core: add new setting Writable= to ListenSpecial= socket units

Writable= is a new boolean setting. If ture, then ListenSpecial= will
open the specified path in O_RDWR mode, rather than just O_RDONLY.

This is useful for implementing services like rfkill, where /dev/rfkill
is more useful when opened in write mode, if we want to not only save
but also restore its state.

man/systemd.socket.xml

@@ -381,6 +381,14 @@
         For SOCK_RAW the port is the IP protocol.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>Writable=</varname></term>
+        <listitem><para>Takes a boolean argument. May only be used in
+        conjunction with <varname>ListenSpecial=</varname>. If true,
+        the specified special file is opened in read-write mode, if
+        false in read-only mode. Defaults to false.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>MaxConnections=</varname></term>
         <listitem><para>The maximum number of connections to

src/core/dbus-socket.c

@@ -95,6 +95,7 @@ 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("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),
         SD_BUS_PROPERTY("KeepAliveIntervalUSec", "t", bus_property_get_usec, offsetof(Socket, keep_alive_interval), SD_BUS_VTABLE_PROPERTY_CONST),

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

@@ -261,6 +261,7 @@ Socket.SocketGroup,              config_parse_unit_string_printf,    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.Writable,                 config_parse_bool,                  0,                             offsetof(Socket, writable)
 Socket.MaxConnections,           config_parse_unsigned,              0,                             offsetof(Socket, max_connections)
 Socket.KeepAlive,                config_parse_bool,                  0,                             offsetof(Socket, keep_alive)
 Socket.KeepAliveTimeSec,         config_parse_sec,                   0,                             offsetof(Socket, keep_alive_time)

src/core/socket.c

@@ -507,6 +507,7 @@ static void socket_dump(Unit *u, FILE *f, const char *prefix) {
                 "%sPassSecurity: %s\n"
                 "%sTCPCongestion: %s\n"
                 "%sRemoveOnStop: %s\n"
+                "%sWritable: %s\n"
                 "%sSELinuxContextFromNet: %s\n",
                 prefix, socket_state_to_string(s->state),
                 prefix, socket_result_to_string(s->result),
@@ -523,6 +524,7 @@ static void socket_dump(Unit *u, FILE *f, const char *prefix) {
                 prefix, yes_no(s->pass_sec),
                 prefix, strna(s->tcp_congestion),
                 prefix, yes_no(s->remove_on_stop),
+                prefix, yes_no(s->writable),
                 prefix, yes_no(s->selinux_context_from_net));
 
         if (s->control_pid > 0)
@@ -1031,14 +1033,14 @@ fail:
         return r;
 }
 
-static int special_address_create(const char *path) {
+static int special_address_create(const char *path, bool writable) {
         _cleanup_close_ int fd = -1;
         struct stat st;
         int r;
 
         assert(path);
 
-        fd = open(path, O_RDONLY|O_CLOEXEC|O_NOCTTY|O_NONBLOCK|O_NOFOLLOW);
+        fd = open(path, (writable ? O_RDWR : O_RDONLY)|O_CLOEXEC|O_NOCTTY|O_NONBLOCK|O_NOFOLLOW);
         if (fd < 0)
                 return -errno;
 
@@ -1280,7 +1282,7 @@ static int socket_open_fds(Socket *s) {
 
                 case SOCKET_SPECIAL:
 
-                        p->fd = special_address_create(p->path);
+                        p->fd = special_address_create(p->path, s->writable);
                         if (p->fd < 0) {
                                 r = p->fd;
                                 goto rollback;

src/core/socket.h

@@ -118,6 +118,7 @@ struct Socket {
 
         bool accept;
         bool remove_on_stop;
+        bool writable;
 
         /* Socket options */
         bool keep_alive;