fix: prevent zombie process accumulation via PID registry and signal propagation (Issue #737) (#806)

* Fix zombie process accumulation (Issue #737)

Problem: Claude haiku subprocesses spawned by the SDK weren't terminating
properly, causing zombie process accumulation (user reported 155 processes
consuming 51GB RAM).

Root causes:
1. SDK's SpawnedProcess interface hides subprocess PIDs
2. deleteSession() didn't verify subprocess exit
3. abort() was fire-and-forget with no confirmation
4. No mechanism to track or clean up orphaned processes

Solution:
- Add ProcessRegistry module to track spawned Claude subprocesses
- Use SDK's spawnClaudeCodeProcess option to capture PIDs via custom spawn
- Pass signal parameter to enable AbortController integration
- Wait for subprocess exit in deleteSession() with 5s timeout
- Escalate to SIGKILL if graceful exit fails
- Add orphan reaper running every 5 minutes as safety net

Files changed:
- src/services/worker/ProcessRegistry.ts (new): PID registry and reaper
- src/services/worker/SDKAgent.ts: Use custom spawn to capture PIDs
- src/services/worker/SessionManager.ts: Verify subprocess exit on delete
- src/services/worker-service.ts: Start/stop orphan reaper

Fixes #737

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

* fix: address code review feedback

- Replace busy-wait polling with event-based proc.once('exit')
- Detect and warn about multiple processes per session (race condition)

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

---------

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

src/services/worker/SessionManager.ts

@@ -14,6 +14,7 @@ import { logger } from '../../utils/logger.js';
 import type { ActiveSession, PendingMessage, PendingMessageWithId, ObservationData } from '../worker-types.js';
 import { PendingMessageStore } from '../sqlite/PendingMessageStore.js';
 import { SessionQueueProcessor } from '../queue/SessionQueueProcessor.js';
+import { getProcessBySession, ensureProcessExit } from './ProcessRegistry.js';
 
 export class SessionManager {
   private dbManager: DatabaseManager;
@@ -256,6 +257,7 @@ export class SessionManager {
 
   /**
    * Delete a session (abort SDK agent and cleanup)
+   * Verifies subprocess exit to prevent zombie process accumulation (Issue #737)
    */
   async deleteSession(sessionDbId: number): Promise<void> {
     const session = this.sessions.get(sessionDbId);
@@ -265,17 +267,27 @@ export class SessionManager {
 
     const sessionDuration = Date.now() - session.startTime;
 
-    // Abort the SDK agent
+    // 1. Abort the SDK agent
     session.abortController.abort();
 
-    // Wait for generator to finish
+    // 2. Wait for generator to finish
     if (session.generatorPromise) {
-      await session.generatorPromise.catch(error => {
+      await session.generatorPromise.catch(() => {
         logger.debug('SYSTEM', 'Generator already failed, cleaning up', { sessionId: session.sessionDbId });
       });
     }
 
-    // Cleanup
+    // 3. Verify subprocess exit with 5s timeout (Issue #737 fix)
+    const tracked = getProcessBySession(sessionDbId);
+    if (tracked && !tracked.process.killed && tracked.process.exitCode === null) {
+      logger.debug('SESSION', `Waiting for subprocess PID ${tracked.pid} to exit`, {
+        sessionId: sessionDbId,
+        pid: tracked.pid
+      });
+      await ensureProcessExit(tracked, 5000);
+    }
+
+    // 4. Cleanup
     this.sessions.delete(sessionDbId);
     this.sessionQueues.delete(sessionDbId);