Function scanPreBoundaryMetadata

src/utils/sessionStorage.ts:3251–3318

* Lightweight forward scan of [0, endOffset) collecting only metadata-entry lines. * Uses raw Buffer chunks and byte-level marker matching — no readline, no per-line * string conversion for the ~99% of lines that are message content. * * Fast path: if a chunk contains zero markers (the common ca

(
  filePath: string,
  endOffset: number,
)

 * are <50 per session), the entire chunk is skipped without line splitting.
 */
async function scanPreBoundaryMetadata(
  filePath: string,
  endOffset: number,
): Promise<string[]> {
  const { createReadStream } = await import('fs')
  const NEWLINE = 0x0a

  const stream = createReadStream(filePath, { end: endOffset - 1 })
  const metadataLines: string[] = []
  let carry: Buffer | null = null

  for await (const chunk of stream) {
    const chunkBuf = chunk as Buffer
    const buf = resolveMetadataBuf(carry, chunkBuf)
    if (buf === null) {
      carry = null
      continue
    }

    // Fast path: most chunks contain zero metadata markers. Skip line splitting.
    let hasAnyMarker = false
    for (const m of METADATA_MARKER_BUFS) {
      if (buf.includes(m)) {
        hasAnyMarker = true
        break
      }
    }

    if (hasAnyMarker) {
      let lineStart = 0
      let nl = buf.indexOf(NEWLINE)
      while (nl !== -1) {
        // Bounded marker check: only look within this line's byte range
        for (const m of METADATA_MARKER_BUFS) {
          const mIdx = buf.indexOf(m, lineStart)
          if (mIdx !== -1 && mIdx < nl) {
            metadataLines.push(buf.toString('utf-8', lineStart, nl))
            break
          }
        }
        lineStart = nl + 1
        nl = buf.indexOf(NEWLINE, lineStart)
      }
      carry = buf.subarray(lineStart)
    } else {
      // No markers in this chunk — just preserve the incomplete trailing line
      const lastNl = buf.lastIndexOf(NEWLINE)
      carry = lastNl >= 0 ? buf.subarray(lastNl + 1) : buf
    }

    // Guard against quadratic carry growth for pathological huge lines
    // (e.g., a 10 MB tool-output line with no newline). Real metadata entries
    // are <1 KB, so if carry exceeds this we're mid-message-content — drop it.
    if (carry.length > 64 * 1024) carry = null
  }

  // Final incomplete line (no trailing newline at endOffset)
  if (carry !== null && carry.length > 0) {