A number of pg_upgrade steps require connecting to every database
in the cluster and running the same query in each one. When there
are many databases, these steps are particularly time-consuming,
especially since they are performed sequentially, i.e., we connect
to a database, run the query, and process the results before moving
on to the next database.
This commit introduces a new framework that makes it easy to
parallelize most of these once-in-each-database tasks by processing
multiple databases concurrently. This framework manages a set of
slots that follow a simple state machine, and it uses libpq's
asynchronous APIs to establish the connections and run the queries.
The --jobs option is used to determine the number of slots to use.
To use this new task framework, callers simply need to provide the
query and a callback function to process its results, and the
framework takes care of the rest. A more complete description is
provided at the top of the new task.c file.
None of the eligible once-in-each-database tasks are converted to
use this new framework in this commit. That will be done via
several follow-up commits.
Reviewed-by: Jeff Davis, Robert Haas, Daniel Gustafsson, Ilya Gladyshev, Corey Huinker
Discussion: https://postgr.es/m/20240516211638.GA1688936%40nathanxps13
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