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.

src/core/dbus-socket.c

@@ -86,6 +86,7 @@ const sd_bus_vtable bus_socket_vtable[] = {
         SD_BUS_PROPERTY("SocketMode", "u", bus_property_get_mode, offsetof(Socket, socket_mode), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("DirectoryMode", "u", bus_property_get_mode, offsetof(Socket, directory_mode), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("Accept", "b", bus_property_get_bool, offsetof(Socket, accept), SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("FlushPending", "b", bus_property_get_bool, offsetof(Socket, flush_pending), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("Writable", "b", bus_property_get_bool, offsetof(Socket, writable), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("KeepAlive", "b", bus_property_get_bool, offsetof(Socket, keep_alive), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("KeepAliveTimeUSec", "t", bus_property_get_usec, offsetof(Socket, keep_alive_time), SD_BUS_VTABLE_PROPERTY_CONST),
@@ -179,6 +180,9 @@ static int bus_socket_set_transient_property(
         if (streq(name, "Accept"))
                 return bus_set_transient_bool(u, name, &s->accept, message, flags, error);
 
+        if (streq(name, "FlushPending"))
+                return bus_set_transient_bool(u, name, &s->flush_pending, message, flags, error);
+
         if (streq(name, "Writable"))
                 return bus_set_transient_bool(u, name, &s->writable, message, flags, error);
 

src/core/load-fragment-gperf.gperf.m4

@@ -391,6 +391,7 @@ Socket.SocketGroup,              config_parse_user_group_compat,     0,
 Socket.SocketMode,               config_parse_mode,                  0,                             offsetof(Socket, socket_mode)
 Socket.DirectoryMode,            config_parse_mode,                  0,                             offsetof(Socket, directory_mode)
 Socket.Accept,                   config_parse_bool,                  0,                             offsetof(Socket, accept)
+Socket.FlushPending,             config_parse_bool,                  0,                             offsetof(Socket, flush_pending)
 Socket.Writable,                 config_parse_bool,                  0,                             offsetof(Socket, writable)
 Socket.MaxConnections,           config_parse_unsigned,              0,                             offsetof(Socket, max_connections)
 Socket.MaxConnectionsPerSource,  config_parse_unsigned,              0,                             offsetof(Socket, max_connections_per_source)

src/core/socket.c

@@ -72,6 +72,7 @@ static const UnitActiveState state_translation_table[_SOCKET_STATE_MAX] = {
 
 static int socket_dispatch_io(sd_event_source *source, int fd, uint32_t revents, void *userdata);
 static int socket_dispatch_timer(sd_event_source *source, usec_t usec, void *userdata);
+static void flush_ports(Socket *s);
 
 static void socket_init(Unit *u) {
         Socket *s = SOCKET(u);
@@ -669,6 +670,11 @@ static void socket_dump(Unit *u, FILE *f, const char *prefix) {
                         prefix, s->n_connections,
                         prefix, s->max_connections,
                         prefix, s->max_connections_per_source);
+        else
+                fprintf(f,
+                        "%sFlushPending: %s\n",
+                         prefix, yes_no(s->flush_pending));
+
 
         if (s->priority >= 0)
                 fprintf(f,
@@ -2201,6 +2207,11 @@ static void socket_enter_listening(Socket *s) {
         int r;
         assert(s);
 
+        if (!s->accept && s->flush_pending) {
+                log_unit_debug(UNIT(s), "Flushing socket before listening.");
+                flush_ports(s);
+        }
+
         r = socket_watch_fds(s);
         if (r < 0) {
                 log_unit_warning_errno(UNIT(s), r, "Failed to watch sockets: %m");

src/core/socket.h

@@ -110,6 +110,7 @@ struct Socket {
         bool accept;
         bool remove_on_stop;
         bool writable;
+        bool flush_pending;
 
         int socket_protocol;