spb/agent-os

1/**2 * Readers/writer gate of one run context. Steps hold it shared while they execute; a suspension3 * (durable wait, timer, reconcile question) takes it exclusively, so a run is never put to sleep —4 * and its lease never released — while one of its steps is still executing or committing.5 * Writers have priority; nothing here is re-entrant (exec must not call ctx.* itself).6 */7export class Gate {8  private readers = 0;9  private writer = false;10  private writersWaiting = 0;11  private parked: (() => void)[] = [];1213  private park(): Promise<void> {14    return new Promise((resolve) => this.parked.push(resolve));15  }16  private wakeAll(): void {17    const p = this.parked;18    this.parked = [];19    for (const r of p) r();20  }21  async shared(): Promise<void> {22    while (this.writer || this.writersWaiting > 0) await this.park();23    this.readers++;24  }25  releaseShared(): void {26    this.readers--;27    this.wakeAll();28  }29  async exclusive(): Promise<void> {30    this.writersWaiting++;31    try {32      while (this.writer || this.readers > 0) await this.park();33    } finally {34      this.writersWaiting--;35    }36    this.writer = true;37  }38  releaseExclusive(): void {39    this.writer = false;40    this.wakeAll();41  }42  /** Resolves when no step holds the gate. */43  async idle(): Promise<void> {44    while (this.writer || this.readers > 0 || this.writersWaiting > 0) await this.park();45  }46}47