Method backpressure

src/db/wal-valve.ts:174–193

* Writer-side backstop, called at a between-transactions boundary. Returns * null (no wait) while growth is under the hard cap; past it, returns a * promise that resolves only once a FULL backfill has landed — see the * header comment for why a single pass is not enough on a saturated disk.

()

   * header comment for why a single pass is not enough on a saturated disk.
   */
  backpressure(): Promise<void> | null {
    if (this.pause) return this.pause;
    if (Date.now() < this.futileUntil) return null; // pinned reader — parking is churn, not progress
    // Two independent triggers:
    //  - growth: un-backfilled BACKLOG past the hard cap (the original valve).
    //  - file size: a WAL can stay fully backfilled and still grow without
    //    bound — the writer only restarts at frame 0 if a commit finds no
    //    reader marks, which §7a.1's instrumented run showed never happens
    //    in practice (file marched 361→721MB through two COMPLETE
    //    backfills). Past the file cap, park and TRUNCATE at the barrier —
    //    the backfill part is instant when the backlog is already folded.
    if (this.growthBytes() <= this.hardBytes && this.db.getWalSizeBytes() <= this.fileCapBytes) return null;
    this.log(`backpressure: wal=${this.mb(this.db.getWalSizeBytes())} baseline=${this.mb(this.sizeAtLastFullBackfill)} — pausing writer for full backfill`);
    const t0 = Date.now();
    this.pause = this.backfillFully().finally(() => {
      this.pause = null;
      this.log(`backpressure released after ${Date.now() - t0}ms: wal=${this.mb(this.db.getWalSizeBytes())} baseline=${this.mb(this.sizeAtLastFullBackfill)}`);
    });
    return this.pause;
  }

  /** Await any in-flight checkpoint and writer pause. */
  async drain(): Promise<void> {