From 9046125563b1b02e9bc0839bde5d77e77c940bbd Mon Sep 17 00:00:00 2001 From: Kyotaro Horiguchi Date: Wed, 1 Feb 2017 16:07:22 +0900 Subject: [PATCH] Fix a bug of physical replication slot. A physical-replication standby can stop just at the boundary of WAL segments. restart_lsn of the slot on the master can be assumed to be the same location. The last segment on the master will be removed after some checkpoints for the case. If the first record of the next segment is a continuation record, it is only on the master and its beginning is only on the standby so the standby cannot restart because the record to read is scattered to two sources. This patch detains restart_lsn in the last sgement when the first page of the next segment is a continuation record. --- src/backend/replication/walsender.c | 104 +++++++++++++++++++++++++++++++++--- 1 file changed, 97 insertions(+), 7 deletions(-) diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c index cfc3fba..6dff18b 100644 --- a/src/backend/replication/walsender.c +++ b/src/backend/replication/walsender.c @@ -210,6 +210,13 @@ static struct WalTimeSample last_read[NUM_SYNC_REP_WAIT_MODE]; } LagTracker; +/* + * This variable corresponds to restart_lsn in pg_replication_slots for a + * physical slot. This has a valid value only when it differs from the current + * flush pointer. + */ +static XLogRecPtr restartLSN = InvalidXLogRecPtr; + /* Signal handlers */ static void WalSndSigHupHandler(SIGNAL_ARGS); static void WalSndXLogSendHandler(SIGNAL_ARGS); @@ -244,7 +251,7 @@ static XLogRecPtr WalSndWaitForWal(XLogRecPtr loc); static TimeOffset LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now); static bool TransactionIdInRecentPast(TransactionId xid, uint32 epoch); -static void XLogRead(char *buf, XLogRecPtr startptr, Size count); +static bool XLogRead(char *buf, XLogRecPtr startptr, Size count, bool noutfoundok); /* Initialize walsender process before entering the main command loop */ @@ -538,6 +545,9 @@ StartReplication(StartReplicationCmd *cmd) ereport(ERROR, (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE), (errmsg("cannot use a logical replication slot for physical replication")))); + + /* Restore restartLSN from replication slot */ + restartLSN = MyReplicationSlot->data.restart_lsn; } /* @@ -553,6 +563,10 @@ StartReplication(StartReplicationCmd *cmd) else FlushPtr = GetFlushRecPtr(); + /* Set InvalidXLogRecPtr if catching up */ + if (restartLSN == FlushPtr) + restartLSN = InvalidXLogRecPtr; + if (cmd->timeline != 0) { XLogRecPtr switchpoint; @@ -767,7 +781,7 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req count = flushptr - targetPagePtr; /* now actually read the data, we know it's there */ - XLogRead(cur_page, targetPagePtr, XLOG_BLCKSZ); + XLogRead(cur_page, targetPagePtr, XLOG_BLCKSZ, false); return count; } @@ -1668,7 +1682,7 @@ static void ProcessStandbyReplyMessage(void) { XLogRecPtr writePtr, - flushPtr, + flushPtr, oldFlushPtr, applyPtr; bool replyRequested; TimeOffset writeLag, @@ -1728,6 +1742,7 @@ ProcessStandbyReplyMessage(void) WalSnd *walsnd = MyWalSnd; SpinLockAcquire(&walsnd->mutex); + oldFlushPtr = walsnd->flush; walsnd->write = writePtr; walsnd->flush = flushPtr; walsnd->apply = applyPtr; @@ -1751,7 +1766,74 @@ ProcessStandbyReplyMessage(void) if (SlotIsLogical(MyReplicationSlot)) LogicalConfirmReceivedLocation(flushPtr); else - PhysicalConfirmReceivedLocation(flushPtr); + { + /* + * Recovery on standby requires that a continuation reocrd must be + * available from single WAL source. For the reason, physical + * replication slot should stay in the first segment for a + * continuation record spanning multiple segments. Since this + * doesn't look into individual record, restartLSN may stay a bit + * too behind. + * + * Since the objective is avoding to remove required segments, + * checking at the beginning of every segment is enough. But once + * restartLSN goes behind, check every page for quick restoration. + * + * restartLSN has a valid value only when it is behind flushPtr. + */ + if (oldFlushPtr != InvalidXLogRecPtr && + (restartLSN == InvalidXLogRecPtr ? + oldFlushPtr / XLOG_SEG_SIZE != flushPtr / XLOG_SEG_SIZE : + restartLSN / XLOG_BLCKSZ != flushPtr / XLOG_BLCKSZ)) + { + XLogRecPtr rp; + + if (restartLSN == InvalidXLogRecPtr) + restartLSN = oldFlushPtr; + + rp = restartLSN - (restartLSN % XLOG_BLCKSZ); + + /* + * We may have let the record at flushPtr sent, so it's worth + * looking + */ + while (rp <= flushPtr) + { + XLogPageHeaderData header; + + /* + * If the page header is not available for now, don't move + * restartLSN forward. We can read it by the next chance. + */ + if(sentPtr - rp >= sizeof(XLogPageHeaderData)) + { + bool found; + /* + * Fetch the page header of the next page. Move + * restartLSN forward only if it is not a continuation + * page. + */ + found = XLogRead((char *)&header, rp, + sizeof(XLogPageHeaderData), true); + if (found && + (header.xlp_info & XLP_FIRST_IS_CONTRECORD) == 0) + restartLSN = rp; + } + rp += XLOG_BLCKSZ; + } + + /* + * If restartLSN is on the same page with flushPtr, it means + * that we are catching up. + */ + if (restartLSN / XLOG_BLCKSZ == flushPtr / XLOG_BLCKSZ) + restartLSN = InvalidXLogRecPtr; + } + + /* restartLSN == InvalidXLogRecPtr means catching up */ + PhysicalConfirmReceivedLocation(restartLSN != InvalidXLogRecPtr ? + restartLSN : flushPtr); + } } } @@ -2217,6 +2299,7 @@ WalSndKill(int code, Datum arg) /* * Read 'count' bytes from WAL into 'buf', starting at location 'startptr' + * Returns false if the segment file is not found when notfoundok is true. * * XXX probably this should be improved to suck data directly from the * WAL buffers when possible. @@ -2226,8 +2309,8 @@ WalSndKill(int code, Datum arg) * always be one descriptor left open until the process ends, but never * more than one. */ -static void -XLogRead(char *buf, XLogRecPtr startptr, Size count) +static bool +XLogRead(char *buf, XLogRecPtr startptr, Size count, bool notfoundok) { char *p; XLogRecPtr recptr; @@ -2304,10 +2387,15 @@ retry: * removed or recycled. */ if (errno == ENOENT) + { + if (notfoundok) + return false; + ereport(ERROR, (errcode_for_file_access(), errmsg("requested WAL segment %s has already been removed", XLogFileNameP(curFileTimeLine, sendSegNo)))); + } else ereport(ERROR, (errcode_for_file_access(), @@ -2389,6 +2477,8 @@ retry: goto retry; } } + + return true; } /* @@ -2619,7 +2709,7 @@ XLogSendPhysical(void) * calls. */ enlargeStringInfo(&output_message, nbytes); - XLogRead(&output_message.data[output_message.len], startptr, nbytes); + XLogRead(&output_message.data[output_message.len], startptr, nbytes, false); output_message.len += nbytes; output_message.data[output_message.len] = '\0'; -- 2.9.2