Truncate UTF-8 in a way that produces valid UTF-8.

tests/test_blendfile_dna_io.py

@@ -0,0 +1,24 @@
+import unittest
+from unittest import mock
+
+from blender_asset_tracer.blendfile import dna, dna_io
+
+
+class StringTest(unittest.TestCase):
+    def test_trim_utf8(self):
+        fileobj = mock.Mock()
+        # Sinhala for 'beer'. This is exactly 15 bytes in UTF-8, so the last
+        # character won't fit in the field (due to the 0-byte required).
+        dna_io.BigEndianTypes.write_string(fileobj, 'බියර්', 15)
+
+        expect_bytes = ('බියර්'[:-1]).encode('utf8') + b'\0'
+        fileobj.write.assert_called_with(expect_bytes)
+
+    def test_utf8(self):
+        fileobj = mock.Mock()
+        # Sinhala for 'beer'. This is exactly 15 bytes in UTF-8,
+        # so with the 0-byte it just fits.
+        dna_io.BigEndianTypes.write_string(fileobj, 'බියර්', 16)
+
+        expect_bytes = 'බියර්'.encode('utf8') + b'\0'
+        fileobj.write.assert_called_with(expect_bytes)