Remove obsolete SLRU wrapping and warnings from predicate.c.

When SSI was developed, slru.c was limited to segment files with names in
the range 0000-FFFF.  This didn't allow enough space for predicate.c to
store every possible XID when spilling old transactions to disk, so it
would wrap around sooner and print warnings.  Since commits 638cf09e and
73c986ad increased the number of segment files slru.c could manage, that
behavior is unnecessary.  Therefore remove that code.

Also remove the macro OldSerXidSegment, which has been unused since
4cd3fb6e.

Thomas Munro, reviewed by Anastasia Lubennikova

Discussion: https://postgr.es/m/CAEepm=3XfsTSxgEbEOmxu0QDiXy0o18NUg2nC89JZcCGE+XFPA@mail.gmail.com

src/backend/storage/lmgr/predicate.c

@@ -318,11 +318,9 @@ static SlruCtlData OldSerXidSlruCtlData;
 #define OLDSERXID_ENTRIESPERPAGE	(OLDSERXID_PAGESIZE / OLDSERXID_ENTRYSIZE)
 
 /*
- * Set maximum pages based on the lesser of the number needed to track all
- * transactions and the maximum that SLRU supports.
+ * Set maximum pages based on the number needed to track all transactions.
  */
-#define OLDSERXID_MAX_PAGE			Min(SLRU_PAGES_PER_SEGMENT * 0x10000 - 1, \
-										(MaxTransactionId) / OLDSERXID_ENTRIESPERPAGE)
+#define OLDSERXID_MAX_PAGE			(MaxTransactionId / OLDSERXID_ENTRIESPERPAGE)
 
 #define OldSerXidNextPage(page) (((page) >= OLDSERXID_MAX_PAGE) ? 0 : (page) + 1)
 
@@ -330,15 +328,13 @@ static SlruCtlData OldSerXidSlruCtlData;
 	(OldSerXidSlruCtl->shared->page_buffer[slotno] + \
 	((((uint32) (xid)) % OLDSERXID_ENTRIESPERPAGE) * OLDSERXID_ENTRYSIZE))))
 
-#define OldSerXidPage(xid)	((((uint32) (xid)) / OLDSERXID_ENTRIESPERPAGE) % (OLDSERXID_MAX_PAGE + 1))
-#define OldSerXidSegment(page)	((page) / SLRU_PAGES_PER_SEGMENT)
+#define OldSerXidPage(xid)	(((uint32) (xid)) / OLDSERXID_ENTRIESPERPAGE)
 
 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;	/* have we issued SLRU wrap-around warning? */
 }			OldSerXidControlData;
 
 typedef struct OldSerXidControlData *OldSerXidControl;
@@ -826,7 +822,6 @@ OldSerXidInit(void)
 		oldSerXidControl->headPage = -1;
 		oldSerXidControl->headXid = InvalidTransactionId;
 		oldSerXidControl->tailXid = InvalidTransactionId;
-		oldSerXidControl->warningIssued = false;
 	}
 }
 
@@ -882,47 +877,6 @@ OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
 	if (isNewPage)
 		oldSerXidControl->headPage = targetPage;
 
-	/*
-	 * 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)
-	{
-		TransactionId lowWatermark;
-
-		lowWatermark = tailXid + 800000000;
-		if (lowWatermark < FirstNormalTransactionId)
-			lowWatermark = FirstNormalTransactionId;
-		if (TransactionIdPrecedes(xid, lowWatermark))
-			oldSerXidControl->warningIssued = false;
-	}
-	else
-	{
-		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 might be an idle transaction or a forgotten prepared transaction causing this.")));
-		}
-	}
-
 	if (isNewPage)
 	{
 		/* Initialize intervening pages. */