Per our policy to support five EOL versions of PostgreSQL, 9.4 is no longer supported by pgBackRest. Remove all logic associated with 9.4 and update the tests.
This includes a small fix in infoPg.c to allow backup.info files with old versions to be saved. This allows expire to function when old versions are present. Even though those older versions cannot be used, they can be expired.
Tests for 9.4 are left in the expire/info tests to demonstrate that these commands work with old versions present.
On certain file systems (e.g. ext4) pg_control may appear torn if there is a concurrent write while reading the file. To prevent an invalid read, retry until the checksum matches the control data.
Special handling is required for the pg-version-force feature since the offset of the checksum is not known. In this case, scan from the default position to the end of the data looking for a checksum match. This is a bit imprecise, but better than nothing, and the chance of a random collision in the control data seems very remote considering the ratio of data size (< 512 bytes) to checksum size (4 bytes).
This was discovered and a possible solution proposed for PostgreSQL in [1]. The proposed solution may work for backup, but pgBackRest needs to be able to read pg_control reliably outside of backup. So no matter what fix is adopted for PostgreSQL, pgBackRest need retries. Further adjustment may be required as the PostgreSQL fix evolves.
[1] https://www.postgresql.org/message-id/20221123014224.xisi44byq3cf5psi%40awork3.anarazel.de
