sd-event: support callback=NULL in IO/child/inotify/defer event sources, too

Also, document this functionality more prominently, including with a
reference from sd_event_exit().

This is mostly to make things complete, as previously we supported NULL
callbacks only in _add_time() and _add_signal(). However, I think this
makes snese for IO event sources too (think: when some fd such as a pipe
end sees SIGHUP or so, exit), as well as defer or post event sources (i.e. exit
once we got nothing else to do). This also adds support for inotify
event sources, simply to complete things (I can't see the immediate use,
but maybe someone else comes up with it).

The only event source type that doesn't allow callback=NULL now are exit
callbacks, but for them they make little sense, as the event loop is
exiting then anyway.

1 files changed, 4 insertions, 6 deletions

diff --git a/man/sd_event_add_time.xml b/man/sd_event_add_time.xml
index 1fc24c8ab0..24a316f9ed 100644
--- a/man/sd_event_add_time.xml
+++ b/man/sd_event_add_time.xml
@@ -161,12 +161,10 @@
     "floating", and will be destroyed implicitly when the event loop
     itself is destroyed.</para>
 
-    <para>If the <parameter>handler</parameter> to
-    <function>sd_event_add_time()</function> is
-    <constant>NULL</constant>, and the event source fires, this will
-    be considered a request to exit the event loop. In this case, the
-    <parameter>userdata</parameter> parameter, cast to an integer, is
-    used for the exit code passed to
+    <para>If the <parameter>handler</parameter> parameter to <function>sd_event_add_time()</function> is
+    <constant>NULL</constant>, and the event source fires, this will be considered a request to exit the
+    event loop. In this case, the <parameter>userdata</parameter> parameter, cast to an integer, is passed as
+    the exit code parameter to
     <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 
     <para>Use <constant>CLOCK_BOOTTIME_ALARM</constant> and