crabka_protocol/records/

borrowed.rs

//! Borrowed `RecordBatch<'a>`, `Record<'a>`, and `RecordHeader<'a>`.

use bytes::Bytes;
use zerocopy::FromBytes as _;

use crate::primitives::varint::{get_varint, get_varlong};
use crate::records::RecordsError;
use crate::records::crc::{crc32c, crc32c_append};
use crate::records::header::{Attributes, HEADER_LEN, RecordBatchHeader};

// batch_length field semantics: bytes after itself.
// Header tail = partition_leader_epoch(4) + magic(1) + crc(4) +
//   attributes(2) + last_offset_delta(4) + base_timestamp(8) +
//   max_timestamp(8) + producer_id(8) + producer_epoch(2) +
//   base_sequence(4) + records_count(4) = 49 bytes.
const HEADER_TAIL_LEN: i32 = 49;

pub struct RecordBatch<'a> {
    pub(crate) header: &'a RecordBatchHeader,
    pub(crate) body: RecordBody<'a>,
}

pub(crate) enum RecordBody<'a> {
    Borrowed(&'a [u8]),
    Owned(Bytes),
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Record<'a> {
    pub attributes: i8,
    pub timestamp_delta: i64,
    pub offset_delta: i32,
    pub key: Option<&'a [u8]>,
    pub value: Option<&'a [u8]>,
    pub headers: Vec<RecordHeader<'a>>,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct RecordHeader<'a> {
    pub key: &'a str,
    pub value: Option<&'a [u8]>,
}

impl RecordBatch<'_> {
    #[must_use]
    pub fn header(&self) -> &RecordBatchHeader {
        self.header
    }

    #[must_use]
    pub fn attributes(&self) -> Attributes {
        Attributes(self.header.attributes.get())
    }
}

impl<'a> Default for RecordBatch<'a> {
    /// Returns an empty record batch with a zeroed header and no records body.
    /// Intended for use in generated `Default` impls and round-trip tests; not
    /// suitable for constructing a real Kafka record batch.
    fn default() -> Self {
        use zerocopy::FromZeros as _;
        // RecordBatchHeader derives zerocopy::FromZeros (via FromBytes), so zeroing is safe.
        let header: &'a RecordBatchHeader = Box::leak(Box::new(RecordBatchHeader::new_zeroed()));
        Self {
            header,
            body: RecordBody::Owned(bytes::Bytes::new()),
        }
    }
}

// ── Decode ────────────────────────────────────────────────────────────────────

impl<'de> crate::DecodeBorrow<'de> for RecordBatch<'de> {
    fn decode_borrow(buf: &mut &'de [u8], _version: i16) -> Result<Self, crate::ProtocolError> {
        decode_borrow_impl(buf).map_err(Into::into)
    }
}

fn decode_borrow_impl<'de>(buf: &mut &'de [u8]) -> Result<RecordBatch<'de>, RecordsError> {
    if buf.len() < HEADER_LEN {
        return Err(RecordsError::HeaderTooShort {
            needed: HEADER_LEN - buf.len(),
        });
    }
    // Split off the header slice — both slices remain tied to 'de.
    let (hdr_slice, rest) = buf.split_at(HEADER_LEN);
    let hdr: &'de RecordBatchHeader =
        RecordBatchHeader::ref_from_bytes(hdr_slice).map_err(|_| RecordsError::ZerocopyFailure)?;
    if hdr.magic != 2 {
        return Err(RecordsError::UnsupportedMagic { found: hdr.magic });
    }

    let body_len = i32::checked_sub(hdr.batch_length.get(), HEADER_TAIL_LEN)
        .and_then(|n| usize::try_from(n).ok())
        .ok_or_else(|| RecordsError::RecordParse("negative or oversized batch_length".into()))?;

    if rest.len() < body_len {
        return Err(RecordsError::BodyTooShort {
            needed: body_len - rest.len(),
        });
    }
    let (raw_body, after) = rest.split_at(body_len);
    *buf = after;

    // CRC: hash header[21..HEADER_LEN] (attributes through records_count)
    // then append the raw_body bytes.
    let expected = hdr.crc.get();
    let mut computed = crc32c(&hdr_slice[21..HEADER_LEN]);
    computed = crc32c_append(computed, raw_body);
    if computed != expected {
        return Err(RecordsError::CrcMismatch { expected, computed });
    }

    let attributes = Attributes(hdr.attributes.get());
    let codec = attributes.compression();
    let body = if codec == crabka_compression::CompressionType::None {
        RecordBody::Borrowed(raw_body)
    } else {
        // Bound decompressed output: generous vs. legit ratios, but finite.
        // A small compressed batch must not be able to expand to gigabytes and
        // OOM the broker (decompression bomb).
        const DECOMPRESS_MIN_CAP: usize = 16 * 1024 * 1024; // 16 MiB floor (small inputs)
        const DECOMPRESS_MAX_RATIO: usize = 100; // ≤100x the compressed size
        const DECOMPRESS_ABSOLUTE_CEILING: usize = 1024 * 1024 * 1024; // 1 GiB hard ceiling
        let max_output = raw_body
            .len()
            .saturating_mul(DECOMPRESS_MAX_RATIO)
            .clamp(DECOMPRESS_MIN_CAP, DECOMPRESS_ABSOLUTE_CEILING);
        let decompressed = crabka_compression::decompress(codec, raw_body, max_output)?;
        RecordBody::Owned(decompressed)
    };

    Ok(RecordBatch { header: hdr, body })
}

/// A single complete v2 batch located within a larger buffer, together
/// with its already-validated header. Returned by
/// [`validate_one_v2_batch`].
#[derive(Debug)]
pub struct ValidatedBatch<'a> {
    /// The fixed 61-byte header, reinterpreted in place (zero-copy).
    pub header: &'a RecordBatchHeader,
    /// Total on-disk/wire length of this batch in bytes
    /// (`12 + batch_length`), i.e. header + body.
    pub total_len: usize,
}

/// Validate exactly one v2 record batch at the start of `buf` **without
/// materializing any records or decompressing the body**.
///
/// This is the produce passthrough fast path: it reinterprets the fixed
/// header in place (zero-copy), checks `magic == 2`, and verifies the
/// producer's CRC over `header[21..61] ++ raw_body` (the compressed body
/// bytes, exactly as stored). Nothing in the body is parsed or
/// decompressed, so the cost is one CRC pass over the bytes already in
/// cache — no allocation.
///
/// Returns the validated header (for offset stamping / idempotent &
/// transactional gating) and the batch's total byte length so the caller
/// can slice out the verbatim bytes.
///
/// # Errors
///
/// - [`RecordsError::HeaderTooShort`] / [`RecordsError::BodyTooShort`]
///   when `buf` does not contain a whole batch.
/// - [`RecordsError::UnsupportedMagic`] for a non-v2 batch.
/// - [`RecordsError::CrcMismatch`] when the stored CRC does not match.
pub fn validate_one_v2_batch(buf: &[u8]) -> Result<ValidatedBatch<'_>, RecordsError> {
    if buf.len() < HEADER_LEN {
        return Err(RecordsError::HeaderTooShort {
            needed: HEADER_LEN - buf.len(),
        });
    }
    let (hdr_slice, rest) = buf.split_at(HEADER_LEN);
    let hdr: &RecordBatchHeader =
        RecordBatchHeader::ref_from_bytes(hdr_slice).map_err(|_| RecordsError::ZerocopyFailure)?;
    if hdr.magic != 2 {
        return Err(RecordsError::UnsupportedMagic { found: hdr.magic });
    }
    let body_len = i32::checked_sub(hdr.batch_length.get(), HEADER_TAIL_LEN)
        .and_then(|n| usize::try_from(n).ok())
        .ok_or_else(|| RecordsError::RecordParse("negative or oversized batch_length".into()))?;
    if rest.len() < body_len {
        return Err(RecordsError::BodyTooShort {
            needed: body_len - rest.len(),
        });
    }
    let raw_body = &rest[..body_len];

    let expected = hdr.crc.get();
    let mut computed = crc32c(&hdr_slice[21..HEADER_LEN]);
    computed = crc32c_append(computed, raw_body);
    if computed != expected {
        return Err(RecordsError::CrcMismatch { expected, computed });
    }

    Ok(ValidatedBatch {
        header: hdr,
        total_len: HEADER_LEN + body_len,
    })
}

/// Sum the `records_count` header field of every concatenated v2 batch in
/// `buf` without decompressing or parsing any record body.
///
/// This is intentionally a best-effort metrics helper: non-v2 input returns
/// 0, malformed/truncated input contributes every complete v2 batch before the
/// first bad boundary, and negative `records_count` contributes 0. Real append
/// validation still goes through [`validate_one_v2_batch`].
#[must_use]
pub fn count_records_in_v2_batches(buf: &[u8]) -> u64 {
    // Magic byte sits at offset 16; only v2 (magic == 2) carries a
    // `records_count` header. Legacy slices contribute 0 here.
    const MAGIC_OFFSET: usize = 16;
    if buf.len() <= MAGIC_OFFSET || buf[MAGIC_OFFSET] != 2 {
        return 0;
    }

    let mut total = 0u64;
    let mut remaining = buf;
    while remaining.len() >= HEADER_LEN {
        if remaining[MAGIC_OFFSET] != 2 {
            break;
        }
        let Ok(hdr) = RecordBatchHeader::ref_from_bytes(&remaining[..HEADER_LEN]) else {
            break;
        };
        let Ok(after_len) = usize::try_from(hdr.batch_length.get()) else {
            break;
        };
        let total_len = 12 + after_len;
        if total_len < HEADER_LEN || total_len > remaining.len() {
            break;
        }
        total += u64::try_from(hdr.records_count.get().max(0)).unwrap_or(0);
        remaining = &remaining[total_len..];
    }
    total
}

// ── Iteration ─────────────────────────────────────────────────────────────────

impl RecordBatch<'_> {
    /// Iterate over records, parsing each lazily.
    ///
    /// The returned `Record<'b>` items borrow from `self` (lifetime `'b`),
    /// not from the original input buffer. For uncompressed batches the
    /// backing memory is the input buffer; for compressed batches it is the
    /// batch's internal decompressed `Bytes`.
    pub fn iter(&self) -> RecordIter<'_> {
        let body: &[u8] = match &self.body {
            RecordBody::Borrowed(s) => s,
            RecordBody::Owned(b) => b.as_ref(),
        };
        #[allow(clippy::cast_sign_loss)] // guarded by .max(0) above
        let count = self.header.records_count.get().max(0) as usize;
        RecordIter {
            remaining: body,
            count,
            index: 0,
        }
    }
}

impl<'a> IntoIterator for &'a RecordBatch<'_> {
    type Item = Result<Record<'a>, RecordsError>;
    type IntoIter = RecordIter<'a>;

    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

pub struct RecordIter<'a> {
    remaining: &'a [u8],
    count: usize,
    index: usize,
}

impl<'a> Iterator for RecordIter<'a> {
    type Item = Result<Record<'a>, RecordsError>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.index >= self.count {
            return None;
        }
        self.index += 1;
        Some(parse_one_record(&mut self.remaining))
    }
}

#[inline]
pub(crate) fn parse_one_record<'a>(buf: &mut &'a [u8]) -> Result<Record<'a>, RecordsError> {
    let body_len =
        get_varlong(buf).map_err(|e| RecordsError::RecordParse(format!("record length: {e}")))?;
    let body_len = usize::try_from(body_len).map_err(|_| {
        RecordsError::RecordParse(format!("record length negative or too large: {body_len}"))
    })?;
    if buf.len() < body_len {
        return Err(RecordsError::BodyTooShort {
            needed: body_len - buf.len(),
        });
    }
    let (body, rest) = buf.split_at(body_len);
    *buf = rest;
    let mut body_cur = body;
    let r = parse_body(&mut body_cur)?;
    if !body_cur.is_empty() {
        return Err(RecordsError::RecordParse(format!(
            "trailing bytes inside record (left={})",
            body_cur.len()
        )));
    }
    Ok(r)
}

fn parse_body<'a>(buf: &mut &'a [u8]) -> Result<Record<'a>, RecordsError> {
    if buf.is_empty() {
        return Err(RecordsError::RecordParse("record body empty".into()));
    }
    #[allow(clippy::cast_possible_wrap)] // intentional: Kafka attributes are i8 on the wire
    let attributes = buf[0] as i8;
    *buf = &buf[1..];
    let timestamp_delta =
        get_varlong(buf).map_err(|e| RecordsError::RecordParse(format!("timestamp_delta: {e}")))?;
    let offset_delta =
        get_varint(buf).map_err(|e| RecordsError::RecordParse(format!("offset_delta: {e}")))?;

    let key = read_nullable_slice(buf, "key")?;
    let value = read_nullable_slice(buf, "value")?;

    let header_count =
        get_varint(buf).map_err(|e| RecordsError::RecordParse(format!("header_count: {e}")))?;
    if header_count < 0 {
        return Err(RecordsError::RecordParse(format!(
            "negative header count {header_count}"
        )));
    }
    // Bound pre-allocation: a record header is at least 1 byte on the wire, so
    // an honest `header_count` can never exceed the bytes left in the record
    // body. Clamp the capacity hint to reject huge declared counts.
    #[allow(clippy::cast_sign_loss)] // guarded by < 0 check above
    let mut headers = Vec::with_capacity((header_count as usize).min(buf.len()));
    for i in 0..header_count {
        let key_len = get_varint(buf)
            .map_err(|e| RecordsError::RecordParse(format!("header[{i}] key length: {e}")))?;
        if key_len < 0 {
            return Err(RecordsError::RecordParse(format!(
                "header[{i}] negative key length"
            )));
        }
        #[allow(clippy::cast_sign_loss)] // guarded by < 0 check above
        let n = key_len as usize;
        if buf.len() < n {
            return Err(RecordsError::BodyTooShort {
                needed: n - buf.len(),
            });
        }
        let (key_bytes, rest) = buf.split_at(n);
        *buf = rest;
        let key_str = std::str::from_utf8(key_bytes)
            .map_err(|e| RecordsError::RecordParse(format!("header[{i}] key utf-8: {e}")))?;

        let value = read_nullable_slice(buf, &format!("header[{i}] value"))?;
        headers.push(RecordHeader {
            key: key_str,
            value,
        });
    }

    Ok(Record {
        attributes,
        timestamp_delta,
        offset_delta,
        key,
        value,
        headers,
    })
}

fn read_nullable_slice<'a>(
    buf: &mut &'a [u8],
    label: &str,
) -> Result<Option<&'a [u8]>, RecordsError> {
    let len =
        get_varint(buf).map_err(|e| RecordsError::RecordParse(format!("{label} length: {e}")))?;
    if len < 0 {
        Ok(None)
    } else {
        #[allow(clippy::cast_sign_loss)] // guarded by < 0 check above
        let n = len as usize;
        if buf.len() < n {
            return Err(RecordsError::BodyTooShort {
                needed: n - buf.len(),
            });
        }
        let (head, rest) = buf.split_at(n);
        *buf = rest;
        Ok(Some(head))
    }
}

// ── to_owned bridge ───────────────────────────────────────────────────────────

impl RecordBatch<'_> {
    /// Materialise an owned `RecordBatch` by copying every byte slice into
    /// `Bytes` / `String`.
    pub fn to_owned(&self) -> Result<super::owned::RecordBatch, RecordsError> {
        let mut records = Vec::new();
        for r in self {
            let r = r?;
            records.push(super::owned::Record {
                attributes: r.attributes,
                timestamp_delta: r.timestamp_delta,
                offset_delta: r.offset_delta,
                key: r.key.map(Bytes::copy_from_slice),
                value: r.value.map(Bytes::copy_from_slice),
                headers: r
                    .headers
                    .into_iter()
                    .map(|h| super::owned::RecordHeader {
                        key: h.key.to_string(),
                        value: h.value.map(Bytes::copy_from_slice),
                    })
                    .collect(),
            });
        }
        Ok(super::owned::RecordBatch {
            base_offset: self.header.base_offset.get(),
            partition_leader_epoch: self.header.partition_leader_epoch.get(),
            attributes: self.attributes(),
            last_offset_delta: self.header.last_offset_delta.get(),
            base_timestamp: self.header.base_timestamp.get(),
            max_timestamp: self.header.max_timestamp.get(),
            producer_id: self.header.producer_id.get(),
            producer_epoch: self.header.producer_epoch.get(),
            base_sequence: self.header.base_sequence.get(),
            records,
        })
    }
}

// ── Debug / Clone / PartialEq / Eq ────────────────────────────────────────────
//
// RecordBatch<'a> holds a header reference and a body slice/bytes, so we can't
// #[derive] these. We provide hand-rolled impls that delegate to the owned type.

impl std::fmt::Debug for RecordBatch<'_> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self.to_owned() {
            Ok(o) => o.fmt(f),
            Err(e) => write!(f, "RecordBatch(<decode error: {e}>)"),
        }
    }
}

impl Clone for RecordBatch<'_> {
    /// Shallow clone: both `header` and `body` share the same underlying
    /// data as `self`.  For a `Borrowed` body this is a reference copy;
    /// for an `Owned` body, `Bytes::clone` is a cheap reference-count bump.
    fn clone(&self) -> Self {
        RecordBatch {
            header: self.header,
            body: match &self.body {
                RecordBody::Borrowed(s) => RecordBody::Borrowed(s),
                RecordBody::Owned(b) => RecordBody::Owned(b.clone()),
            },
        }
    }
}

impl PartialEq for RecordBatch<'_> {
    fn eq(&self, other: &Self) -> bool {
        match (self.to_owned(), other.to_owned()) {
            (Ok(a), Ok(b)) => a == b,
            _ => false,
        }
    }
}

impl Eq for RecordBatch<'_> {}

// ── Encode trait impl ─────────────────────────────────────────────────────────

impl crate::Encode for RecordBatch<'_> {
    fn encode<B: bytes::BufMut>(
        &self,
        buf: &mut B,
        version: i16,
    ) -> Result<(), crate::ProtocolError> {
        let owned = self.to_owned().map_err(crate::ProtocolError::from)?;
        crate::Encode::encode(&owned, buf, version)
    }

    fn encoded_len(&self, version: i16) -> usize {
        match self.to_owned() {
            Ok(o) => crate::Encode::encoded_len(&o, version),
            Err(_) => 0,
        }
    }
}

// ── Tests ─────────────────────────────────────────────────────────────────────

#[cfg(test)]
mod tests {
    use super::*;
    use crate::DecodeBorrow;
    use assert2::assert;
    use bytes::BytesMut;
    use crabka_compression::CompressionType;

    fn encode_owned_then_borrow(b: &super::super::owned::RecordBatch) -> Vec<u8> {
        let mut buf = BytesMut::new();
        b.encode(&mut buf).unwrap();
        buf.to_vec()
    }

    macro_rules! borrowed_roundtrip {
        ($name:ident, $codec:expr) => {
            #[test]
            fn $name() {
                let mut owned = super::super::owned::RecordBatch::default();
                owned.attributes = owned.attributes.with_compression($codec);
                owned.records.push(super::super::owned::Record {
                    key: Some(Bytes::from_static(b"key")),
                    value: Some(Bytes::from_static(b"value")),
                    ..Default::default()
                });

                let encoded = encode_owned_then_borrow(&owned);
                let mut cur: &[u8] = &encoded[..];
                let borrowed = RecordBatch::decode_borrow(&mut cur, 0).unwrap();
                assert!(cur.is_empty());
                assert!(borrowed.attributes() == owned.attributes);

                let records: Vec<_> = borrowed.iter().collect::<Result<_, _>>().unwrap();
                assert!(records.len() == 1);
                assert!(records[0].key == Some(b"key".as_slice()));
                assert!(records[0].value == Some(b"value".as_slice()));

                let back_owned = borrowed.to_owned().unwrap();
                assert!(back_owned == owned);
            }
        };
    }

    borrowed_roundtrip!(roundtrip_none, CompressionType::None);
    borrowed_roundtrip!(roundtrip_gzip, CompressionType::Gzip);
    borrowed_roundtrip!(roundtrip_snappy, CompressionType::Snappy);
    borrowed_roundtrip!(roundtrip_lz4, CompressionType::Lz4);
    borrowed_roundtrip!(roundtrip_zstd, CompressionType::Zstd);

    #[test]
    fn zero_copy_for_uncompressed() {
        // Pointer-identity: record key/value slices must point into the
        // input buffer for uncompressed batches.
        let mut owned = super::super::owned::RecordBatch::default();
        owned.records.push(super::super::owned::Record {
            key: Some(Bytes::from_static(b"k")),
            value: Some(Bytes::from_static(b"v")),
            ..Default::default()
        });
        let encoded = encode_owned_then_borrow(&owned);
        let encoded_start = encoded.as_ptr() as usize;
        let encoded_end = encoded_start + encoded.len();

        let mut cur: &[u8] = &encoded[..];
        let borrowed = RecordBatch::decode_borrow(&mut cur, 0).unwrap();
        let records: Vec<_> = borrowed.iter().collect::<Result<_, _>>().unwrap();

        let v_ptr = records[0].value.unwrap().as_ptr() as usize;
        assert!(
            v_ptr >= encoded_start && v_ptr < encoded_end,
            "value slice does not point into the input buffer: \
             input range [{encoded_start:#x}, {encoded_end:#x}), value ptr {v_ptr:#x}",
        );
    }

    #[test]
    fn validate_one_v2_batch_reads_header_and_len() {
        let mut owned = super::super::owned::RecordBatch {
            base_offset: 7,
            partition_leader_epoch: 3,
            last_offset_delta: 0,
            producer_id: 99,
            producer_epoch: 1,
            base_sequence: 5,
            max_timestamp: 1_234,
            ..Default::default()
        };
        owned.records.push(super::super::owned::Record {
            value: Some(Bytes::from_static(b"payload")),
            ..Default::default()
        });
        let encoded = encode_owned_then_borrow(&owned);

        let v = validate_one_v2_batch(&encoded).unwrap();
        assert!(v.total_len == encoded.len());
        assert!(v.header.base_offset.get() == 7);
        assert!(v.header.partition_leader_epoch.get() == 3);
        assert!(v.header.producer_id.get() == 99);
        assert!(v.header.producer_epoch.get() == 1);
        assert!(v.header.base_sequence.get() == 5);
        assert!(v.header.max_timestamp.get() == 1_234);
        assert!(v.header.magic == 2);
    }

    #[test]
    fn validate_one_v2_batch_rejects_corrupt_crc() {
        let owned = super::super::owned::RecordBatch {
            records: vec![super::super::owned::Record {
                value: Some(Bytes::from_static(b"x")),
                ..Default::default()
            }],
            ..Default::default()
        };
        let mut encoded = encode_owned_then_borrow(&owned);
        // Flip a body byte (after the 61-byte header) → CRC mismatch.
        encoded[HEADER_LEN] ^= 0xFF;
        let err = validate_one_v2_batch(&encoded).unwrap_err();
        assert!(matches!(err, RecordsError::CrcMismatch { .. }));
    }

    #[test]
    fn validate_one_v2_batch_rejects_truncated() {
        let owned = super::super::owned::RecordBatch {
            records: vec![super::super::owned::Record {
                value: Some(Bytes::from_static(b"value")),
                ..Default::default()
            }],
            ..Default::default()
        };
        let encoded = encode_owned_then_borrow(&owned);
        let err = validate_one_v2_batch(&encoded[..encoded.len() - 2]).unwrap_err();
        assert!(matches!(err, RecordsError::BodyTooShort { .. }));
    }

    #[test]
    fn count_records_in_v2_batches_counts_single_and_concatenated_batches() {
        let one = encode_owned_then_borrow(&super::super::owned::RecordBatch {
            records: vec![super::super::owned::Record::default()],
            ..Default::default()
        });
        let three = encode_owned_then_borrow(&super::super::owned::RecordBatch {
            last_offset_delta: 2,
            records: vec![
                super::super::owned::Record::default(),
                super::super::owned::Record::default(),
                super::super::owned::Record::default(),
            ],
            ..Default::default()
        });
        let mut both = Vec::with_capacity(one.len() + three.len());
        both.extend_from_slice(&one);
        both.extend_from_slice(&three);

        assert!(count_records_in_v2_batches(&one) == 1);
        assert!(count_records_in_v2_batches(&both) == 4);
    }

    #[test]
    fn count_records_in_v2_batches_stops_at_bad_or_non_v2_input() {
        assert!(count_records_in_v2_batches(&[]) == 0);
        assert!(count_records_in_v2_batches(&[0u8; HEADER_LEN]) == 0);

        let encoded = encode_owned_then_borrow(&super::super::owned::RecordBatch {
            records: vec![super::super::owned::Record::default()],
            ..Default::default()
        });
        let mut with_truncated_tail = encoded.clone();
        with_truncated_tail.extend_from_slice(&encoded[..HEADER_LEN - 1]);

        assert!(count_records_in_v2_batches(&with_truncated_tail) == 1);
    }

    /// Build a bare 61-byte v2 batch header (magic == 2, all other fields zero)
    /// with the given big-endian `batch_length` and `records_count`. The helper
    /// drives `count_records_in_v2_batches`'s `total_len` boundary checks
    /// precisely: `records_count` is read straight from the header, so the body
    /// need not contain any real records.
    fn v2_header_only(batch_length: i32, records_count: i32) -> Vec<u8> {
        let mut buf = vec![0u8; HEADER_LEN];
        buf[16] = 2; // magic == v2
        buf[8..12].copy_from_slice(&batch_length.to_be_bytes());
        buf[57..61].copy_from_slice(&records_count.to_be_bytes());
        buf
    }

    #[test]
    fn count_records_in_v2_batches_accepts_batch_that_exactly_fills_buffer() {
        // total_len = 12 + 49 = 61 = HEADER_LEN, exactly filling the slice, so
        // the batch is accepted and its header count is summed. Guards the
        // `total_len < HEADER_LEN` lower bound: a `<`→`==`/`<=` mutation would
        // wrongly break here and report 0.
        let buf = v2_header_only(49, 7);
        assert!(buf.len() == HEADER_LEN);
        assert!(count_records_in_v2_batches(&buf) == 7);
    }

    #[test]
    fn count_records_in_v2_batches_rejects_batch_overrunning_buffer() {
        // total_len = 12 + 100 = 112 > 61 bytes available, so the batch is
        // rejected and contributes nothing. Guards the
        // `total_len > remaining.len()` arm: an `||`→`&&` mutation would skip
        // the break and index past the buffer.
        let buf = v2_header_only(100, 9);
        assert!(count_records_in_v2_batches(&buf) == 0);
    }

    #[test]
    fn borrowed_encode_via_trait_roundtrips() {
        use crate::Encode as _;
        let owned_in = super::super::owned::RecordBatch {
            records: vec![super::super::owned::Record {
                key: Some(Bytes::from_static(b"x")),
                value: Some(Bytes::from_static(b"y")),
                ..Default::default()
            }],
            ..Default::default()
        };
        let bytes_in = encode_owned_then_borrow(&owned_in);
        let mut cur: &[u8] = &bytes_in[..];
        let borrowed = RecordBatch::decode_borrow(&mut cur, 0).unwrap();

        let mut out = BytesMut::new();
        borrowed.encode(&mut out, 0).unwrap();
        assert!(&out[..] == &bytes_in[..]);
    }
}