man: document the new sd_bus_slot_set_floating() call

Also extend the memory management description of sd-bus highlighting the
effect of "floating" slot objects a bit.

4 files changed, 144 insertions, 17 deletions

diff --git a/man/rules/meson.build b/man/rules/meson.build
index d3ed9f8a6a..0fb7978c21 100644
--- a/man/rules/meson.build
+++ b/man/rules/meson.build
@@ -231,6 +231,7 @@ manpages = [
  ['sd_bus_set_connected_signal', '3', ['sd_bus_get_connected_signal'], ''],
  ['sd_bus_set_sender', '3', ['sd_bus_get_sender'], ''],
  ['sd_bus_set_watch_bind', '3', ['sd_bus_get_watch_bind'], ''],
+ ['sd_bus_slot_set_floating', '3', ['sd_bus_slot_get_floating'], ''],
  ['sd_bus_track_add_name',
   '3',
   ['sd_bus_track_add_sender',
diff --git a/man/sd_bus_add_match.xml b/man/sd_bus_add_match.xml
index 07a7b0ea17..13f2b06af0 100644
--- a/man/sd_bus_add_match.xml
+++ b/man/sd_bus_add_match.xml
@@ -132,7 +132,9 @@
     at a later time with
     <citerefentry><refentrytitle>sd_bus_slot_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>. If specified
     as <constant>NULL</constant> the lifetime of the match is bound to the lifetime of the bus object itself, and the
-    match cannot be removed independently.</para>
+    match is generally not removed independently. See
+    <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry> for
+    details.</para>
 
     <para>The message <parameter>m</parameter> passed to the callback is only borrowed, that is, the callback should
     not call <citerefentry><refentrytitle>sd_bus_message_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
@@ -175,7 +177,8 @@
       <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>sd_bus_slot_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>sd_bus_message_ref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
-      <citerefentry><refentrytitle>sd_bus_set_bus_client</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+      <citerefentry><refentrytitle>sd_bus_set_bus_client</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>
     </para>
   </refsect1>
 
diff --git a/man/sd_bus_default.xml b/man/sd_bus_default.xml
index f36272e854..da06fb9cbf 100644
--- a/man/sd_bus_default.xml
+++ b/man/sd_bus_default.xml
@@ -206,21 +206,26 @@
     <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
     to drop the reference.</para>
 
-    <para>Queued but unwritten/unread messages also keep a reference
-    to their bus connection object. For this reason, even if an
-    application dropped all references to a bus connection, it might
-    not get destroyed right away. Until all incoming queued
-    messages are read, and until all outgoing unwritten messages are
-    written, the bus object will stay
-    alive. <function>sd_bus_flush()</function> may be used to write
-    all outgoing queued messages so they drop their references. To
-    flush the unread incoming messages, use
-    <function>sd_bus_close()</function>, which will also close the bus
-    connection. When using the default bus logic, it is a good idea to
-    first invoke <function>sd_bus_flush()</function> followed by
-    <function>sd_bus_close()</function> when a thread or process
-    terminates, and thus its bus connection object should be
-    freed.</para>
+    <para>Queued but unwritten/unread messages keep a reference to their bus connection object. For this reason, even
+    if an application dropped all references to a bus connection, it might not get destroyed right away. Until all
+    incoming queued messages are read, and until all outgoing unwritten messages are written, the bus object will stay
+    alive. <function>sd_bus_flush()</function> may be used to write all outgoing queued messages so they drop their
+    references. To flush the unread incoming messages, use <function>sd_bus_close()</function>, which will also close
+    the bus connection. When using the default bus logic, it is a good idea to first invoke
+    <function>sd_bus_flush()</function> followed by <function>sd_bus_close()</function> when a thread or process
+    terminates, and thus its bus connection object should be freed.</para>
+
+    <para>Normally, slot objects (as created by
+    <citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry> and similar
+    calls) keep a reference to their bus connection object, too. Thus, as long as a bus slot object remains referenced
+    its bus object will remain allocated too. Optionally, bus slot objects may be placed in "floating" mode. When in
+    floating mode the life cycle of the bus slot object is bound to the bus object, i.e. when the bus object is freed
+    the bus slot object is automatically unreferenced too. The floating state of a slot object may be controlled
+    explicitly with
+    <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+    though usually floating bus slot objects are created by passing <constant>NULL</constant> as the
+    <parameter>slot</parameter> parameter of <function>sd_bus_add_match()</function> and related calls, thus indicating
+    that the caller is not directly interested in referencing and managing the bus slot object.</para>
 
     <para>The life cycle of the default bus connection should be the
     responsibility of the code that creates/owns the thread the
diff --git a/man/sd_bus_slot_set_floating.xml b/man/sd_bus_slot_set_floating.xml
new file mode 100644
index 0000000000..2ac5090193
--- /dev/null
+++ b/man/sd_bus_slot_set_floating.xml
@@ -0,0 +1,118 @@
+<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
+<!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_slot_set_floating">
+
+  <refentryinfo>
+    <title>sd_bus_slot_set_floating</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>sd_bus_slot_set_floating</refentrytitle>
+    <manvolnum>3</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>sd_bus_slot_set_floating</refname>
+    <refname>sd_bus_slot_get_floating</refname>
+
+    <refpurpose>Control whether a bus slot object is "floating".</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <funcsynopsis>
+      <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_slot_set_floating</function></funcdef>
+        <paramdef>sd_bus_slot *<parameter>bus</parameter></paramdef>
+        <paramdef>int <parameter>b</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_slot_get_floating</function></funcdef>
+        <paramdef>sd_bus_slot *<parameter>slot</parameter></paramdef>
+      </funcprototype>
+
+    </funcsynopsis>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><function>sd_bus_slot_set_floating()</function> controls whether the specified bus slot object
+    <parameter>slot</parameter> shall be "floating" or not. A floating bus slot object's lifetime is bound to the
+    lifetime of the bus object it is associated with, meaning that it remains allocated as long as the bus object
+    itself and is freed automatically when the bus object is freed. Regular (i.e. non-floating) bus slot objects keep
+    the bus referenced, hence the bus object remains allocated at least as long as there remains at least one
+    referenced bus slot object around. The floating state hence controls the direction of referencing between the bus
+    object and the bus slot objects: if floating the bus pins the the bus slot, and otherwise the bus slot pins the bus
+    objects. Use <function>sd_bus_slot_set_floating()</function> to switch between both modes: if the
+    <parameter>b</parameter> parameter is zero, the slot object is considered floating, otherwise it is made a regular
+    (non-floating) slot object.</para>
+
+    <para>Bus slot objects may be allocated with calls such as
+    <citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry>. If the
+    <parameter>slot</parameter> of these functions is non-<constant>NULL</constant> the slot object will be of the
+    regular kind (i.e. non-floating), otherwise it will be created floating. With
+    <function>sd_bus_slot_set_floating()</function> a bus slot object allocated as regular can be converted into a
+    floating object and back. This is particularly useful for creating a bus slot object, then changing parameters of
+    it, and then turning it into a floating object, whose lifecycle is managed by the bus object.</para>
+
+    <para><function>sd_bus_slot_get_floating()</function> returns the current floating state of the specified bus slot
+    object. It returns negative on error, zero if the bus slot object is a regular (non-floating) object and positive
+    otherwise.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Return Value</title>
+
+    <para>On success, these functions return 0 or a positive integer. On failure, they return 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>
+
+      <varlistentry>
+        <term><constant>-ESTALE</constant></term>
+
+        <listitem><para>The bus object the specified bus slot object is associated with has already been freed, and
+        hence no change in the floating state can be made anymore.</para></listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
+  <refsect1>
+    <title>Notes</title>
+
+    <para><function>sd_bus_slot_set_floating()</function> and <function>sd_bus_slot_get_floating()</function> are
+    available as a shared library, which can be compiled and linked to with the
+    <constant>libsystemd</constant> <citerefentry
+    project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry> file.</para>
+  </refsect1>
+
+  <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_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>