Function try_read_preamble

nodedb-wal/src/reader.rs:171–195

Attempt to read a WAL segment preamble from the start of a file. Returns `(Some(preamble), PREAMBLE_SIZE)` if a valid `WALP` preamble is found, or `(None, 0)` if the file does not start with the preamble magic (unencrypted segment — seek back to 0).

(file: &mut File)

/// found, or `(None, 0)` if the file does not start with the preamble magic
/// (unencrypted segment — seek back to 0).
fn try_read_preamble(file: &mut File) -> Result<(Option<SegmentPreamble>, u64)> {
    use std::io::Seek;

    let mut buf = [0u8; PREAMBLE_SIZE];
    match file.read_exact(&mut buf) {
        Ok(()) => {}
        Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => {
            // File is too short to hold a preamble — no preamble present.
            file.seek(std::io::SeekFrom::Start(0))?;
            return Ok((None, 0));
        }
        Err(e) => return Err(WalError::Io(e)),
    }

    if buf[0..4] == WAL_PREAMBLE_MAGIC {
        // Parse and validate the preamble. An unsupported version is a hard
        // error — do not silently fall through to record scanning.
        let preamble = SegmentPreamble::from_bytes(&buf, &WAL_PREAMBLE_MAGIC)?;
        Ok((Some(preamble), PREAMBLE_SIZE as u64))
    } else {
        // First bytes are not the preamble magic — rewind and read as records.
        file.seek(std::io::SeekFrom::Start(0))?;
        Ok((None, 0))
    }
}

/// Iterator over WAL records.
pub struct WalRecordIter {