Handle invalid UTF-8 encoding in strings (#295)

Log a warning and then retry with the replacement error handling
approach.

nptdms/test/test_types.py

@@ -3,6 +3,7 @@
 from datetime import date, datetime
 import io
 import numpy as np
+import struct
 import pytest
 
 from nptdms import types
@@ -54,3 +55,33 @@ def test_timestamp_from_date():
     read_datetime = types.TimeStamp.read(data_file)
 
     assert expected_datetime == read_datetime.as_datetime64()
+
+
+def test_invalid_utf8_string_read(caplog):
+    """ Test reading a single invalid string value"""
+    file = io.BytesIO(struct.pack("<L", 3) + b'0 \xb0')
+    string_value = types.String.read(file)
+
+    assert string_value == "0 �"
+    assert "WARNING" in caplog.text
+    assert "0 \\xb0" in caplog.text
+
+
+def test_invalid_utf8_strings_read(caplog):
+    """Test reading multiple string values where one is invalid"""
+    string_bytes = [
+        b'hello',
+        b'0 \xb0',
+        b'world',
+    ]
+    offset = 0
+    offsets = []
+    for val in string_bytes:
+        offset += len(val)
+        offsets.append(struct.pack("<L", offset))
+    file = io.BytesIO(b''.join(offsets + string_bytes))
+    string_values = types.String.read_values(file, len(string_bytes))
+
+    assert string_values == ["hello", "0 �", "world"]
+    assert "WARNING" in caplog.text
+    assert "0 \\xb0" in caplog.text

nptdms/types.py

@@ -3,6 +3,10 @@
 import numpy as np
 import struct
 from nptdms.timestamp import TdmsTimestamp, TimestampArray
+from nptdms.log import log_manager
+
+
+log = log_manager.get_logger(__name__)
 
 
 __all__ = [
@@ -205,7 +209,7 @@ def __init__(self, value):
     def read(file, endianness="<"):
         size_bytes = file.read(4)
         size = _struct_unpack(endianness + 'L', size_bytes)[0]
-        return file.read(size).decode('utf-8')
+        return String._decode(file.read(size))
 
     @classmethod
     def read_values(cls, file, number_values, endianness="<"):
@@ -220,9 +224,19 @@ def read_values(cls, file, number_values, endianness="<"):
         strings = []
         for i in range(number_values):
             s = file.read(offsets[i + 1] - offsets[i])
-            strings.append(s.decode('utf-8'))
+            strings.append(String._decode(s))
         return strings
 
+    @staticmethod
+    def _decode(string_bytes):
+        try:
+            return string_bytes.decode('utf-8')
+        except UnicodeDecodeError as exc:
+            log.warning(
+                "Error decoding string from bytes %s, retrying with replace handler: %s",
+                string_bytes, exc)
+            return string_bytes.decode('utf-8', errors='replace')
+
 
 @tds_data_type(0x21, np.bool_)
 class Boolean(StructType):