I'll admit I hadn't really considered pipelining, but I'm tempted to say that it's probably not worth the complexity. Not only do most of the tasks have only one step, but even tasks like the data types check are unlikely to require more than a few queries for upgrades from supported versions.
Can you point me to a complex multi-step task that you think wouldn't work for pipelining? My skimming of the other patches all seemed to be one query with one result set to be processed by one callback.
Furthermore, most of the callbacks should do almost nothing for a given upgrade, and since pg_upgrade runs on the server, client/server round-trip time should be pretty low.
To my mind, that makes pipelining make more sense, you throw out N queries, most of which are trivial, and by the time you cycle back around and start digesting result sets via callbacks, more of the queries have finished because they were waiting on the query ahead of them in the pipeline, not waiting on a callback to finish consuming its assigned result set and then launching the next task query.
Perhaps pipelining would make more sense if we consolidated the tasks a bit better, but when I last looked into that, I didn't see a ton of great opportunities that would help anything except for upgrades from really old versions. Even then, I'm not sure if pipelining is worth it.
I think you'd want to do the opposite of consolidating the tasks. If anything, you'd want to break them down in known single-query operations, and if the callback function for one of them happens to queue up a subsequent query (with subsequent callback) then so be it.
