/**
 * Nexus DB handle pool — LRU cache for per-project graph DB handles (T9150 ADR-072).
 *
 * Cross-project queries open multiple per-project nexus-graph/<id>.db files.
 * This module provides a soft-cap LRU pool of 32 Database handles to bound
 * file descriptor usage regardless of project count.
 *
 * When the 33rd handle is requested, the least-recently-used handle is closed
 * first. This provides O(1) amortized access with bounded resource usage.
 *
 * Usage:
 * ```ts
 * const pool = getNexusHandlePool();
 * const db = pool.acquire('/path/to/nexus-graph/projId.db');
 * // use db...
 * // pool.release() is a no-op — the pool manages lifecycle
 * ```
 *
 * @task T9150
 * @see ADR-072 docs/adr/ADR-072-nexus-db-split.md
 */
import { DatabaseSync } from 'node:sqlite';
/**
 * LRU pool of `DatabaseSync` handles for per-project nexus-graph files.
 *
 * Thread-safety note: Node.js is single-threaded; concurrent `await` calls
 * do not interleave synchronous sections. The pool is safe for async workloads.
 */
export declare class NexusHandlePool {
    private readonly cap;
    private readonly map;
    private head;
    private tail;
    constructor(softCap?: number);
    /**
     * Acquire a `DatabaseSync` handle for the given path.
     *
     * If the handle is already in the pool, it is promoted to MRU position.
     * If the pool is full, the LRU handle is closed and evicted first.
     */
    acquire(dbPath: string): DatabaseSync;
    /**
     * Explicitly close and evict a handle (e.g. after a project is unregistered).
     */
    evict(dbPath: string): void;
    /** Close all handles and clear the pool. */
    closeAll(): void;
    /** Number of open handles. */
    get size(): number;
    private evictLru;
    private insertHead;
    private promoteToHead;
    private removeNode;
}
/** Return the global singleton LRU handle pool. */
export declare function getNexusHandlePool(): NexusHandlePool;
/** Reset the singleton (for testing). */
export declare function resetNexusHandlePool(): void;
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