Function ReservePrivateRefCountEntry

src/backend/storage/buffer/bufmgr.c:230–291

* Ensure that the PrivateRefCountArray has sufficient space to store one more * entry. This has to be called before using NewPrivateRefCountEntry() to fill * a new entry - but it's perfectly fine to not use a reserved entry. */

 * a new entry - but it's perfectly fine to not use a reserved entry.
 */
static void
ReservePrivateRefCountEntry(void)
{
	/* Already reserved (or freed), nothing to do */
	if (ReservedRefCountEntry != NULL)
		return;

	/*
	 * First search for a free entry the array, that'll be sufficient in the
	 * majority of cases.
	 */
	{
		int			i;

		for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
		{
			PrivateRefCountEntry *res;

			res = &PrivateRefCountArray[i];

			if (res->buffer == InvalidBuffer)
			{
				ReservedRefCountEntry = res;
				return;
			}
		}
	}

	/*
	 * No luck. All array entries are full. Move one array entry into the hash
	 * table.
	 */
	{
		/*
		 * Move entry from the current clock position in the array into the
		 * hashtable. Use that slot.
		 */
		PrivateRefCountEntry *hashent;
		bool		found;

		/* select victim slot */
		ReservedRefCountEntry =
			&PrivateRefCountArray[PrivateRefCountClock++ % REFCOUNT_ARRAY_ENTRIES];

		/* Better be used, otherwise we shouldn't get here. */
		Assert(ReservedRefCountEntry->buffer != InvalidBuffer);

		/* enter victim array entry into hashtable */
		hashent = hash_search(PrivateRefCountHash,
							  (void *) &(ReservedRefCountEntry->buffer),
							  HASH_ENTER,
							  &found);
		Assert(!found);
		hashent->refcount = ReservedRefCountEntry->refcount;

		/* clear the now free array slot */
		ReservedRefCountEntry->buffer = InvalidBuffer;
		ReservedRefCountEntry->refcount = 0;