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

[Mael-Abgrall]

Just adding to this, I'm facing the same issue with the @sentry/cloudflare package, where as a result the runtime throws an error.

I tried with and without a timeout, but both generate the following error:

✘ [ERROR] Uncaught Error: The Workers runtime canceled this request because it detected that your Worker's code had hung and would never generate a response. Refer to: https://developers.cloudflare.com/workers/observability/errors/ 

[Lms24]

@Mael-Abgrall Let's see if #19062 solves this for you once it's released. If you follow this issue, you'll get an automatic reply once a release has been made. If it doesn't fix the error, please open a new issue. Coudlfare has its own flushing problems, so this might not be fully related.

[Lms24]

Closing since #19062 was merged which should fix the reported hang with the node SDK (forgot to link the issue)