man: document that sd_notify() is racy in some cases

diff --git a/man/sd_notify.xml b/man/sd_notify.xml
index ae6792dd708185650163e9c2239edac8e8173b73..f3f3269769bdee77fd56f301a63d81444c013df0 100644 (file)
--- a/man/sd_notify.xml
+++ b/man/sd_notify.xml
@@ -268,6 +268,15 @@
     <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
     for details.</para>
 
     <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
     for details.</para>
 

+    <para>Note that <function>sd_notify()</function> notifications may be attributed to units correctly only if either
+    the sending process is still around at the time PID 1 processes the message, or if the sending process is
+    explicitly runtime-tracked by the service manager. The latter is the case if the service manager originally forked
+    off the process, i.e. on all processes that match <varname>NotifyAccess=</varname><option>main</option> or
+    <varname>NotifyAccess=</varname><option>exec</option>. Conversely, if an auxiliary process of the unit sends an
+    <function>sd_notify()</function> message and immediately exits, the service manager might not be able to properly
+    attribute the message to the unit, and thus will ignore it, even if
+    <varname>NotifyAccess=</varname><option>all</option> is set for it.</para>
+

     <para><function>sd_notifyf()</function> is similar to
     <function>sd_notify()</function> but takes a
     <function>printf()</function>-like format string plus
     <para><function>sd_notifyf()</function> is similar to
     <function>sd_notify()</function> but takes a
     <function>printf()</function>-like format string plus