man: sd_notify() race is gone with sd_notify_barrier()

Add note for change of behaviour in systemd-notify, where parent pid trick
is only used when --no-block is passed, and with enough privileges ofcourse.

Also, fix a small error in systemd(1).

Author: kkdwivedi

man/systemd-notify.xml

@@ -54,15 +54,19 @@
     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><command>systemd-notify</command> will first attempt to invoke <function>sd_notify()</function> pretending to
-    have the PID of the invoking process. This will only succeed when invoked with sufficient privileges. On failure,
-    it will then fall back to invoking it under its own PID. This behaviour is useful in order that when the tool is
-    invoked from a shell script the shell process — and not the <command>systemd-notify</command> process — appears as
-    sender of the message, which in turn is helpful if the shell process is the main process of a service, due to the
-    limitations of <varname>NotifyAccess=</varname><option>all</option> described above.</para>
+    attribute the message to the unit, and thus will ignore it, even if <varname>NotifyAccess=</varname><option>all
+    </option> is set for it. When <option>--no-block</option> is used, all synchronization for reception of notifications
+    is disabled, and hence the aforementioned race may occur if the invoking process is not the service manager or spawned
+    by the service manager.</para>
+
+    <para>Hence, <command>systemd-notify</command> will first attempt to invoke <function>sd_notify()</function>
+    pretending to have the PID of the invoking process. This will only succeed when invoked with sufficient privileges.
+    On failure, it will then fall back to invoking it under its own PID. This behaviour is useful in order that when
+    the tool is invoked from a shell script the shell process — and not the <command>systemd-notify</command> process
+    — appears as sender of the message, which in turn is helpful if the shell process is the main process of a service,
+    due to the limitations of <varname>NotifyAccess=</varname><option>all</option>. Use the <option>--pid=</option>
+    switch to tweak this behaviour.</para>
+
   </refsect1>
 
   <refsect1>
@@ -129,6 +133,17 @@
         with systemd.  </para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>--no-block</option></term>
+
+        <listitem><para>Do not synchronously wait for the requested operation to finish.
+        Use of this option is only recommended when <command>systemd-notify</command>
+        is spawned by the service manager, or when the invoking process is directly spawned
+        by the service manager and has enough privileges to allow <command>systemd-notify
+        </command> to send the notification on its behalf. Sending notifications with
+        this option set is prone to race conditions in all other cases.</para></listitem>
+      </varlistentry>
+
       <xi:include href="standard-options.xml" xpointer="help" />
       <xi:include href="standard-options.xml" xpointer="version" />
     </variablelist>

man/systemd.service.xml

@@ -959,7 +959,14 @@
         <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>
+        <varname>NotifyAccess=</varname><option>all</option> is set for it.</para>
+
+        <para>Hence, to eliminate all race conditions involving lookup of the client's unit and attribution of notifications
+        to units correctly, <function>sd_notify_barrier()</function> may be used. This call acts as a synchronization point
+        and ensures all notifications sent before this call have been picked up by the service manager when it returns
+        successfully. Use of <function>sd_notify_barrier()</function> is needed for clients which are not invoked by the
+        service manager, otherwise this synchronization mechanism is unnecessary for attribution of notifications to the
+        unit.</para></listitem>
       </varlistentry>
 
       <varlistentry>

man/systemd.xml

@@ -257,7 +257,7 @@
     execution compared to the target unit's state and is marked successful and
     complete when both satisfy. However, this job also pulls in other
     dependencies due to the defined relationships and thus leads to, in our
-    our example, start jobs for any of those inactive units getting queued as
+    example, start jobs for any of those inactive units getting queued as
     well.</para>
 
     <para>systemd contains native implementations of various tasks