mrrc 0.8.0

"""Tests for the typed exception hierarchy and the structured positional
attributes attached to each exception class.

Snapshot tests (via syrupy) pin the externally-visible str/repr/detailed
output for representative exception classes. Property tests cover pickle
round-trip, hierarchy assertions, and bare-constructor compatibility.
"""

import pickle

import pytest

import mrrc


# ---------------------------------------------------------------------------
# Hierarchy assertions
# ---------------------------------------------------------------------------


class TestExceptionHierarchy:
    """The new mrrc-specific subclasses must extend the closest pymarc
    parent so existing pymarc-style ``except`` clauses keep catching them.
    """

    def test_invalid_indicator_subclass_of_record_directory_invalid(self):
        assert issubclass(mrrc.InvalidIndicator, mrrc.RecordDirectoryInvalid)

    def test_bad_subfield_code_subclass_of_record_directory_invalid(self):
        assert issubclass(mrrc.BadSubfieldCode, mrrc.RecordDirectoryInvalid)

    def test_invalid_field_subclass_of_record_directory_invalid(self):
        assert issubclass(mrrc.InvalidField, mrrc.RecordDirectoryInvalid)

    def test_truncated_record_subclass_of_end_of_record_not_found(self):
        assert issubclass(mrrc.TruncatedRecord, mrrc.EndOfRecordNotFound)

    def test_all_mrrc_specific_classes_subclass_mrrc_exception(self):
        for cls in (
            mrrc.InvalidIndicator,
            mrrc.BadSubfieldCode,
            mrrc.InvalidField,
            mrrc.TruncatedRecord,
            mrrc.EncodingError,
            mrrc.XmlError,
            mrrc.JsonError,
            mrrc.WriterError,
        ):
            assert issubclass(cls, mrrc.MrrcException), cls

    def test_pymarc_style_catch_catches_new_subclasses(self):
        """``except RecordDirectoryInvalid`` must catch the new subclasses
        unchanged, so existing pymarc-style code keeps working.
        """
        try:
            raise mrrc.InvalidIndicator(
                record_index=1,
                field_tag="245",
                indicator_position=0,
                found=b":",
                expected="digit or space",
            )
        except mrrc.RecordDirectoryInvalid as caught:
            assert isinstance(caught, mrrc.InvalidIndicator)


# ---------------------------------------------------------------------------
# Stable error codes
# ---------------------------------------------------------------------------


# Mapping of Python exception class to its (code, slug). Cross-checks the
# class-level constants against the Rust MarcError side via the FFI tests
# below. Update both lists together when adding a new variant.
_CODE_TABLE = [
    (mrrc.RecordLengthInvalid, "E001", "record_length_invalid"),
    (mrrc.RecordLeaderInvalid, "E002", "leader_invalid"),
    (mrrc.BaseAddressInvalid, "E003", "base_address_invalid"),
    (mrrc.BaseAddressNotFound, "E004", "base_address_not_found"),
    (mrrc.TruncatedRecord, "E005", "truncated_record"),
    (mrrc.EndOfRecordNotFound, "E006", "end_of_record_not_found"),
    (mrrc.FatalReaderError, "E099", "fatal_reader_error"),
    (mrrc.RecordDirectoryInvalid, "E101", "directory_invalid"),
    (mrrc.FieldNotFound, "E105", "field_not_found"),
    (mrrc.InvalidField, "E106", "invalid_field"),
    (mrrc.InvalidIndicator, "E201", "invalid_indicator"),
    (mrrc.BadSubfieldCode, "E202", "bad_subfield_code"),
    (mrrc.EncodingError, "E301", "utf8_invalid"),
    (mrrc.XmlError, "E401", "marcxml_invalid"),
    (mrrc.JsonError, "E402", "marcjson_invalid"),
    (mrrc.WriterError, "E404", "record_too_large_for_iso2709"),
    (mrrc.BadSubfieldCodeWarning, "W001", "bad_subfield_code_warning"),
]


class TestErrorCodes:
    """Stable error codes. Codes and slugs must never change for an existing
    variant — see ``CONTRIBUTING.md`` for the stability policy.
    """

    @pytest.mark.parametrize("cls, code, slug", _CODE_TABLE)
    def test_class_carries_canonical_code_and_slug(self, cls, code, slug):
        assert cls.code == code, f"{cls.__name__}.code != {code!r}"
        assert cls.slug == slug, f"{cls.__name__}.slug != {slug!r}"

    @pytest.mark.parametrize("cls, code, _slug", _CODE_TABLE)
    def test_help_url_anchors_on_docs_page(self, cls, code, _slug):
        if cls is mrrc.BadSubfieldCodeWarning:
            pytest.skip("BadSubfieldCodeWarning is a warning, not an exception")
        instance = cls()
        from mrrc.exceptions import DOCS_BASE_URL
        assert (
            instance.help_url()
            == f"{DOCS_BASE_URL}/reference/error-codes/#{code}"
        )

    def test_codes_are_unique(self):
        codes = [code for _cls, code, _slug in _CODE_TABLE]
        assert len(codes) == len(set(codes)), "duplicate error codes detected"

    def test_slugs_are_unique(self):
        slugs = [slug for _cls, _code, slug in _CODE_TABLE]
        assert len(slugs) == len(set(slugs)), "duplicate error slugs detected"


# ---------------------------------------------------------------------------
# Bare-constructor + kwarg-only compatibility
# ---------------------------------------------------------------------------


class TestBareConstructor:
    """All exception classes must accept ``raise Foo()`` for pymarc
    bare-constructor compatibility, with every positional kwarg defaulting
    to ``None``.
    """

    @pytest.mark.parametrize(
        "cls",
        [
            mrrc.MrrcException,
            mrrc.RecordLengthInvalid,
            mrrc.RecordLeaderInvalid,
            mrrc.BaseAddressInvalid,
            mrrc.BaseAddressNotFound,
            mrrc.RecordDirectoryInvalid,
            mrrc.EndOfRecordNotFound,
            mrrc.FieldNotFound,
            mrrc.FatalReaderError,
            mrrc.InvalidIndicator,
            mrrc.BadSubfieldCode,
            mrrc.InvalidField,
            mrrc.TruncatedRecord,
            mrrc.EncodingError,
            mrrc.XmlError,
            mrrc.JsonError,
            mrrc.WriterError,
        ],
    )
    def test_bare_construct(self, cls):
        instance = cls()
        assert instance.record_index is None
        assert instance.byte_offset is None
        assert instance.field_tag is None

    def test_unknown_kwarg_raises_type_error(self):
        with pytest.raises(TypeError, match="unexpected keyword"):
            mrrc.InvalidIndicator(not_a_real_field="oops")


# ---------------------------------------------------------------------------
# Pickle round-trip
# ---------------------------------------------------------------------------


class TestPickleRoundTrip:
    def test_round_trip_preserves_all_positional_attrs(self):
        original = mrrc.InvalidIndicator(
            record_index=847,
            record_control_number="ocm01234567",
            field_tag="245",
            indicator_position=1,
            found=b":",
            expected="digit or space",
            byte_offset=7217,
            record_byte_offset=42,
            source="harvest.mrc",
        )
        restored = pickle.loads(pickle.dumps(original))
        assert restored.record_index == 847
        assert restored.record_control_number == "ocm01234567"
        assert restored.field_tag == "245"
        assert restored.indicator_position == 1
        assert restored.found == b":"
        assert restored.expected == "digit or space"
        assert restored.byte_offset == 7217
        assert restored.record_byte_offset == 42
        assert restored.source == "harvest.mrc"

    def test_round_trip_preserves_subclass_extras(self):
        original = mrrc.TruncatedRecord(
            record_index=12,
            expected_length=1024,
            actual_length=640,
        )
        restored = pickle.loads(pickle.dumps(original))
        assert restored.expected_length == 1024
        assert restored.actual_length == 640
        assert restored.record_index == 12

    def test_round_trip_preserves_fatal_reader_error_cap_fields(self):
        original = mrrc.FatalReaderError(
            cap=100,
            errors_seen=101,
            record_index=7,
            source="stream.mrc",
        )
        restored = pickle.loads(pickle.dumps(original))
        assert restored.cap == 100
        assert restored.errors_seen == 101
        assert restored.record_index == 7
        assert restored.source == "stream.mrc"
        d = restored.to_dict()
        assert d["cap"] == 100
        assert d["errors_seen"] == 101

    def test_round_trip_preserves_invalid_field_message(self):
        original = mrrc.InvalidField(
            record_index=5,
            field_tag="245",
            message="exceeds data area",
        )
        restored = pickle.loads(pickle.dumps(original))
        assert restored.message == "exceeds data area"

    def test_round_trip_bare_instance(self):
        original = mrrc.RecordLeaderInvalid()
        restored = pickle.loads(pickle.dumps(original))
        assert restored.record_index is None
        assert restored.byte_offset is None

    def test_setstate_rejects_unexpected_keys(self):
        """Defense-in-depth: restoring a state dict with attribute names
        outside the per-class allowed set must raise rather than blindly
        setattr (which could shadow methods on the instance).
        """
        instance = mrrc.RecordLeaderInvalid()
        with pytest.raises(TypeError, match="unexpected"):
            instance.__setstate__({"_format": "evil_lambda_replacement"})


# ---------------------------------------------------------------------------
# Snapshot tests for str / repr / detailed
# ---------------------------------------------------------------------------


@pytest.fixture
def invalid_indicator_full():
    """Fully-populated InvalidIndicator instance used in several snapshot
    tests; mirrors the Rust-side test fixture so the two sides can be
    cross-compared.
    """
    return mrrc.InvalidIndicator(
        record_index=847,
        record_control_number="ocm01234567",
        field_tag="245",
        indicator_position=1,
        found=b":",
        expected="digit or space",
        byte_offset=7217,
        record_byte_offset=42,
        source="harvest.mrc",
    )


class TestSnapshotFormats:
    def test_str_invalid_indicator_full_context(
        self, invalid_indicator_full, snapshot
    ):
        assert str(invalid_indicator_full) == snapshot

    def test_repr_invalid_indicator_full_context(
        self, invalid_indicator_full, snapshot
    ):
        assert repr(invalid_indicator_full) == snapshot

    def test_detailed_invalid_indicator_full_context(
        self, invalid_indicator_full, snapshot
    ):
        assert invalid_indicator_full.detailed() == snapshot

    def test_str_no_context_falls_back_to_class_name(self, snapshot):
        err = mrrc.BaseAddressNotFound()
        assert str(err) == snapshot

    def test_str_truncated_record(self, snapshot):
        err = mrrc.TruncatedRecord(
            record_index=12,
            record_control_number="oc00000012",
            byte_offset=16384,
            record_byte_offset=128,
            source="partial.mrc",
            expected_length=1024,
            actual_length=640,
        )
        assert str(err) == snapshot

    def test_detailed_truncated_record(self, snapshot):
        err = mrrc.TruncatedRecord(
            record_index=12,
            record_control_number="oc00000012",
            byte_offset=16384,
            record_byte_offset=128,
            source="partial.mrc",
            expected_length=1024,
            actual_length=640,
        )
        assert err.detailed() == snapshot

    def test_str_writer_error(self, snapshot):
        err = mrrc.WriterError(
            record_index=99,
            record_control_number="oc00000099",
            message="Record length exceeds 4GB limit (5000000000 bytes)",
        )
        assert str(err) == snapshot


# ---------------------------------------------------------------------------
# Structured serialization (to_dict / to_json)
# ---------------------------------------------------------------------------


class TestStructuredSerialization:
    """``to_dict()`` / ``to_json()`` produce a versioned schema suitable for
    structured logging platforms. See ``docs/reference/error-handling.md``
    for the full schema contract.
    """

    def test_to_dict_full_schema_invalid_indicator(self):
        err = mrrc.InvalidIndicator(
            record_index=847,
            record_control_number="ocm01234567",
            field_tag="245",
            indicator_position=1,
            found=b":",
            expected="digit or space",
            byte_offset=7217,
            record_byte_offset=42,
            source="harvest.mrc",
        )
        d = err.to_dict()

        # Schema fixed-position keys
        assert d["schema_version"] == 1
        assert d["class"] == "InvalidIndicator"
        assert d["code"] == "E201"
        assert d["slug"] == "invalid_indicator"
        assert d["severity"] == "error"
        assert d["help_url"].endswith("#E201")

        # Positional fields
        assert d["record_index"] == 847
        assert d["record_control_number"] == "ocm01234567"
        assert d["field_tag"] == "245"
        assert d["indicator_position"] == 1
        assert d["expected"] == "digit or space"
        assert d["byte_offset"] == 7217
        assert d["record_byte_offset"] == 42
        assert d["source"] == "harvest.mrc"

        # Bytes get hex-encoded under _hex suffix; original key is null
        assert d["found"] is None
        assert d["found_hex"] == "3a"

        # No cause
        assert d["_cause"] is None

    def test_to_dict_truncated_record_includes_length_extras(self):
        err = mrrc.TruncatedRecord(expected_length=1024, actual_length=640)
        d = err.to_dict()
        assert d["expected_length"] == 1024
        assert d["actual_length"] == 640
        assert d["class"] == "TruncatedRecord"
        assert d["code"] == "E005"

    def test_to_dict_invalid_field_includes_message(self):
        err = mrrc.InvalidField(message="exceeds data area", field_tag="245")
        d = err.to_dict()
        assert d["message"] == "exceeds data area"
        assert d["field_tag"] == "245"

    def test_to_dict_cause_chain_populates_underscore_cause(self):
        try:
            try:
                raise FileNotFoundError("test path")
            except FileNotFoundError as e:
                raise mrrc.WriterError(message="failed") from e
        except mrrc.WriterError as e:
            d = e.to_dict()
        assert d["_cause"] == "test path"

    def test_to_dict_no_cause_chain_yields_null(self):
        err = mrrc.InvalidIndicator()
        assert err.to_dict()["_cause"] is None

    def test_to_dict_include_traceback(self):
        try:
            raise mrrc.InvalidIndicator()
        except mrrc.InvalidIndicator as e:
            d = e.to_dict(include_traceback=True)
        assert "traceback" in d
        assert isinstance(d["traceback"], list)
        assert any("InvalidIndicator" in line for line in d["traceback"])

    def test_to_dict_default_omits_traceback(self):
        try:
            raise mrrc.InvalidIndicator()
        except mrrc.InvalidIndicator as e:
            d = e.to_dict()
        assert "traceback" not in d

    def test_to_json_returns_valid_json_string(self):
        import json

        err = mrrc.InvalidIndicator(
            record_index=1,
            field_tag="245",
            found=b":",
        )
        s = err.to_json()
        parsed = json.loads(s)
        assert parsed["class"] == "InvalidIndicator"
        assert parsed["found_hex"] == "3a"

    def test_to_json_forwards_kwargs_to_json_dumps(self):
        err = mrrc.InvalidIndicator()
        pretty = err.to_json(indent=2)
        # Pretty-printed JSON has newlines
        assert "\n" in pretty

    def test_to_json_include_traceback_routes_to_to_dict(self):
        import json

        try:
            raise mrrc.InvalidIndicator()
        except mrrc.InvalidIndicator as e:
            s = e.to_json(include_traceback=True)
        parsed = json.loads(s)
        assert "traceback" in parsed

    def test_to_dict_payload_size_bounded(self):
        """The schema's bounded-size guarantee depends on `found` being
        capped (32 bytes max via the Rust truncate_bytes helper). Verify
        the Python side honors this when bytes are provided directly.
        """
        # Even if a caller passes a larger bytes value, the resulting
        # found_hex stays small as long as upstream truncation works.
        err = mrrc.InvalidIndicator(found=b"a" * 32)
        d = err.to_dict()
        assert len(d["found_hex"]) == 64  # 32 bytes * 2 hex chars

    def test_to_dict_bytes_near_surfaces_hex_and_offset(self):
        err = mrrc.InvalidIndicator(
            bytes_near=b" :0",
            bytes_near_offset=99,
        )
        d = err.to_dict()
        assert d["bytes_near"] is None
        assert d["bytes_near_hex"] == "203a30"
        assert d["bytes_near_offset"] == 99

    def test_to_dict_bytes_near_null_when_absent(self):
        err = mrrc.InvalidIndicator()
        d = err.to_dict()
        assert d["bytes_near"] is None
        assert "bytes_near_hex" not in d
        assert d["bytes_near_offset"] is None


class TestHexDumpRendering:
    """Python ``MrrcException.detailed()`` renders a hex dump matching the
    Rust ``MarcError::detailed()`` format byte-for-byte when a byte window
    is attached via ``bytes_near`` / ``bytes_near_offset``.
    """

    def test_detailed_includes_hex_dump_when_bytes_near_set(self):
        # 32-byte window: 16 before + 16 after the offending byte at 0x1C31
        window = b"2023nyu         " + b":0\x000 0 eng d\x1e245"
        err = mrrc.InvalidIndicator(
            record_index=847,
            field_tag="245",
            indicator_position=0,
            byte_offset=0x1C31,
            bytes_near=window,
            bytes_near_offset=0x1C21,
        )
        d = err.detailed()
        assert "bytes near offset 0x1C31:" in d, d
        assert "0x1C21:" in d, d
        assert "0x1C31:" in d, d
        assert "^^ offending byte" in d, d
        assert "|2023nyu" in d, d

    def test_detailed_no_dump_when_bytes_near_absent(self):
        err = mrrc.InvalidIndicator(record_index=1)
        assert "bytes near offset" not in err.detailed()

    def test_detailed_no_caret_when_offset_outside_window(self):
        err = mrrc.InvalidIndicator(
            byte_offset=9999,
            bytes_near=b"abcdef",
            bytes_near_offset=0,
        )
        d = err.detailed()
        assert "bytes near offset 0x270F:" in d
        assert "^^ offending byte" not in d

    def test_pickle_preserves_bytes_near(self):
        import pickle

        err = mrrc.InvalidIndicator(
            bytes_near=b" :0",
            bytes_near_offset=99,
        )
        restored = pickle.loads(pickle.dumps(err))
        assert restored.bytes_near == b" :0"
        assert restored.bytes_near_offset == 99


# ---------------------------------------------------------------------------
# FFI integration: Rust → PyO3 → Python typed exceptions with attrs
# ---------------------------------------------------------------------------


def _build_minimal_marc_record(record_type: bytes = b"a") -> bytes:
    """Construct a minimal valid ISO 2709 MARC record with a single 245
    field. Used as a starting point that downstream tests mutate to trigger
    specific MarcError variants.
    """
    field_245 = b"10\x1fa" + b"Test" + b"\x1e"
    directory = b"245" + format(len(field_245), "04d").encode() + b"00000"
    base_address = 24 + len(directory) + 1
    record_length = base_address + len(field_245) + 1
    leader = (
        format(record_length, "05d").encode()
        + b"n"
        + record_type
        + b"m a2"
        + b"2"
        + format(base_address, "05d").encode()
        + b"   4500"
    )
    return leader + directory + b"\x1e" + field_245 + b"\x1d"


class TestFfiTypedExceptions:
    """Drive the Rust → PyO3 → Python conversion path and verify each
    MarcError variant surfaces as the corresponding typed Python exception
    with positional attributes populated.
    """

    def test_truncated_record_surfaces_as_typed_exception_with_byte_counts(self):
        """A truncated record must surface as ``mrrc.TruncatedRecord``
        (a subclass of ``mrrc.EndOfRecordNotFound``) with
        ``expected_length`` and ``actual_length`` populated.

        Note: ``record_index``, ``byte_offset``, and ``source`` are not yet
        populated on this path because the boundary scanner that catches
        truncation does not currently track a records-read counter. That's
        captured in the ``ParseError`` builder methods landed in this PR;
        wiring per-site context tracking is intentionally separate work.
        """
        import io

        full = _build_minimal_marc_record()
        # Truncate the record bytes mid-record
        truncated = full[: len(full) - 10]

        reader = mrrc.MARCReader(io.BytesIO(truncated))
        with pytest.raises(mrrc.EndOfRecordNotFound) as excinfo:
            list(reader)
        err = excinfo.value
        assert isinstance(err, mrrc.TruncatedRecord)
        assert err.expected_length is not None
        assert err.actual_length is not None
        assert err.actual_length < err.expected_length

    def test_invalid_leader_record_length_too_small(self):
        """A leader claiming a record length below 24 is structurally
        invalid; surface as ``mrrc.RecordLeaderInvalid``.
        """
        import io

        leader = b"00010nam a2200025 i 4500"  # record_length=10 < 24
        reader = mrrc.MARCReader(io.BytesIO(leader))
        with pytest.raises(mrrc.MrrcException):
            list(reader)

    def test_wrong_authority_record_type_raises_invalid_field(self):
        """An `AuthorityMARCReader` fed a bibliographic-typed record
        rejects it via a typed exception.
        """
        import io

        bib_record = _build_minimal_marc_record(record_type=b"a")
        reader = mrrc.AuthorityMARCReader(io.BytesIO(bib_record))
        with pytest.raises(mrrc.MrrcException) as excinfo:
            list(reader)
        # Should carry record_index and at least the type-mismatch message
        err = excinfo.value
        assert err.record_index == 1
        # InvalidField surfaces with a message naming the type mismatch
        if isinstance(err, mrrc.InvalidField):
            assert err.message is not None
            assert "authority" in err.message.lower()

    def test_typed_exception_carries_code_slug_help_url(self):
        """An FFI-surfaced typed exception must carry the same `code`,
        `slug`, and `help_url()` values as a Python-constructed instance.
        """
        import io
        from mrrc.exceptions import DOCS_BASE_URL

        leader = b"00010nam a2200025 i 4500"
        reader = mrrc.MARCReader(io.BytesIO(leader))
        with pytest.raises(mrrc.MrrcException) as excinfo:
            list(reader)
        err = excinfo.value
        # Whatever variant fired, code/slug should be populated and the
        # help URL should anchor onto the docs page.
        assert err.code.startswith("E"), err.code
        assert err.slug
        assert (
            err.help_url()
            == f"{DOCS_BASE_URL}/reference/error-codes/#{err.code}"
        )

    def test_pre_parse_error_from_buffered_reader_carries_bytes_near(self):
        """The buffered-reader boundary-scan path (pre-parse validation
        for short record-length headers, record_length < 24, missing
        terminator) previously collapsed its `ParseError` into an
        `InvalidField` with no positional context. The ParseError path
        now carries `bytes_near` / `byte_offset` through to the typed
        Python exception.
        """
        import io

        # record_length=10 < 24 → caught by buffered_reader before the
        # parser runs. bytes_near should contain the 5-byte length header
        # + surrounding leader bytes.
        bad = b"00010nam a2200025 i 4500"
        reader = mrrc.MARCReader(io.BytesIO(bad))
        with pytest.raises(mrrc.MrrcException) as excinfo:
            list(reader)
        err = excinfo.value
        assert err.byte_offset == 0
        assert err.bytes_near is not None
        assert len(err.bytes_near) > 0
        assert b"00010" in err.bytes_near
        assert "bytes near offset" in err.detailed()

    def test_leader_error_carries_bytes_near_via_with_bytes_near(self):
        """Leader errors bypass `ParseContext` (constructed directly in
        `leader.rs`), but the reader enriches them via
        ``MarcError::with_bytes_near`` so `detailed()` still renders a
        hex dump of the offending leader bytes.
        """
        import io

        # record_length=50 clears the buffered_reader's >= 24 guard, but
        # base_address=00010 fails Leader::validate_for_reading.
        bad_leader = b"00050nam a2200010 i 4500" + b"\x00" * 26
        reader = mrrc.MARCReader(io.BytesIO(bad_leader))
        with pytest.raises(mrrc.MrrcException) as excinfo:
            list(reader)
        err = excinfo.value
        assert isinstance(err, mrrc.RecordLeaderInvalid)
        assert err.bytes_near is not None
        assert len(err.bytes_near) > 0
        assert err.byte_offset == 0
        assert err.bytes_near_offset == 0
        d = err.detailed()
        assert "bytes near offset" in d
        assert "^^ offending byte" in d
        assert "00050nam" in d

    def test_ffi_error_carries_bytes_near_window_for_hex_dump(self):
        """End-to-end: a malformed record raised via the `MARCReader` FFI
        path carries a populated `bytes_near` window and a precise
        `byte_offset` that points inside the window — so ``detailed()``
        renders a caret at the actual offending byte rather than column 0
        of the first row.
        """
        import io

        # Directory entry claims field length=9999 — past the data area.
        # Raises `err_invalid_field` with ctx populated, which carries
        # bytes_near captured from the record buffer.
        field_245 = b"10\x1faT\x1e"
        directory = b"245999900000"
        base_address = 24 + len(directory) + 1
        record_length = base_address + len(field_245) + 1
        leader = (
            f"{record_length:05d}".encode()
            + b"nam a22"
            + f"{base_address:05d}".encode()
            + b" i 4500"
        )
        record = leader + directory + b"\x1e" + field_245 + b"\x1d"
        reader = mrrc.MARCReader(io.BytesIO(record))
        with pytest.raises(mrrc.MrrcException) as excinfo:
            list(reader)
        err = excinfo.value
        # bytes_near populated from reader buffer
        assert err.bytes_near is not None, err
        assert len(err.bytes_near) > 0
        # byte_offset falls inside the captured window
        assert err.byte_offset is not None
        assert err.bytes_near_offset is not None
        assert (
            err.bytes_near_offset
            <= err.byte_offset
            <= err.bytes_near_offset + len(err.bytes_near)
        )
        # detailed() renders the hex dump + caret
        d = err.detailed()
        assert "bytes near offset" in d
        assert "^^ offending byte" in d

    def test_to_dict_on_surfaced_exception_carries_positional_fields(self):
        """FFI integration: a Rust parse error surfaced as a Python typed
        exception round-trips through ``to_dict()`` with all positional
        fields populated. Regression guard for the structured-serialization
        path end-to-end.
        """
        import io

        full = _build_minimal_marc_record()
        truncated = full[: len(full) - 10]
        reader = mrrc.MARCReader(io.BytesIO(truncated))
        with pytest.raises(mrrc.MrrcException) as excinfo:
            list(reader)
        d = excinfo.value.to_dict()
        # Schema spine always populated
        assert d["schema_version"] == 1
        assert d["class"]
        assert d["code"].startswith("E")
        assert d["slug"]
        assert d["help_url"].endswith(f"#{d['code']}")
        # Positional fields surface (keys always present; values may be null)
        for key in (
            "record_index",
            "record_control_number",
            "field_tag",
            "byte_offset",
            "record_byte_offset",
            "source",
            "bytes_near",
            "bytes_near_offset",
        ):
            assert key in d, f"missing key {key}"
        # TruncatedRecord carries expected/actual length extras
        if d["class"] == "TruncatedRecord":
            assert d["expected_length"] is not None
            assert d["actual_length"] is not None
        # _cause present; may be None or string
        assert "_cause" in d

    def test_no_silent_drops_in_pyo3_conversion(self):
        """Catch-all: confirm that whatever Rust raises always surfaces as
        an MrrcException subclass (or OSError for I/O), never an
        unhandled bare PyValueError when the input is recognizably a
        MARC parse failure.
        """
        import io

        # Garbage that parses as a leader but has no directory
        garbage = b"00050nam a22000000 i 4500" + b"\x00" * 26
        reader = mrrc.MARCReader(io.BytesIO(garbage))
        try:
            list(reader)
        except mrrc.MrrcException:
            pass  # expected — typed exception class raised
        except OSError:
            pass  # also acceptable — I/O errors map to built-in
        # If neither, the test will fail with the actual exception type
        # in the traceback; that is the diagnostic.