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

man/sd_notify.xml

@@ -268,6 +268,15 @@
     <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

man/systemd-notify.xml

@@ -72,10 +72,17 @@
     <para>The command line may carry a list of environment variables
     to send as part of the status update.</para>
 
-    <para>Note that systemd will refuse reception of status updates
-    from this command unless <varname>NotifyAccess=all</varname> is
-    set for the service unit this command is called from.</para>
-
+    <para>Note that systemd will refuse reception of status updates from this command unless
+    <varname>NotifyAccess=</varname> is set for the service unit this command is called from.</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>
   </refsect1>
 
   <refsect1>

man/systemd.service.xml

@@ -792,26 +792,26 @@
 
       <varlistentry>
         <term><varname>NotifyAccess=</varname></term>
-        <listitem><para>Controls access to the service status
-        notification socket, as accessible via the
-        <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
-        call. Takes one of <option>none</option> (the default),
-        <option>main</option>, <option>exec</option> or
-        <option>all</option>. If <option>none</option>, no daemon status
-        updates are accepted from the service processes, all status
-        update messages are ignored. If <option>main</option>, only
-        service updates sent from the main process of the service are
-        accepted. If <option>exec</option>, only service updates sent
-        from any of the control processes originating from one of the
-        <varname>Exec*=</varname> commands are accepted. If
-        <option>all</option>, all services updates from all members of
-        the service's control group are accepted. This option should
-        be set to open access to the notification socket when using
-        <varname>Type=notify</varname> or
-        <varname>WatchdogSec=</varname> (see above). If those options
-        are used but <varname>NotifyAccess=</varname> is not
-        configured, it will be implicitly set to
-        <option>main</option>.</para></listitem>
+        <listitem><para>Controls access to the service status notification socket, as accessible via the
+        <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> call. Takes one
+        of <option>none</option> (the default), <option>main</option>, <option>exec</option> or
+        <option>all</option>. If <option>none</option>, no daemon status updates are accepted from the service
+        processes, all status update messages are ignored. If <option>main</option>, only service updates sent from the
+        main process of the service are accepted. If <option>exec</option>, only service updates sent from any of the
+        main or control processes originating from one of the <varname>Exec*=</varname> commands are accepted. If
+        <option>all</option>, all services updates from all members of the service's control group are accepted. This
+        option should be set to open access to the notification socket when using <varname>Type=notify</varname> or
+        <varname>WatchdogSec=</varname> (see above). If those options are used but <varname>NotifyAccess=</varname> is
+        not configured, it will be implicitly set to <option>main</option>.</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 <option>main</option> or
+        <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></listitem>
       </varlistentry>
 
       <varlistentry>