Fix the truncation logic of the OldSerXid SLRU mechanism. We can't pass

SimpleLruTruncate() a page number that's "in the future", because it will
issue a warning and refuse to truncate anything. Instead, we leave behind
the latest segment. If the slru is not needed before XID wrap-around, the
segment will appear as new again, and not be cleaned up until it gets old
enough again. That's a bit unpleasant, but better than not cleaning up
anything.

Also, fix broken calculation to check and warn if the span of the OldSerXid
SLRU is getting too large to fit in the 64k SLRU pages that we have
available. It was not XID wraparound aware.

Kevin Grittner and me.

src/backend/storage/lmgr/predicate.c

@@ -330,7 +330,7 @@ typedef struct OldSerXidControlData
 	int			headPage;		/* newest initialized page */
 	TransactionId headXid;		/* newest valid Xid in the SLRU */
 	TransactionId tailXid;		/* oldest xmin we might be interested in */
-	bool		warningIssued;
+	bool		warningIssued;	/* have we issued SLRU wrap-around warning? */
 }	OldSerXidControlData;
 
 typedef struct OldSerXidControlData *OldSerXidControl;
@@ -738,7 +738,6 @@ OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
 	int			targetPage;
 	int			slotno;
 	int			firstZeroPage;
-	int			xidSpread;
 	bool		isNewPage;
 
 	Assert(TransactionIdIsValid(xid));
@@ -779,18 +778,45 @@ OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
 	if (isNewPage)
 		oldSerXidControl->headPage = targetPage;
 
-	xidSpread = (((uint32) xid) - ((uint32) tailXid));
+	/*
+	 * Give a warning if we're about to run out of SLRU pages.
+	 *
+	 * slru.c has a maximum of 64k segments, with 32 (SLRU_PAGES_PER_SEGMENT)
+	 * pages each. We need to store a 64-bit integer for each Xid, and with
+	 * default 8k block size, 65536*32 pages is only enough to cover 2^30
+	 * XIDs. If we're about to hit that limit and wrap around, warn the user.
+	 *
+	 * To avoid spamming the user, we only give one warning when we've used 1
+	 * billion XIDs, and stay silent until the situation is fixed and the
+	 * number of XIDs used falls below 800 million again.
+	 *
+	 * XXX: We have no safeguard to actually *prevent* the wrap-around,
+	 * though. All you get is a warning.
+	 */
 	if (oldSerXidControl->warningIssued)
 	{
-		if (xidSpread < 800000000)
+		TransactionId lowWatermark;
+
+		lowWatermark = tailXid + 800000000;
+		if (lowWatermark < FirstNormalTransactionId)
+			lowWatermark = FirstNormalTransactionId;
+		if (TransactionIdPrecedes(xid, lowWatermark))
 			oldSerXidControl->warningIssued = false;
 	}
-	else if (xidSpread >= 1000000000)
+	else
 	{
-		oldSerXidControl->warningIssued = true;
-		ereport(WARNING,
-				(errmsg("memory for serializable conflict tracking is nearly exhausted"),
-				 errhint("There may be an idle transaction or a forgotten prepared transaction causing this.")));
+		TransactionId highWatermark;
+
+		highWatermark = tailXid + 1000000000;
+		if (highWatermark < FirstNormalTransactionId)
+			highWatermark = FirstNormalTransactionId;
+		if (TransactionIdFollows(xid, highWatermark))
+		{
+			oldSerXidControl->warningIssued = true;
+			ereport(WARNING,
+					(errmsg("memory for serializable conflict tracking is nearly exhausted"),
+					 errhint("There may be an idle transaction or a forgotten prepared transaction causing this.")));
+		}
 	}
 
 	if (isNewPage)
@@ -931,18 +957,16 @@ CheckPointPredicate(void)
 	else
 	{
 		/*
-		 * The SLRU is no longer needed. Truncate everything.  If we try to
-		 * leave the head page around to avoid re-zeroing it, we might not use
-		 * the SLRU again until we're past the wrap-around point, which makes
-		 * SLRU unhappy.
+		 * The SLRU is no longer needed. Truncate to head before we set head
+		 * invalid.
 		 *
-		 * While the API asks you to specify truncation by page, it silently
-		 * ignores the request unless the specified page is in a segment past
-		 * some allocated portion of the SLRU.	We don't care which page in a
-		 * later segment we hit, so just add the number of pages per segment
-		 * to the head page to land us *somewhere* in the next segment.
+		 * XXX: It's possible that the SLRU is not needed again until XID
+		 * wrap-around has happened, so that the segment containing headPage
+		 * that we leave behind will appear to be new again. In that case it
+		 * won't be removed until XID horizon advances enough to make it
+		 * current again.
 		 */
-		tailPage = oldSerXidControl->headPage + SLRU_PAGES_PER_SEGMENT;
+		tailPage = oldSerXidControl->headPage;
 		oldSerXidControl->headPage = -1;
 	}