socket: New option 'FlushPending' (boolean) to flush socket before entering listening state

Disabled by default. When Enabled, before listening on the socket, flush the content.
Applies when Accept=no only.

man/org.freedesktop.systemd1.xml

@@ -3950,6 +3950,8 @@ node /org/freedesktop/systemd1/unit/avahi_2ddaemon_2esocket {
       @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
       readonly u NRefused = ...;
       @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
+      readonly u FlushPending = ...;
+      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
       readonly s FileDescriptorName = '...';
       @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
       readonly i SocketProtocol = ...;
@@ -5031,6 +5033,8 @@ node /org/freedesktop/systemd1/unit/avahi_2ddaemon_2esocket {
 
     <variablelist class="dbus-property" generated="True" extra-ref="NRefused"/>
 
+    <variablelist class="dbus-property" generated="True" extra-ref="FlushPending"/>
+
     <variablelist class="dbus-property" generated="True" extra-ref="FileDescriptorName"/>
 
     <variablelist class="dbus-property" generated="True" extra-ref="SocketProtocol"/>
@@ -5508,6 +5512,10 @@ node /org/freedesktop/systemd1/unit/avahi_2ddaemon_2esocket {
       meaning as they have for the corresponding field of service units (see above). In addition to that,
       the value <literal>service-failed-permanent</literal> indicates that the service of this socket failed
       continuously.</para>
+
+      <para><varname>FlushPending</varname> specifies whether to flush the socket
+      just before entering the listening state. This setting only applies to sockets with
+      <varname>Accept=</varname> set to <literal>no</literal>.</para>
     </refsect2>
   </refsect1>
 

man/systemd.socket.xml

@@ -427,6 +427,18 @@
         false, in read-only mode. Defaults to false.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>FlushPending=</varname></term>
+        <listitem><para>Takes a boolean argument. May only be used when
+        <option>Accept=no</option>. If yes, the socket's buffers are cleared after the
+        triggered service exited. This causes any pending data to be
+        flushed and any pending incoming connections to be rejected. If no, the
+        socket's buffers won't be cleared, permitting the service to handle any
+        pending connections after restart, which is the usually expected behaviour.
+        Defaults to <option>no</option>.
+        </para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>MaxConnections=</varname></term>
         <listitem><para>The maximum number of connections to