bus **ret) {
 
         assert_return(ret, -EINVAL);
 
-        b = new0(sd_bus, 1);
+        b = new(sd_bus, 1);
         if (!b)
                 return -ENOMEM;
 
-        b->n_ref = REFCNT_INIT;
-        b->input_fd = b->output_fd = -1;
-        b->inotify_fd = -1;
-        b->message_version = 1;
-        b->creds_mask |= SD_BUS_CREDS_WELL_KNOWN_NAMES|SD_BUS_CREDS_UNIQUE_NAME;
-        b->accept_fd = true;
-        b->original_pid = getpid_cached();
-        b->n_groups = (size_t) -1;
+        *b = (sd_bus) {
+                .n_ref = REFCNT_INIT,
+                .input_fd = -1,
+                .output_fd = -1,
+                .inotify_fd = -1,
+                .message_version = 1,
+                .creds_mask = SD_BUS_CREDS_WELL_KNOWN_NAMES|SD_BUS_CREDS_UNIQUE_NAME,
+                .accept_fd = true,
+                .original_pid = getpid_cached(),
+                .n_groups = (size_t) -1,
+                .close_on_exit = true,
+        };
 
         assert_se(pthread_mutex_init(&b->memfd_cache_mutex, NULL) == 0);
 
@@ -3409,8 +3413,10 @@ static int quit_callback(sd_event_source *event, void *userdata) {
 
         assert(event);
 
-        sd_bus_flush(bus);
-        sd_bus_close(bus);
+        if (bus->close_on_exit) {
+                sd_bus_flush(bus);
+                sd_bus_close(bus);
+        }
 
         return 1;
 }
@@ -4135,3 +4141,18 @@ _public_ int sd_bus_get_method_call_timeout(sd_bus *bus, uint64_t *ret) {
         *ret = bus->method_call_timeout = BUS_DEFAULT_TIMEOUT;
         return 0;
 }
+
+_public_ int sd_bus_set_close_on_exit(sd_bus *bus, int b) {
+        assert_return(bus, -EINVAL);
+        assert_return(bus = bus_resolve(bus), -ENOPKG);
+
+        bus->close_on_exit = b;
+        return 0;
+}
+
+_public_ int sd_bus_get_close_on_exit(sd_bus *bus) {
+        assert_return(bus, -EINVAL);
+        assert_return(bus = bus_resolve(bus), -ENOPKG);
+
+        return bus->close_on_exit;
+}