With that fix applied to REL_17_5 things are working well. Limiting the search sounds like an improvement too.
As an experiment I added a log for when semget in InternalIpcSemaphoreCreate returns -1. When I'm running `pg_ctl init` for this local build concurrently with `pg_ctl init` from PostgreSQL 15 (or another version prior to 17), I saw ~8 logged failures when there was contention. As I increased the concurrency, the maximum number of logged failures looked to be ~8 times concurrency, roughly. For me, then, running `pg_ctl init` with a concurrency of 125 would be needed to even begin exceeding the max retries of 1000 – in the worst case. That sounds high enough.
Then I thought: I'm only seeing the log from one of those instances, yet they're all going through the same search for free semaphore sets. That's a few system calls going to waste. Maybe not important in the big picture, but it gave me an idea to left shift nextSemaKey in PGReserveSemaphores, i.e. `nextSemaKey = statbuf.st_ino << 4`, to give each pg_ctl process a few guaranteed uncontested keys (at least, uncontested between themselves). In a small test this eliminated contention for semaphore sets due to concurrency. It is more of an optimisation though, rather than a bug fix.
