ratbagd: Add an APIVersion property to make version checks easier

While the DBus API is still in flux, things can break at any time.

Keeping piper and libratbag in version-step works terribly, doubly so where
piper is flatpaked but ratbagd comes from some repo somewhere. We can't check
at build-time, we can't run ratbagd --version at runtime.

so let's add a version field for now and drop it if we ever get to stable.
Meanwhile, ratbagd.py will throw an exception if the expected version doesn't
match exactly.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

doc/dbus.rst

@@ -80,6 +80,19 @@ org.freedesktop.ratbag1.Manager
 The **org.freedesktop.ratbag1.Manager** interface is the entry point to
 interact with ratbagd.
 
+.. attribute:: APIVersion
+
+        :type: i
+        :flags: read-only,constant
+
+	The DBus API version. This is a stopgap feature while the DBus API
+	is still in flux. The version has no backwards or
+	forward-compatibilty guarantees - your client must understand
+	**exactly** the same version, it is not enough to support an older
+	or a newer version.
+
+	This API will be removed once the DBus API is declared stable.
+
 .. attribute:: Devices
 
         :type: ao

meson.build

@@ -4,6 +4,12 @@ project('libratbag', 'c',
 	default_options : [ 'c_std=gnu99', 'warning_level=2' ],
 	meson_version : '>= 0.40.0')
 
+# The DBus API version. Increase this every time the DBus API changes.
+# No backwards/forwards guarantee, clients are expected to understand
+# whatever ratbagd speaks or bail out. This should be removed if we ever
+# finish the API and declare it stable.
+ratbagd_api_version = 1
+
 # We use libtool-version numbers because it's easier to understand.
 # Before making a release, the libratbag_so_* and liblur_so_*
 # numbers should be modified. The components are of the form C:R:A.
@@ -35,6 +41,7 @@ add_project_arguments(cflags, language: 'c')
 config_h = configuration_data()
 config_h.set('_GNU_SOURCE', '1')
 config_h.set_quoted('RATBAG_VERSION', meson.project_version())
+config_h.set('RATBAGD_API_VERSION', ratbagd_api_version)
 libratbag_data_dir = join_paths(get_option('prefix'),
 				get_option('datadir'),
 				'libratbag')
@@ -520,15 +527,22 @@ else
   py3 = pymod.find_installation()
 endif
 
+config_ratbagctl = configuration_data()
+config_ratbagctl.set('RATBAGD_API_VERSION', ratbagd_api_version)
+config_ratbagctl.set('version', meson.project_version())
+
+ratbagctl_in = configure_file(input : 'tools/ratbagctl.in.in',
+			      output : 'ratbagctl.in',
+			      configuration: config_ratbagctl)
+
 # ratbagctl is the commandline tool to interact with ratbagd over DBus.
 custom_target('ratbagctl',
   output : 'ratbagctl',
-  input : ['tools/ratbagctl.in', 'tools/ratbagd.py'],
+  input : [ratbagctl_in, 'tools/ratbagd.py'],
   build_by_default : true,
   command : [py3, join_paths(meson.source_root(), 'tools', 'merge_ratbagd.py'),
 				'@INPUT@',
-				'--output', '@OUTPUT@',
-				'--version', meson.project_version()],
+				'--output', '@OUTPUT@'],
   install: true,
   install_dir: get_option('bindir'))
 
@@ -543,6 +557,7 @@ man_ratbagctl = configure_file (
 config_ratbagctl_devel = configuration_data()
 config_ratbagctl_devel.set('MESON_BUILD_ROOT', meson.build_root())
 config_ratbagctl_devel.set('LIBRATBAG_DATA_DIR', libratbag_data_dir_devel)
+config_ratbagctl_devel.set('RATBAGD_API_VERSION', ratbagd_api_version)
 
 configure_file(input : 'tools/toolbox.py',
 	       output : 'toolbox.py',
@@ -610,12 +625,11 @@ ratbagc_py = configure_file(input: 'tools/ratbagc.py.in',
 # development tool for protocol debugging etc.
 custom_target('ratbag-command',
   output : 'ratbag-command',
-  input : ['tools/ratbagctl.in', ratbagc_py],
+  input : [ratbagctl_in, ratbagc_py],
   build_by_default : true,
   command : [py3, join_paths(meson.source_root(), 'tools', 'merge_ratbagd.py'),
 				'@INPUT@',
-				'--output', '@OUTPUT@',
-				'--version', meson.project_version()],
+				'--output', '@OUTPUT@'],
   install: false)
 
 #### output files ####

ratbagd/ratbagd.c

@@ -166,6 +166,7 @@ static int ratbagd_get_devices(sd_bus *bus,
 
 static const sd_bus_vtable ratbagd_vtable[] = {
 	SD_BUS_VTABLE_START(0),
+	SD_BUS_PROPERTY("APIVersion", "i", 0, offsetof(struct ratbagd, api_version), SD_BUS_VTABLE_PROPERTY_CONST),
 	SD_BUS_PROPERTY("Devices", "ao", ratbagd_get_devices, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
 #ifdef RATBAG_DEVELOPER_EDITION
 	SD_BUS_METHOD("LoadTestDevice", "s", "i", ratbagd_load_test_device, SD_BUS_VTABLE_UNPRIVILEGED),
@@ -345,6 +346,7 @@ static int ratbagd_new(struct ratbagd **out)
 	int r;
 
 	ctx = zalloc(sizeof(*ctx));
+	ctx->api_version = RATBAGD_API_VERSION;
 
 	r = sd_event_default(&ctx->event);
 	if (r < 0)

ratbagd/ratbagd.h

@@ -218,6 +218,8 @@ struct ratbagd_device *ratbagd_device_next(struct ratbagd_device *device);
  */
 
 struct ratbagd {
+	int api_version;
+
 	sd_event *event;
 	struct ratbag *lib_ctx;
 	struct udev_monitor *monitor;

tools/ratbagctl.in.in

@@ -1465,7 +1465,7 @@ def on_device_added(ratbagd, device):
 
 def open_ratbagd(ratbagd_process=None, verbose=0):
     try:
-        r = Ratbagd()
+        r = Ratbagd(@RATBAGD_API_VERSION@)
         r.verbose = verbose
         try:
             r.connect('device-added', on_device_added)

tools/ratbagd.py

@@ -62,6 +62,18 @@ class RatbagErrorCode(IntEnum):
     IMPLEMENTATION = -1004
 
 
+class RatbagdIncompatible(Exception):
+    """ratbagd is incompatible with this client"""
+    def __init__(self, ratbagd_version, required_version):
+        super().__init__()
+        self.ratbagd_version = ratbagd_version
+        self.required_version = required_version
+        self.message = "ratbagd API version is {} but we require {}".format(ratbagd_version, required_version)
+
+    def __str__(self):
+        return self.message
+
+
 class RatbagdUnavailable(Exception):
     """Signals DBus is unavailable or the ratbagd daemon is not available."""
     pass
@@ -238,11 +250,13 @@ class Ratbagd(_RatbagdDBus):
             (GObject.SignalFlags.RUN_FIRST, None, ()),
     }
 
-    def __init__(self):
+    def __init__(self, api_version):
         super().__init__("Manager", None)
         result = self._get_dbus_property("Devices") or []
         self._devices = [RatbagdDevice(objpath) for objpath in result]
         self._proxy.connect("notify::g-name-owner", self._on_name_owner_changed)
+        if self.api_version != api_version:
+            raise RatbagdIncompatible(self.api_version or -1, api_version)
 
     def _on_name_owner_changed(self, *kwargs):
         self.emit("daemon-disappeared")
@@ -261,6 +275,10 @@ class Ratbagd(_RatbagdDBus):
                     self.emit("device-removed", device)
             self.notify("devices")
 
+    @GObject.Property
+    def api_version(self):
+        return self._get_dbus_property("APIVersion")
+
     @GObject.Property
     def devices(self):
         """A list of RatbagdDevice objects supported by ratbagd."""