Function seek_to_offset

crates/commitlog/src/segment.rs:485–512

Advances the `segment` reader to the position corresponding to the `start_tx_offset` using the `index_file` for efficient seeking. Input: - `segment` - segment reader - `min_tx_offset` - minimum transaction offset in the segment - `start_tx_offset` - transaction offset to advance to Returns the byte position `segment` is at after seeking.

(
    mut segment: &mut R,
    index_file: &TxOffsetIndex,
    start_tx_offset: u64,
)

///
/// Returns the byte position `segment` is at after seeking.
pub fn seek_to_offset<R: io::Read + io::Seek>(
    mut segment: &mut R,
    index_file: &TxOffsetIndex,
    start_tx_offset: u64,
) -> Result<u64, IndexError> {
    let (index_key, byte_offset) = index_file.key_lookup(start_tx_offset)?;

    // If the index_key is 0, it means the index file is empty, return error without seeking
    if index_key == 0 {
        return Err(IndexError::KeyNotFound);
    }
    debug!("index lookup for key={start_tx_offset}: found key={index_key} at byte-offset={byte_offset}");
    // returned `index_key` should never be greater than `start_tx_offset`
    debug_assert!(index_key <= start_tx_offset);

    // Check if the offset index is pointing to the right commit.
    let hdr = validate_commit_header(&mut segment, byte_offset)?;
    if hdr.min_tx_offset == index_key {
        // Advance the segment Seek if expected commit is found.
        segment.seek(SeekFrom::Start(byte_offset))
    } else {
        Err(io::Error::new(
            io::ErrorKind::InvalidData,
            "mismatched key in offset index file",
        ))
    }
    .map_err(Into::into)
}

/// Try to extract the commit header from the asked position without advancing seek.
/// `IndexFileMut` fsync asynchoronously, which makes it important for reader to verify its entry