PGReserveSemaphores() called from CreateSharedMemoryAndSemaphores()

Before e256266, PGReserveSemaphores()
was required to be called before SpinlockSemaInit() since spinlocks may
be implemented using semaphores on some platforms.  SpinlockSemaInit()
was required to be called before InitShmemAllocation() since the latter
initialized a spinlock to synchronize shared memory allocations.
e256266 removed the call to
SpinlockSemaInit() from CreateSharedMemoryAndSemaphores() but left the
call to PGReserveSemaphores() in CreateSharedMemoryAndSemaphores(), even
though it fits in CreateOrAttachShmemStructs() along with the calls to
other functions allocating shared memory structures. Add a comment
explaining this absurdity.

To be backpatched to PG 18.

Author: Ashutosh Bapat <[email protected]>
Discussion: https://www.postgresql.org/message-id/CAExHW5seSZpPx-znjidVZNzdagGHOk06F+Ds88MpPUbxd1kTaA@mail.gmail.com

Author: ashutosh-bapat

src/backend/storage/ipc/ipci.c

@@ -225,7 +225,12 @@ CreateSharedMemoryAndSemaphores(void)
 	InitShmemAccess(seghdr);
 
 	/*
-	 * Create semaphores
+	 * Create semaphores. This is done here because of a now-non-existent
+	 * dependency between spinlocks, which were required for shared memory
+	 * allocation, and semaphores, which were allocated in the shared memory
+	 * on some platforms. Ideally it should be done in
+	 * CreateOrAttachShmemStructs() where we allocate other shared memory
+	 * structures.
 	 */
 	PGReserveSemaphores(numSemas);