Method append

nodedb-wal/src/writer.rs:274–339

Append a record to the WAL. Returns the assigned LSN. The record is written to the in-memory buffer. Call `sync()` to flush to disk and make the write durable. `database_id` is stored in header bytes 34-41. Pass `0` for the default database (backward-compatible with pre-existing records).

(
        &mut self,
        record_type: u32,
        tenant_id: u64,
        vshard_id: u32,
        database_id: u64,
        payload: &[u8],
    )

    /// `database_id` is stored in header bytes 34-41. Pass `0` for the
    /// default database (backward-compatible with pre-existing records).
    pub fn append(
        &mut self,
        record_type: u32,
        tenant_id: u64,
        vshard_id: u32,
        database_id: u64,
        payload: &[u8],
    ) -> Result<u64> {
        if self.sealed {
            return Err(WalError::Sealed);
        }

        let lsn = self.next_lsn.fetch_add(1, Ordering::Relaxed);
        let preamble_bytes = self.segment_preamble.as_ref().map(|p| p.to_bytes());
        let record = WalRecord::new(
            record_type,
            lsn,
            tenant_id,
            vshard_id,
            database_id,
            payload.to_vec(),
            self.encryption_ring.as_ref().map(|r| r.current()),
            preamble_bytes.as_ref(),
        )?;

        // Write to double-write buffer (deferred — no fsync yet).
        // The DWB is fsynced in batch during `sync()`, before the WAL fsync.
        // This amortizes DWB fsync cost across the entire group commit batch.
        //
        // DWB failure is non-fatal for the write itself (the WAL is the
        // authoritative store), but we log a warning because it means
        // torn-write recovery is degraded. If the DWB is persistently
        // broken, we detach it to avoid repeated error noise.
        if let Some(dwb) = &mut self.double_write
            && let Err(e) = dwb.write_record_deferred(&record)
        {
            tracing::warn!(
                lsn = lsn,
                error = %e,
                "DWB write failed — torn-write protection degraded, detaching DWB"
            );
            self.double_write = None;
        }

        let header_bytes = record.header.to_bytes();
        let total_size = HEADER_SIZE + record.payload.len();

        // If this record doesn't fit in the remaining buffer, flush first.
        if self.buffer.remaining() < total_size {
            self.flush_buffer()?;
        }

        // If the record is larger than the entire buffer, we have a problem.
        // This shouldn't happen with MAX_WAL_PAYLOAD_SIZE checks, but guard anyway.
        if total_size > self.buffer.capacity() {
            return Err(WalError::PayloadTooLarge {
                size: record.payload.len(),
                max: self.buffer.capacity() - HEADER_SIZE,