Function ConditionalLockBufferForCleanup

src/backend/storage/buffer/bufmgr.c:4522–4568

* ConditionalLockBufferForCleanup - as above, but don't wait to get the lock * * We won't loop, but just check once to see if the pin count is OK. If * not, return false with no lock held. */

 * not, return false with no lock held.
 */
bool
ConditionalLockBufferForCleanup(Buffer buffer)
{
	BufferDesc *bufHdr;
	uint32		buf_state,
				refcount;

	Assert(BufferIsValid(buffer));

	if (BufferIsLocal(buffer))
	{
		refcount = LocalRefCount[-buffer - 1];
		/* There should be exactly one pin */
		Assert(refcount > 0);
		if (refcount != 1)
			return false;
		/* Nobody else to wait for */
		return true;
	}

	/* There should be exactly one local pin */
	refcount = GetPrivateRefCount(buffer);
	Assert(refcount);
	if (refcount != 1)
		return false;

	/* Try to acquire lock */
	if (!ConditionalLockBuffer(buffer))
		return false;

	bufHdr = GetBufferDescriptor(buffer - 1);
	buf_state = LockBufHdr(bufHdr);
	refcount = BUF_STATE_GET_REFCOUNT(buf_state);

	Assert(refcount > 0);
	if (refcount == 1)
	{
		/* Successfully acquired exclusive lock with pincount 1 */
		UnlockBufHdr(bufHdr, buf_state);
		return true;
	}

	/* Failed, so release the lock */
	UnlockBufHdr(bufHdr, buf_state);
	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
	return false;
}

/*
 * IsBufferCleanupOK - as above, but we already have the lock