Affected versions

• Firefox 68.0a1 (20190415213626)

Affected platforms

• Windows 10 x64
• Windows 7 x86

Prerequisites:

• Set app.update.BITS.enabled and app.update.log to true
• Use Netlimiter to set the Internet bandwidth limit to about 20kb/s.

Steps to reproduce

1. Launch Firefox.
2. Open the browser console and check if update download is started via BITS.
3. Close browser while the update download is in progress.
4. Wait for about 30 seconds for the download to finish in the background.
5. Disable and Stop BITS service.
6. Open Firefox and check the browser console.

Expected result

• Firefox update download is completed and the browser is attempting to stage the update.

Actual result

• The update download gets restarted via nsIINcremental.

Regression range

• I don’t think this is a regression.

Additional Notes

• Attached a screen recording with the issue: link

This is actually the expected behavior. When a BITS download finishes transferring, it has not actually been "completed" yet. Firefox has to call the complete() method on the job for the job to be completed and saved to the disk.

Firefox does not see the competed download because there is none. It also does not see the BITS job, because BITS is not available. Therefore, falling back to an alternate download mechanism is the expected behavior.