I want to suggest a patch against master (it may also be worth backporting it) that makes it possible to use longer filenames (such as those with absolute paths) in `BackgroundWorker.bgw_library_name`.
`BackgroundWorker.bgw_library_name` currently allows names up to BGW_MAXLEN-1, which is generally sufficient if `$libdir` expansion is used.
However, there are use cases where [potentially] longer names are expected/desired; for example, test benches (where library files may not [or can not] be copied to Postgres installation) or alternative library installation methods that do not put them into $libdir.
The patch is backwards-compatible and ensures that bgw_library_name stays *at least* as long as BGW_MAXLEN. Existing external code that uses BGW_MAXLEN is a length boundary (for example, in `strncpy`) will continue to work as expected.
The trade-off of this patch is that the `BackgroundWorker` structure becomes larger. From my perspective, this is a reasonable cost (less than a kilobyte of extra space per worker).
