ratbagd: pick the ID from a list of preselected names

We don't want anyone to rely on a specific content of the ID, e.g. by
hardcoding hidraw/event node numbers. So let's make this an opaque list with
no meaning other than "this is a rodent".

This is solely for the benefit of ratbagctl which uses these IDs as access
identifier.

This doesn't affect ratbag-command which still uses device paths. It also does
not affect the object path. And piper shows the device name anyway.

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

ratbagd/ratbagd-device.c

@@ -44,6 +44,7 @@ struct ratbagd_device {
 	struct ratbagd *ctx;
 	RBNode node;
 	char *sysname;
+	char *id;
 	char *path;
 	struct ratbag_device *lib_device;
 
@@ -274,7 +275,7 @@ ratbagd_device_get_capabilities(sd_bus *bus,
 
 const sd_bus_vtable ratbagd_device_vtable[] = {
 	SD_BUS_VTABLE_START(0),
-	SD_BUS_PROPERTY("Id", "s", NULL, offsetof(struct ratbagd_device, sysname), SD_BUS_VTABLE_PROPERTY_CONST),
+	SD_BUS_PROPERTY("Id", "s", NULL, offsetof(struct ratbagd_device, id), SD_BUS_VTABLE_PROPERTY_CONST),
 	SD_BUS_PROPERTY("Capabilities", "au", ratbagd_device_get_capabilities, 0, SD_BUS_VTABLE_PROPERTY_CONST),
 	SD_BUS_PROPERTY("Name", "s", ratbagd_device_get_device_name, 0, SD_BUS_VTABLE_PROPERTY_CONST),
 	SD_BUS_PROPERTY("Profiles", "ao", ratbagd_device_get_profiles, 0, SD_BUS_VTABLE_PROPERTY_CONST),
@@ -287,6 +288,7 @@ const sd_bus_vtable ratbagd_device_vtable[] = {
 int ratbagd_device_new(struct ratbagd_device **out,
 		       struct ratbagd *ctx,
 		       const char *sysname,
+		       const char *id,
 		       struct ratbag_device *lib_device)
 {
 	_cleanup_(ratbagd_device_freep) struct ratbagd_device *device = NULL;
@@ -297,6 +299,7 @@ int ratbagd_device_new(struct ratbagd_device **out,
 	assert(out);
 	assert(ctx);
 	assert(sysname);
+	assert(id);
 
 	device = zalloc(sizeof(*device));
 	device->ctx = ctx;
@@ -304,6 +307,7 @@ int ratbagd_device_new(struct ratbagd_device **out,
 	device->lib_device = ratbag_device_ref(lib_device);
 
 	device->sysname = strdup_safe(sysname);
+	device->id = strdup_safe(id);
 
 	r = sd_bus_path_encode(RATBAGD_OBJ_ROOT "/device",
 			       device->sysname,
@@ -357,12 +361,19 @@ struct ratbagd_device *ratbagd_device_free(struct ratbagd_device *device)
 	device->lib_device = ratbag_device_unref(device->lib_device);
 	device->path = mfree(device->path);
 	device->sysname = mfree(device->sysname);
+	device->id = mfree(device->id);
 
 	assert(!device->lib_device); /* ratbag yields !NULL if still pinned */
 
 	return mfree(device);
 }
 
+const char *ratbagd_device_get_id(struct ratbagd_device *device)
+{
+	assert(device);
+	return device->sysname;
+}
+
 const char *ratbagd_device_get_sysname(struct ratbagd_device *device)
 {
 	assert(device);

ratbagd/ratbagd-test.c

@@ -190,7 +190,7 @@ int ratbagd_reset_test_device(sd_bus_message *m,
 
 	device = ratbag_device_new_test_device(ctx->lib_ctx, &ratbagd_test_device_descr);
 
-	r = ratbagd_device_new(&ratbagd_test_device, ctx, "test_device", device);
+	r = ratbagd_device_new(&ratbagd_test_device, ctx, "test_device", "test_device", device);
 
 	/* the ratbagd_device takes its own reference, drop ours */
 	ratbag_device_unref(device);

ratbagd/ratbagd.c

@@ -204,6 +204,67 @@ static const sd_bus_vtable ratbagd_vtable[] = {
 	SD_BUS_VTABLE_END,
 };
 
+/* 32 pre-picked names. If you have more than 32 devices connected and need
+ * ratbag to control them all, congratulations, you are official a niche
+ * use-case.
+ */
+static const char *device_ids[] = {
+"mara", "capybara", "porcupine", "paca",
+"vole", "woodrat", "gerbil", "shrew",
+"hare", "beaver", "gopher", "chinchilla",
+"mouse", "rat", "rabbit", "guineapig",
+"hutia", "degu", "gundi", "acouchy",
+"nutria", "tuco-tuco", "paca", "hamster",
+"zokor", "chipmunk", "squirrel", "marmot",
+"groundhog", "suslik", "agouti", "blesmol",
+};
+
+/* bitmask telling us which ones of the above are free. */
+static uint32_t device_ids_free = (1UL << ARRAY_LENGTH(device_ids)) - 1;
+
+static inline const char *
+pick_device_id(const char *sysname)
+{
+	int index;
+
+	/* search for the first number in the sysname and where we find it,
+	 * use that as lookup index. This way we get some sort of
+	 * consistency, hidraw2 is always "rat", etc.
+	 * Totally not necessary but we might as well have some fun here.
+	 * Where this fails or picks an id already chosen, we fall back to
+	 * "choose the first one free".
+	 */
+	if (sscanf(sysname, "%*[^0-9]%d", &index) == 1 &&
+	    index >= 0 && (unsigned)index < sizeof(device_ids_free) * 8 &&
+	    !!(device_ids_free & (1 << index)))
+	    goto assign;
+
+	index = ffs(device_ids_free);
+
+	/* This can only happen if you have >32 devices, so let's not worry
+	 * about this too much */
+	if (index == 0)
+		return sysname;
+
+	index -= 1; /* ffs is 1-based */
+
+assign:
+	device_ids_free &= ~(1 << index);
+
+	return device_ids[index];
+}
+
+static inline void
+release_device_id(const char *id)
+{
+	for (size_t i = 0; i < ARRAY_LENGTH(device_ids); i++) {
+		if (streq(id, device_ids[i])) {
+			device_ids_free |= (1 << i);
+			break;
+		}
+	}
+}
+
 static void ratbagd_process_device(struct ratbagd *ctx,
 				   struct udev_device *udevice)
 {
@@ -230,9 +291,11 @@ static void ratbagd_process_device(struct ratbagd *ctx,
 	if (streq_ptr("remove", udev_device_get_action(udevice))) {
 		/* device was removed, unlink it and destroy our context */
 		if (device) {
+			release_device_id(ratbagd_device_get_id(device));
 			ratbagd_device_unlink(device);
 			ratbagd_device_free(device);
 
+
 			(void) sd_bus_emit_properties_changed(ctx->bus,
 							      RATBAGD_OBJ_ROOT,
 							      RATBAGD_NAME_ROOT ".Manager",
@@ -242,6 +305,7 @@ static void ratbagd_process_device(struct ratbagd *ctx,
 	} else if (device) {
 		/* device already known, refresh our view of the device */
 	} else {
+		const char *id;
 		enum ratbag_error_code error;
 
 		/* device unknown, create new one and link it */
@@ -251,7 +315,8 @@ static void ratbagd_process_device(struct ratbagd *ctx,
 		if (error != RATBAG_SUCCESS)
 			return; /* unsupported device */
 
-		r = ratbagd_device_new(&device, ctx, sysname, lib_device);
+		id = pick_device_id(sysname);
+		r = ratbagd_device_new(&device, ctx, sysname, id, lib_device);
 
 		/* the ratbagd_device takes its own reference, drop ours */
 		ratbag_device_unref(lib_device);

ratbagd/ratbagd.h

@@ -169,8 +169,10 @@ extern const sd_bus_vtable ratbagd_device_vtable[];
 int ratbagd_device_new(struct ratbagd_device **out,
 		       struct ratbagd *ctx,
 		       const char *sysname,
+		       const char *id,
 		       struct ratbag_device *lib_device);
 struct ratbagd_device *ratbagd_device_free(struct ratbagd_device *device);
+const char *ratbagd_device_get_id(struct ratbagd_device *device);
 const char *ratbagd_device_get_sysname(struct ratbagd_device *device);
 const char *ratbagd_device_get_path(struct ratbagd_device *device);
 unsigned int ratbagd_device_get_num_buttons(struct ratbagd_device *device);