client.flush() blocks process exit for full timeout duration due to non-unref'd timers

[BYK]

Is there an existing issue for this?

• I have checked for existing issues https://github.com/getsentry/sentry-javascript/issues
• I have reviewed the documentation https://docs.sentry.io/
• I am using the latest SDK release https://github.com/getsentry/sentry-javascript/releases

How do you use Sentry?

Sentry SaaS (sentry.io)

Which SDK are you using?

@sentry/node

SDK Version

10.36.0

Framework Version

Bun 1.3.3 / Node.js 22

Link to Sentry event

No response

Reproduction Example/SDK Setup

import * as Sentry from "@sentry/node";

const client = Sentry.init({
  dsn: "https://your-dsn@sentry.io/123",
  enabled: true,
  defaultIntegrations: false,
});

// This will block for 3 seconds even when there's nothing to send
await client?.flush(3000);

Steps to Reproduce

1. Initialize Sentry in a CLI application
2. Call client.flush(3000) before exiting
3. Observe the process hangs for ~3 seconds even when there's nothing to flush

Expected Result

flush(timeout) should:

1. Return immediately if there's nothing to process (_numProcessing === 0)
2. Use unref() on internal timers so the process can exit naturally when work is complete

Actual Result

The process blocks for the full timeout duration because:

1. _isClientDoneProcessing() in client.js checks _numProcessing after the first setTimeout, not before:

   async _isClientDoneProcessing(timeout) {
     let ticked = 0;
     while (!timeout || ticked < timeout) {
       await new Promise(resolve => setTimeout(resolve, 1));  // Always waits first
       if (!this._numProcessing) {
         return true;
       }
       ticked++;
     }
     return false;
   }

2. The setTimeout calls don't use .unref(), keeping the Node.js event loop alive:

   • client.js line 650: setTimeout(resolve, 1) - polling loop
   • promisebuffer.js line 72: setTimeout(() => resolve(false), timeout) - drain timeout

Other parts of the codebase already use .unref() correctly (e.g., httpServerIntegration.js line 234).

Suggested Fix

1. Add early exit check in _isClientDoneProcessing():

   async _isClientDoneProcessing(timeout) {
     if (!this._numProcessing) {
       return true;  // Early exit
     }
     // ... rest of polling loop
   }

2. Add .unref() to timers with browser-safe check:

   await new Promise(resolve => {
     const t = setTimeout(resolve, 1);
     if (typeof t !== 'number' && t.unref) t.unref();
   });

[linear]

JS-1563