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.

man/rules/meson.build

@@ -289,6 +289,7 @@ manpages = [
    'sd_bus_release_name_async',
    'sd_bus_request_name_async'],
   ''],
+ ['sd_bus_set_close_on_exit', '3', ['sd_bus_get_close_on_exit'], ''],
  ['sd_bus_set_connected_signal', '3', ['sd_bus_get_connected_signal'], ''],
  ['sd_bus_set_description',
   '3',

man/sd-bus.xml

@@ -84,6 +84,7 @@
 <citerefentry><refentrytitle>sd_bus_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 <citerefentry><refentrytitle>sd_bus_set_sender</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 <citerefentry><refentrytitle>sd_bus_set_watch_bind</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+<citerefentry><refentrytitle>sd_bus_set_close_on_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 <citerefentry><refentrytitle>sd_bus_slot_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 <citerefentry><refentrytitle>sd_bus_slot_set_destroy_callback</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

man/sd_bus_set_close_on_exit.xml

@@ -0,0 +1,105 @@
+<?xml version='1.0'?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+  SPDX-License-Identifier: LGPL-2.1+
+-->
+
+<refentry id="sd_bus_set_close_on_exit"
+          xmlns:xi="http://www.w3.org/2001/XInclude">
+
+  <refentryinfo>
+    <title>sd_bus_set_close_on_exit</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>sd_bus_set_close_on_exit</refentrytitle>
+    <manvolnum>3</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>sd_bus_set_close_on_exit</refname>
+    <refname>sd_bus_get_close_on_exit</refname>
+
+    <refpurpose>Control whether to close the bus connection during the event loop exit phase</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <funcsynopsis>
+      <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_set_close_on_exit</function></funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+        <paramdef>int <parameter>b</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_get_close_on_exit</function></funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+      </funcprototype>
+
+    </funcsynopsis>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><function>sd_bus_set_close_on_exit()</function> may be used to enable or disable whether the bus connection
+    is automatically flushed (as in
+    <citerefentry><refentrytitle>sd_bus_flush</refentrytitle><manvolnum>3</manvolnum></citerefentry>) and closed (as in
+    <citerefentry><refentrytitle>sd_bus_close</refentrytitle><manvolnum>3</manvolnum></citerefentry>) during the exit
+    phase of the event loop. This logic only applies to bus connections that are attached to an
+    <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry> event loop, see
+    <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>. By default
+    this mechanism is enabled and makes sure that any pending messages that have not been written to the bus connection
+    are written out when the event loop is shutting down. In some cases this behaviour is not desirable, for example
+    when the bus connection shall remain usable until after the event loop exited. If <parameter>b</parameter> is
+    true, the feature is enabled (which is the default), otherwise disabled.</para>
+
+    <para><function>sd_bus_get_close_on_exit()</function> may be used to query the current setting of this feature. It
+    returns zero when the feature is disabled, and positive if enabled.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Return Value</title>
+
+    <para>On success, <function>sd_bus_set_close_on_exit()</function> returns 0 or a positive integer. On failure, it returns a negative errno-style
+    error code.</para>
+
+    <para><function>sd_bus_get_close_on_exit()</function> returns 0 if the feature is currently turned off or a
+    positive integer if it is on. On failure, it returns a negative errno-style error code.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Errors</title>
+
+    <para>Returned errors may indicate the following problems:</para>
+
+    <variablelist>
+      <varlistentry>
+        <term><constant>-ECHILD</constant></term>
+
+        <listitem><para>The bus connection has been created in a different process.</para></listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
+  <xi:include href="libsystemd-pkgconfig.xml" />
+
+  <refsect1>
+    <title>See Also</title>
+
+    <para>
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_flush</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_event_add_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>

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;
+}

src/systemd/sd-bus.h

@@ -154,6 +154,8 @@ int sd_bus_set_allow_interactive_authorization(sd_bus *bus, int b);
 int sd_bus_get_allow_interactive_authorization(sd_bus *bus);
 int sd_bus_set_exit_on_disconnect(sd_bus *bus, int b);
 int sd_bus_get_exit_on_disconnect(sd_bus *bus);
+int sd_bus_set_close_on_exit(sd_bus *bus, int b);
+int sd_bus_get_close_on_exit(sd_bus *bus);
 int sd_bus_set_watch_bind(sd_bus *bus, int b);
 int sd_bus_get_watch_bind(sd_bus *bus);
 int sd_bus_set_connected_signal(sd_bus *bus, int b);