fix: critical memory leak from per-session database connections (#554)

* fix: critical memory leak from per-session database connections (#542)

Each MCP session was creating its own database connection (~900MB),
causing OOM kills every ~20 minutes with 3-4 concurrent sessions.

Changes:
- Add SharedDatabase singleton pattern - all sessions share ONE connection
- Reduce session timeout from 30 min to 5 min (configurable)
- Add eager cleanup for reconnecting instances
- Fix telemetry event listener leak

Memory impact: ~900MB/session → ~68MB shared + ~5MB/session overhead

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Conceived by Romuald Czlonkowski - https://www.aiadvisors.pl/en

* fix: resolve test failures from shared database race conditions

- Fix `shutdown()` to respect shared database pattern (was directly closing)
- Add `await this.initialized` in both `close()` and `shutdown()` to prevent
  race condition where cleanup runs while initialization is in progress
- Add double-shutdown protection with `isShutdown` flag
- Export `SharedDatabaseState` type for proper typing
- Include error details in debug logs
- Add MCP server close to `shutdown()` for consistency with `close()`
- Null out `earlyLogger` in `shutdown()` for consistency

The CI test failure "The database connection is not open" was caused by:
1. `shutdown()` directly calling `this.db.close()` which closed the SHARED
   database connection, breaking subsequent tests
2. Race condition where `shutdown()` ran before initialization completed

Conceived by Romuald Członkowski - www.aiadvisors.pl/en

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* test: add unit tests for shared-database module

Add comprehensive unit tests covering:
- getSharedDatabase: initialization, reuse, different path error, concurrent requests
- releaseSharedDatabase: refCount decrement, double-release guard
- closeSharedDatabase: state clearing, error handling, re-initialization
- Helper functions: isSharedDatabaseInitialized, getSharedDatabaseRefCount

21 tests covering the singleton database connection pattern used to prevent
~900MB memory leaks per session.

Conceived by Romuald Członkowski - www.aiadvisors.pl/en

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/database/shared-database.ts

@@ -0,0 +1,197 @@
+/**
+ * Shared Database Manager - Singleton for cross-session database connection
+ *
+ * This module implements a singleton pattern to share a single database connection
+ * across all MCP server sessions. This prevents memory leaks caused by each session
+ * creating its own database connection (~900MB per session).
+ *
+ * Memory impact: Reduces per-session memory from ~900MB to near-zero by sharing
+ * a single ~68MB database connection across all sessions.
+ *
+ * Issue: https://github.com/czlonkowski/n8n-mcp/issues/XXX
+ */
+
+import { DatabaseAdapter, createDatabaseAdapter } from './database-adapter';
+import { NodeRepository } from './node-repository';
+import { TemplateService } from '../templates/template-service';
+import { EnhancedConfigValidator } from '../services/enhanced-config-validator';
+import { logger } from '../utils/logger';
+
+/**
+ * Shared database state - holds the singleton connection and services
+ */
+export interface SharedDatabaseState {
+  db: DatabaseAdapter;
+  repository: NodeRepository;
+  templateService: TemplateService;
+  dbPath: string;
+  refCount: number;
+  initialized: boolean;
+}
+
+// Module-level singleton state
+let sharedState: SharedDatabaseState | null = null;
+let initializationPromise: Promise<SharedDatabaseState> | null = null;
+
+/**
+ * Get or create the shared database connection
+ *
+ * Thread-safe initialization using a promise lock pattern.
+ * Multiple concurrent calls will wait for the same initialization.
+ *
+ * @param dbPath - Path to the SQLite database file
+ * @returns Shared database state with connection and services
+ */
+export async function getSharedDatabase(dbPath: string): Promise<SharedDatabaseState> {
+  // If already initialized with the same path, increment ref count and return
+  if (sharedState && sharedState.initialized && sharedState.dbPath === dbPath) {
+    sharedState.refCount++;
+    logger.debug('Reusing shared database connection', {
+      refCount: sharedState.refCount,
+      dbPath
+    });
+    return sharedState;
+  }
+
+  // If already initialized with a DIFFERENT path, this is a configuration error
+  if (sharedState && sharedState.initialized && sharedState.dbPath !== dbPath) {
+    logger.error('Attempted to initialize shared database with different path', {
+      existingPath: sharedState.dbPath,
+      requestedPath: dbPath
+    });
+    throw new Error(`Shared database already initialized with different path: ${sharedState.dbPath}`);
+  }
+
+  // If initialization is in progress, wait for it
+  if (initializationPromise) {
+    try {
+      const state = await initializationPromise;
+      state.refCount++;
+      logger.debug('Reusing shared database (waited for init)', {
+        refCount: state.refCount,
+        dbPath
+      });
+      return state;
+    } catch (error) {
+      // Initialization failed while we were waiting, clear promise and rethrow
+      initializationPromise = null;
+      throw error;
+    }
+  }
+
+  // Start new initialization
+  initializationPromise = initializeSharedDatabase(dbPath);
+
+  try {
+    const state = await initializationPromise;
+    // Clear the promise on success to allow future re-initialization after close
+    initializationPromise = null;
+    return state;
+  } catch (error) {
+    // Clear promise on failure to allow retry
+    initializationPromise = null;
+    throw error;
+  }
+}
+
+/**
+ * Initialize the shared database connection and services
+ */
+async function initializeSharedDatabase(dbPath: string): Promise<SharedDatabaseState> {
+  logger.info('Initializing shared database connection', { dbPath });
+
+  const db = await createDatabaseAdapter(dbPath);
+  const repository = new NodeRepository(db);
+  const templateService = new TemplateService(db);
+
+  // Initialize similarity services for enhanced validation
+  EnhancedConfigValidator.initializeSimilarityServices(repository);
+
+  sharedState = {
+    db,
+    repository,
+    templateService,
+    dbPath,
+    refCount: 1,
+    initialized: true
+  };
+
+  logger.info('Shared database initialized successfully', {
+    dbPath,
+    refCount: sharedState.refCount
+  });
+
+  return sharedState;
+}
+
+/**
+ * Release a reference to the shared database
+ *
+ * Decrements the reference count. Does NOT close the database
+ * as it's shared across all sessions for the lifetime of the process.
+ *
+ * @param state - The shared database state to release
+ */
+export function releaseSharedDatabase(state: SharedDatabaseState): void {
+  if (!state || !sharedState) {
+    return;
+  }
+
+  // Guard against double-release (refCount going negative)
+  if (sharedState.refCount <= 0) {
+    logger.warn('Attempted to release shared database with refCount already at or below 0', {
+      refCount: sharedState.refCount
+    });
+    return;
+  }
+
+  sharedState.refCount--;
+  logger.debug('Released shared database reference', {
+    refCount: sharedState.refCount
+  });
+
+  // Note: We intentionally do NOT close the database even when refCount hits 0
+  // The database should remain open for the lifetime of the process to handle
+  // new sessions. Only process shutdown should close it.
+}
+
+/**
+ * Force close the shared database (for graceful shutdown only)
+ *
+ * This should only be called during process shutdown, not during normal
+ * session cleanup. Closing the database would break other active sessions.
+ */
+export async function closeSharedDatabase(): Promise<void> {
+  if (!sharedState) {
+    return;
+  }
+
+  logger.info('Closing shared database connection', {
+    refCount: sharedState.refCount
+  });
+
+  try {
+    sharedState.db.close();
+  } catch (error) {
+    logger.warn('Error closing shared database', {
+      error: error instanceof Error ? error.message : String(error)
+    });
+  }
+
+  sharedState = null;
+  initializationPromise = null;
+}
+
+/**
+ * Check if shared database is initialized
+ */
+export function isSharedDatabaseInitialized(): boolean {
+  return sharedState !== null && sharedState.initialized;
+}
+
+/**
+ * Get current reference count (for debugging/monitoring)
+ */
+export function getSharedDatabaseRefCount(): number {
+  return sharedState?.refCount ?? 0;
+}