A single line that tries to read an HTTP response body and fails is very hard to test without mocking, and the mock does not add value to a single line of code to check for an error that can rarely ever be triggered. Going to settle for 99.9% for now.
Using some designs unearthed from building the higher level model, I have brough that design down to the lower level to speed things up. It only took 8 years, but finally, I think I have mastered mult-threading. No more deadlocks, and no more need for waitgroups for everything.