Don't create a default match for fallback devices (#723)

If we end up using a fallback device, creating a default match triggers
two complaints: an assert in bus_to_str() for the WBUSTYPE_UNKNOWN
and one in libwacom_set_default_match() because the match we compose
based on bustype/vid/pid does doesn't exist in the list of the generic
device's matches.

Closes #721, #722

libwacom/libwacom.c

@@ -740,7 +740,6 @@ libwacom_new_from_builder(const WacomDeviceDatabase *db, const WacomBuilder *bui
 		char *name, *uniq;
 		const char *used_match_name = NULL;
 		const char *used_match_uniq = NULL;
-		WacomMatch *used_match;
 
 		vendor_id = builder->vendor_id;
 		product_id = builder->product_id;
@@ -786,9 +785,9 @@ libwacom_new_from_builder(const WacomDeviceDatabase *db, const WacomBuilder *bui
 			bus++;
 		}
 		ret = fallback_or_device(db, device, builder->device_name, fallback);
-		if (ret) {
-			/* for multiple-match devices, set to the one we requested */
-			used_match = libwacom_match_new(used_match_name, used_match_uniq, *bus, vendor_id, product_id);
+		if (ret && device != NULL) {
+			/* If this isn't the fallback device: for multiple-match devices, set to the one we requested */
+			WacomMatch *used_match = libwacom_match_new(used_match_name, used_match_uniq, *bus, vendor_id, product_id);
 			libwacom_set_default_match(ret, used_match);
 			libwacom_match_unref(used_match);
 		}

test/test_libwacom.py

@@ -377,3 +377,25 @@ def test_dont_ignore_exact_matches(custom_datadir):
     builder = WacomBuilder.create(usbid=USBID, uniq="whatever", match_name=NAME)
     device = db.new_from_builder(builder)
     assert device is None
+
+
+# Emulates the behavior of new_from_path for an unknown device
+@pytest.mark.parametrize("fallback", (WacomDatabase.Fallback.NONE, WacomDatabase.Fallback.GENERIC))
+@pytest.mark.parametrize("bustype", (WacomBustype.USB, WacomBustype.BLUETOOTH))
+def test_new_unknown_device_with_fallback(custom_datadir, fallback, bustype):
+    USBID = (0x1234, 0x5678)
+    NAME = "nameval"
+    db = WacomDatabase(path=custom_datadir)
+    builder = WacomBuilder.create(usbid=USBID, bus=bustype, match_name=NAME)
+
+    device = db.new_from_builder(builder, fallback=fallback)
+    if fallback:
+        assert device is not None
+        match = device.match
+        assert match.decode("utf-8") == "generic"
+        # Generic device always has 0, 0, 0 triple for bus/vid/pid
+        assert device.bustype == WacomBustype.UNKNOWN
+        assert device.vendor_id == 0
+        assert device.product_id == 0
+    else:
+        assert device is None