man: substantially update the docs regarding hooking sd-bus objects up with external event loops

Prompted by https://lists.freedesktop.org/archives/systemd-devel/2018-December/041817.html

This also drops all references to select() from our manpages. It's 2018
after all, people should use poll(), or ppoll() or epoll().

5 files changed, 299 insertions, 54 deletions

diff --git a/man/rules/meson.build b/man/rules/meson.build
index b091859829..0c990a0c5d 100644
--- a/man/rules/meson.build
+++ b/man/rules/meson.build
@@ -194,7 +194,7 @@ manpages = [
   '3',
   ['SD_BUS_ERROR_END', 'SD_BUS_ERROR_MAP', 'sd_bus_error_map'],
   ''],
- ['sd_bus_get_fd', '3', [], ''],
+ ['sd_bus_get_fd', '3', ['sd_bus_get_events', 'sd_bus_get_timeout'], ''],
  ['sd_bus_get_n_queued_read', '3', ['sd_bus_get_n_queued_write'], ''],
  ['sd_bus_is_open', '3', ['sd_bus_is_ready'], ''],
  ['sd_bus_message_append', '3', ['sd_bus_message_appendv'], ''],
@@ -347,6 +347,7 @@ manpages = [
    'sd_bus_track_unref',
    'sd_bus_track_unrefp'],
   ''],
+ ['sd_bus_wait', '3', [], ''],
  ['sd_event_add_child',
   '3',
   ['sd_event_child_handler_t', 'sd_event_source_get_child_pid'],
diff --git a/man/sd_bus_attach_event.xml b/man/sd_bus_attach_event.xml
index 45f034910f..a2f7297f21 100644
--- a/man/sd_bus_attach_event.xml
+++ b/man/sd_bus_attach_event.xml
@@ -71,6 +71,13 @@
 
     <para>The <function>sd_bus_get_event()</function> returns the event loop object the specified bus object is
     currently attached to, or <constant>NULL</constant> if it is currently not attached to any.</para>
+
+    <para>Note that <function>sd_bus_attach_event()</function> is only one of three supported ways to implement I/O
+    event handling for bus connections. Alternatively use
+    <citerefentry><refentrytitle>sd_bus_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry> for hooking up a
+    bus connection object with external or manual event loops. Or use
+    <citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry> as a simple
+    synchronous, blocking I/O waiting call.</para>
   </refsect1>
 
   <refsect1>
@@ -107,7 +114,8 @@
       <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>sd_bus_set_close_on_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
-      <citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+      <citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>
     </para>
   </refsect1>
 
diff --git a/man/sd_bus_get_fd.xml b/man/sd_bus_get_fd.xml
index 6f709df4c0..2a81bddaa0 100644
--- a/man/sd_bus_get_fd.xml
+++ b/man/sd_bus_get_fd.xml
@@ -8,7 +8,7 @@
   Copyright © 2016 Julian Orth
 -->
 
-<refentry id="sd_bus_get_fd">
+<refentry id="sd_bus_get_fd" xmlns:xi="http://www.w3.org/2001/XInclude">
 
   <refentryinfo>
     <title>sd_bus_get_fd</title>
@@ -22,8 +22,10 @@
 
   <refnamediv>
     <refname>sd_bus_get_fd</refname>
+    <refname>sd_bus_get_events</refname>
+    <refname>sd_bus_get_timeout</refname>
 
-    <refpurpose>Get the file descriptor connected to the message bus</refpurpose>
+    <refpurpose>Get the file descriptor, I/O events and time-out to wait for from a message bus object</refpurpose>
   </refnamediv>
 
   <refsynopsisdiv>
@@ -34,46 +36,124 @@
         <funcdef>int <function>sd_bus_get_fd</function></funcdef>
         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
       </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_get_events</function></funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_get_timeout</function></funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+        <paramdef>uint64_t *<parameter>timeout_usec</parameter></paramdef>
+      </funcprototype>
     </funcsynopsis>
   </refsynopsisdiv>
 
   <refsect1>
     <title>Description</title>
 
-    <para>
-      <function>sd_bus_get_fd()</function> returns the file descriptor used to
-      communicate with the message bus. This descriptor can be used with
-      <citerefentry
-        project='die-net'><refentrytitle>select</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
-      <citerefentry
-        project='die-net'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
-      or similar functions to wait for incoming messages.
-    </para>
-
-    <para>
-      If the bus was created with the
-      <citerefentry><refentrytitle>sd_bus_set_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>
-      function, then the <parameter>input_fd</parameter> used in that call is
-      returned.
-    </para>
+    <para><function>sd_bus_get_fd()</function> returns the file descriptor used to communicate from a message bus
+    object. This descriptor can be used with <citerefentry
+    project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry> or a similar
+    function to wait for I/O events on the specified bus connection object. If the bus object was configured with the
+    <citerefentry><refentrytitle>sd_bus_set_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry> function, then
+    the <parameter>input_fd</parameter> file descriptor used in that call is returned.</para>
+
+    <para><function>sd_bus_get_events()</function> returns the I/O events to wait for, suitable for passing to
+    <function>poll()</function> or a similar call. Returns a combination of <constant>POLLIN</constant>,
+    <constant>POLLOUT</constant>, … events, or negative on error.</para>
+
+    <para><function>sd_bus_get_timeout()</function> returns the time-out in µs to pass to to
+    <function>poll()</function> or a similar call when waiting for events on the specified bus connection. The returned
+    time-out may be zero, in which case a subsequent I/O polling call should be invoked in non-blocking mode. The
+    returned timeout may be <constant>UINT64_MAX</constant> in which case the I/O polling call may block indefinitely,
+    without any applied time-out. Note that the returned time-out should be considered only a maximum sleeping time. It
+    is permissible (and even expected) that shorter time-outs are used by the calling program, in case other event
+    sources are polled in the same event loop. Note that the returned time-value is relative and specified in
+    microseconds. When converting this value in order to pass it as third argument to <function>poll()</function>
+    (which expects milliseconds), care should be taken to use a division that rounds up to ensure the I/O polling
+    operation doesn't sleep for shorter than necessary, which might result in unintended busy looping (alternatively,
+    use <citerefentry project='man-pages'><refentrytitle>ppoll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    instead of plain <function>poll()</function>, which understands time-outs with nano-second granularity).</para>
+
+    <para>These three functions are useful to hook up a bus connection object with an external or manual event loop
+    involving <function>poll()</function> or a similar I/O polling call. Before each invocation of the I/O polling
+    call, all three functions should be invoked: the file descriptor returned by <function>sd_bus_get_fd()</function>
+    should be polled for the events indicated by <function>sd_bus_get_events()</function>, and the I/O call should
+    block for that up to the time-out returned by <function>sd_bus_get_timeout()</function>. After each I/O polling
+    call the bus connection needs to process incoming or outgoing data, by invoking
+    <citerefentry><refentrytitle>sd_bus_process</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
+
+    <para>Note that these function are only one of three supported ways to implement I/O event handling for bus
+    connections. Alternatively use
+    <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry> to attach a
+    bus connection to an <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    event loop. Or use <citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    as a simple synchronous, blocking I/O waiting call.</para>
   </refsect1>
 
   <refsect1>
     <title>Return Value</title>
 
-    <para>
-      Returns the file descriptor used for incoming messages from the message
-      bus.
-    </para>
+    <para><function>sd_bus_get_fd()</function> returns the file descriptor used for communication, or a negative
+    <varname>errno</varname>-style error code on error.</para>
+
+    <para><function>sd_bus_get_events()</function> returns the I/O event mask to use for I/O event watching, or a
+    negative <varname>errno</varname>-style error code on error.</para>
+
+    <para><function>sd_bus_get_timeout()</function> returns zero or positive on success, or a negative
+    <varname>errno</varname>-style error code on error.</para>
   </refsect1>
 
   <refsect1>
+    <title>Errors</title>
+
+    <para>Returned errors may indicate the following problems:</para>
+
+    <variablelist>
+      <varlistentry>
+        <term><constant>-EINVAL</constant></term>
+
+        <listitem><para>An invalid bus object was passed.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>-ECHILD</constant></term>
+
+        <listitem><para>The bus connection was allocated in a parent process and is being reused in a child process
+        after <function>fork()</function>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>-ENOTCONN</constant></term>
+
+        <listitem><para>The bus connection has been terminated.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>-EPERM</constant></term>
+
+        <listitem><para>Two distinct file descriptors were passed for input and output using
+        <function>sd_bus_set_fd()</function>, which <function>sd_bus_get_fd()</function> cannot
+        return.</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_set_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_process</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
     </para>
   </refsect1>
 
diff --git a/man/sd_bus_process.xml b/man/sd_bus_process.xml
index b78192990c..66f22c29a6 100644
--- a/man/sd_bus_process.xml
+++ b/man/sd_bus_process.xml
@@ -8,7 +8,7 @@
   Copyright © 2016 Julian Orth
 -->
 
-<refentry id="sd_bus_process">
+<refentry id="sd_bus_process" xmlns:xi="http://www.w3.org/2001/XInclude">
 
   <refentryinfo>
     <title>sd_bus_process</title>
@@ -33,7 +33,7 @@
       <funcprototype>
         <funcdef>int <function>sd_bus_process</function></funcdef>
         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
-        <paramdef>sd_bus_message **<parameter>r</parameter></paramdef>
+        <paramdef>sd_bus_message **<parameter>ret</parameter></paramdef>
       </funcprototype>
     </funcsynopsis>
   </refsynopsisdiv>
@@ -41,49 +41,92 @@
   <refsect1>
     <title>Description</title>
 
-    <para>
-      <function>sd_bus_process()</function> drives the connection between the
-      message bus and the client. That is, it handles connecting,
-      authentication, and message processing. It should be called in a loop
-      until no further progress can be made or an error occurs.
-    </para>
-
-    <para>
-      Once no further progress can be made,
-      <citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>
-      should be called. Alternatively the user can wait for incoming data on
-      the file descriptor returned by
-      <citerefentry><refentrytitle>sd_bus_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
+    <para><function>sd_bus_process()</function> drives the connection between the client and the message bus. That is,
+    it handles connecting, authentication, and message processing. When invoked pending I/O work is executed, and
+    queued incoming messages are dispatched to registered callbacks. Each time it is invoked a single operation is
+    executed. It returns zero when no operations were pending and positive if a message was processed. When zero is
+    returned the caller should synchronously poll for I/O events before calling into
+    <function>sd_bus_process()</function> again. For that either user the simple, synchronous
+    <citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry> call, or hook up
+    the bus connection object to an external or manual event loop using
+    <citerefentry><refentrytitle>sd_bus_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
     </para>
 
-    <para>
-      <function>sd_bus_process</function> processes at most one incoming
-      message per call.  If the parameter <parameter>r</parameter> is not NULL
-      and the call processed a message, <code>*r</code> is set to this message.
-      The caller owns a reference to this message and should call
-      <citerefentry><refentrytitle>sd_bus_message_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
-      when the message is no longer needed. If <parameter>r</parameter> is not
-      NULL, progress was made, but no message was processed, <code>*r</code> is
-      set to NULL.
-    </para>
+    <para><function>sd_bus_process()</function> processes at most one incoming message per call.  If the parameter
+    <parameter>ret</parameter> is not <constant>NULL</constant> and the call processed a message,
+    <parameter>*ret</parameter> is set to this message.  The caller owns a reference to this message and should call
+    <citerefentry><refentrytitle>sd_bus_message_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry> when the
+    message is no longer needed. If <parameter>ret</parameter> is not NULL, progress was made, but no message was
+    processed, <parameter>*ret</parameter> is set to <constant>NULL</constant>.</para>
+
+    <para>If a the bus object is connected to an
+    <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry> event loop (with
+    <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>), it is not
+    necessary to call <function>sd_bus_process()</function> directly as it is invoked automatically when
+    necessary.</para>
   </refsect1>
 
   <refsect1>
     <title>Return Value</title>
 
-    <para>
-      If progress was made, a positive integer is returned. If no progress was
-      made, 0 is returned. If an error occurs, a negative errno-style error code
-      is returned.
-    </para>
+    <para>If progress was made, a positive integer is returned. If no progress was made, 0 is returned. If an error
+    occurs, a negative <varname>errno</varname>-style error code is returned.</para>
   </refsect1>
 
   <refsect1>
+    <title>Errors</title>
+
+    <para>Returned errors may indicate the following problems:</para>
+
+    <variablelist>
+      <varlistentry>
+        <term><constant>-EINVAL</constant></term>
+
+        <listitem><para>An invalid bus object was passed.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>-ECHILD</constant></term>
+
+        <listitem><para>The bus connection was allocated in a parent process and is being reused in a child process
+        after <function>fork()</function>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>-ENOTCONN</constant></term>
+
+        <listitem><para>The bus connection has been terminated already.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>-ECONNRESET</constant></term>
+
+        <listitem><para>The bus connection has been terminated just now.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>-EBUSY</constant></term>
+
+        <listitem><para>This function is already being called, i.e. <function>sd_bus_process()</function> has been
+        called from a callback function that itself was called by
+        <function>sd_bus_process()</function>.</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_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_message_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>
     </para>
   </refsect1>
 
diff --git a/man/sd_bus_wait.xml b/man/sd_bus_wait.xml
new file mode 100644
index 0000000000..e866eeb20b
--- /dev/null
+++ b/man/sd_bus_wait.xml
@@ -0,0 +1,113 @@
+<?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+
+
+  Copyright © 2016 Julian Orth
+-->
+
+<refentry id="sd_bus_wait" xmlns:xi="http://www.w3.org/2001/XInclude">
+
+  <refentryinfo>
+    <title>sd_bus_wait</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>sd_bus_wait</refentrytitle>
+    <manvolnum>3</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>sd_bus_wait</refname>
+
+    <refpurpose>Wait for I/O on a bus connection</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <funcsynopsis>
+      <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_wait</function></funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+        <paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef>
+      </funcprototype>
+    </funcsynopsis>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><function>sd_bus_wait()</function> synchronously waits for I/O on the specified bus connection object. This
+    function is supposed to be called whenever
+    <citerefentry><refentrytitle>sd_bus_process</refentrytitle><manvolnum>3</manvolnum></citerefentry> returns zero,
+    indicating that no work is pending on the connection. Internally, this call invokes <citerefentry
+    project='man-pages'><refentrytitle>ppoll</refentrytitle><manvolnum>3</manvolnum></citerefentry>, to wait for I/O on
+    the bus connection. If the <parameter>timeout_sec</parameter> parameter is specified, the call will block at most
+    for the specified amount of time in µs. Pass <constant>UINT64_MAX</constant> to permit it to sleep
+    indefinitely.</para>
+
+    <para>After each invocation of <function>sd_bus_wait()</function> the <function>sd_bus_process()</function> call
+    should be invoked in order to process any now pending I/O work.</para>
+
+    <para>Note that <function>sd_bus_wait()</function> is suitable only for simple programs as it does not permit
+    waiting for other I/O events. For more complex programs either connect the bus connection object to an external
+    event loop using <citerefentry><refentrytitle>sd_bus_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    or to an <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry> event loop
+    using
+    <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Return Value</title>
+
+    <para>If any I/O was seen, a positive value is returned, zero otherwise. If an error occurs, a negative
+    <varname>errno</varname>-style error code is returned.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Errors</title>
+
+    <para>Returned errors may indicate the following problems:</para>
+
+    <variablelist>
+      <varlistentry>
+        <term><constant>-EINVAL</constant></term>
+
+        <listitem><para>An invalid bus object was passed.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>-ECHILD</constant></term>
+
+        <listitem><para>The bus connection was allocated in a parent process and is being reused in a child process
+        after <function>fork()</function>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>-ENOTCONN</constant></term>
+
+        <listitem><para>The bus connection has been terminated already.</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_process</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>