Confirmed vBulletin Blog importer gets 'stuck' importing blog comments when using parallel importer

Affected version
v2.1.10 Patch 2, XFI 1.3.2


Full disclosure upfront: I'm trying to import blogs from vBulletin 3.6, so I've hacked together my own importer and blog trait that extend the existing vBulletin 3.7 / 3.8 importer. The only difference is that blog tags do not exist in 3.6, so that step is removed in my trait's getSteps function. I believe this issue would exist with the stock 3.7/3.8 importer, but unfortunately I do not have access to a vB3.7 forum with blogs to test.

When importing using the CLI with multiple processes, the import gets stuck on the blog comments step. Once the blog comments have been migrated (you can see them in the database), the child processes just keep running in the background. The reporter gets stuck on:

Step 18 of 18: Blog comments             00:00:01 [1,234] 0.00%

If you kill the importer and re-run it without the --processes argument, the import will then finish successfully. (If you run it again with the processes argument it just gets stuck again).

In case it's relevant, there were 3,703 blog comments to migrate and I was running it with 6 processes.