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

blender_asset_tracer/blendfile/dna_io.py

@@ -73,9 +73,25 @@ class EndianIO:
         :returns: the number of bytes written.
         """
         assert isinstance(astring, str)
-        # TODO: truncate the string on a UTF-8 character boundary to avoid creating invalid UTF-8.
-        encoded = astring.encode('utf-8')[:fieldlen-1] + b'\0'
-        return fileobj.write(encoded)
+        encoded = astring.encode('utf-8')
+
+        # Take into account we also need space for a trailing 0-byte.
+        maxlen = fieldlen - 1
+
+        if len(encoded) >= maxlen:
+            encoded = encoded[:maxlen]
+
+            # Keep stripping off the last byte until the string
+            # is valid UTF-8 again.
+            while True:
+                try:
+                    encoded.decode('utf8')
+                except UnicodeDecodeError:
+                    encoded = encoded[:-1]
+                else:
+                    break
+
+        return fileobj.write(encoded + b'\0')
 
     @classmethod
     def write_bytes(cls, fileobj: typing.BinaryIO, data: bytes, fieldlen: int) -> int:

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)