Fix #142 and #143 (#144)

Fix bugs that manifest when reading and writing empty protocol streams in Python.

cpp/test/roundtrip_test.cc

@@ -459,6 +459,24 @@ TEST_P(RoundTripTests, SimpleDatasets) {
   tw->Close();
 }
 
+TEST_P(RoundTripTests, SimpleDatasets_Mixed) {
+  auto tw = CreateValidatingWriter<StreamsWriterBase>();
+
+  tw->WriteIntData({});
+  tw->EndIntData();
+
+  tw->WriteOptionalIntData({10, 11, {}, 13, 14, 15, {}, 17, 18, 19});
+  tw->EndOptionalIntData();
+
+  tw->EndRecordWithOptionalVectorData();
+
+  tw->WriteFixedVector({1, 2, 3});
+  tw->WriteFixedVector({4, 5, 6});
+  tw->EndFixedVector();
+
+  tw->Close();
+}
+
 TEST_P(RoundTripTests, SimpleDatasets_Empty) {
   auto tw = CreateValidatingWriter<StreamsWriterBase>();
 

python/tests/test_protocol_roundtrip.py

@@ -584,6 +584,7 @@ def days():
 def test_simple_streams(format: Format):
     c = create_validating_writer_class(format, tm.StreamsWriterBase)
 
+    # non-empty streams
     with c() as w:
         w.write_int_data(range(10))
         w.write_int_data(range(20))
@@ -600,6 +601,13 @@ def test_simple_streams(format: Format):
         )
         w.write_fixed_vector(([1, 2, 3] for _ in range(4)))
 
+    # mixed empty and non-empty streams
+    with c() as w:
+        w.write_int_data(range(0))
+        w.write_optional_int_data([1, 2, None, 4, 5, None, 7, 8, 9, 10])
+        w.write_record_with_optional_vector_data([])
+        w.write_fixed_vector(([1, 2, 3] for _ in range(4)))
+
     # empty streams
     with c() as w:
         w.write_int_data(range(0))

tooling/internal/python/static_files/_binary.py

@@ -77,7 +77,7 @@ def __init__(
     ) -> None:
         self._stream = CodedInputStream(stream)
         magic_bytes = self._stream.read_view(len(MAGIC_BYTES))
-        if magic_bytes != MAGIC_BYTES: # pyright: ignore [reportUnnecessaryComparison]
+        if magic_bytes != MAGIC_BYTES:  # pyright: ignore [reportUnnecessaryComparison]
             raise RuntimeError("Invalid magic bytes")
 
         version = read_fixed_int32(self._stream)
@@ -955,7 +955,7 @@ def __init__(self, element_serializer: TypeSerializer[T, T_NP]) -> None:
     def write(self, stream: CodedOutputStream, value: Iterable[T]) -> None:
         # Note that the final 0 is missing and will be added before the next protocol step
         # or the protocol is closed.
-        if isinstance(value, list):
+        if isinstance(value, list) and len(value) > 0:
             stream.write_unsigned_varint(len(value))
             for element in value:
                 self._element_serializer.write(stream, element)

tooling/internal/python/static_files/_ndjson.py

@@ -136,6 +136,7 @@ def _read_json_line(self, stepName: str, required: bool) -> object:
                 return value
             if required:
                 raise ValueError(f"Expected protocol step '{stepName}' not found.")
+            return MISSING_SENTINEL
 
         line = self._stream.readline()
         if line == "":