LockBufferForCleanup()
-> ResolveRecoveryConflictWithBufferPin()

/*
 * Walreceiver is active, so see if new data has arrived.
 *
 * We only advance XLogReceiptTime when we obtain fresh
 * WAL from walreceiver and observe that we had already
 * processed everything before the most recent "chunk"
 * that it flushed to disk.  In steady state where we are
 * keeping up with the incoming data, XLogReceiptTime will
 * be updated on each cycle. When we are behind,
 * XLogReceiptTime will not advance, so the grace time
 * allotted to conflicting queries will decrease.
 */
if (RecPtr < flushedUpto)
	havedata = true;
else
{
	XLogRecPtr	latestChunkStart;

	flushedUpto = GetWalRcvFlushRecPtr(&latestChunkStart, &receiveTLI);
	if (RecPtr < flushedUpto && receiveTLI == curFileTLI)
	{
		havedata = true;
		if (latestChunkStart <= RecPtr)
		{
			XLogReceiptTime = GetCurrentTimestamp();
			SetCurrentChunkStartTime(XLogReceiptTime);
		}
	}
	else
		havedata = false;

Replace max_standby_delay with two parameters, max_standby_archive_delay and
max_standby_streaming_delay, and revise the implementation to avoid assuming
that timestamps found in WAL records can meaningfully be compared to clock
time on the standby server.  Instead, the delay limits are compared to the
elapsed time since we last obtained a new WAL segment from archive or since
we were last "caught up" to WAL data arriving via streaming replication.
This avoids problems with clock skew between primary and standby, as well
as other corner cases that the original coding would misbehave in, such
as the primary server having significant idle time between transactions.
Per my complaint some time ago and considerable ensuing discussion.

Do some desultory editing on the hot standby documentation, too.