/* adapted from Postgres' txid.c#convert_xid function */
uint64 convert_xid(TransactionId xid) {
TxidEpoch state;
uint64 epoch;
GetNextXidAndEpoch(&state.last_xid, &state.epoch);
/* return special xid's as-is */
if (!TransactionIdIsNormal(xid))
return (uint64) xid;
/* xid can be on either side when near wrap-around */
epoch = (uint64) state.epoch;
if (xid > state.last_xid && TransactionIdPrecedes(xid, state.last_xid))
epoch--;
else if (xid < state.last_xid && TransactionIdFollows(xid, state.last_xid))
epoch++;
return (epoch << 32) | xid;
}
/*
* Hot Standby feedback
*/
static void
ProcessStandbyHSFeedbackMessage(void)
{
StandbyHSFeedbackMessage reply;
TransactionId newxmin = InvalidTransactionId;
pq_copymsgbytes(&reply_message, (char *) &reply, sizeof(StandbyHSFeedbackMessage));
elog(DEBUG2, "hot standby feedback xmin %u epoch %u",
reply.xmin,
reply.epoch);
/*
* Update the WalSender's proc xmin to allow it to be visible to
* snapshots. This will hold back the removal of dead rows and thereby
* prevent the generation of cleanup conflicts on the standby server.
*/
if (TransactionIdIsValid(reply.xmin))
{
TransactionId nextXid;
uint32 nextEpoch;
bool epochOK = false;
GetNextXidAndEpoch(&nextXid, &nextEpoch);
/*
* Epoch of oldestXmin should be same as standby or if the counter has
* wrapped, then one less than reply.
*/
if (reply.xmin <= nextXid)
{
if (reply.epoch == nextEpoch)
epochOK = true;
}
else
{
if (nextEpoch > 0 && reply.epoch == nextEpoch - 1)
epochOK = true;
}
/*
* Feedback from standby must not go backwards, nor should it go
* forwards further than our most recent xid.
*/
if (epochOK && TransactionIdPrecedesOrEquals(reply.xmin, nextXid))
{
if (!TransactionIdIsValid(MyProc->xmin))
{
TransactionId oldestXmin = GetOldestXmin(true, true);
if (TransactionIdPrecedes(oldestXmin, reply.xmin))
newxmin = reply.xmin;
else
newxmin = oldestXmin;
}
else
{
if (TransactionIdPrecedes(MyProc->xmin, reply.xmin))
newxmin = reply.xmin;
else
newxmin = MyProc->xmin; /* stay the same */
}
}
}
/*
* Grab the ProcArrayLock to set xmin, or invalidate for bad reply
*/
if (MyProc->xmin != newxmin)
{
LWLockAcquire(ProcArrayLock, LW_SHARED);
MyProc->xmin = newxmin;
LWLockRelease(ProcArrayLock);
}
}
/*
* Fetch epoch data from xact.c.
*/
static void
load_xid_epoch(TxidEpoch *state)
{
GetNextXidAndEpoch(&state->last_xid, &state->epoch);
}
/*
* Hot Standby feedback
*/
static void
ProcessStandbyHSFeedbackMessage(void)
{
StandbyHSFeedbackMessage reply;
TransactionId nextXid;
uint32 nextEpoch;
/* Decipher the reply message */
pq_copymsgbytes(&reply_message, (char *) &reply,
sizeof(StandbyHSFeedbackMessage));
elog(DEBUG2, "hot standby feedback xmin %u epoch %u",
reply.xmin,
reply.epoch);
/* Ignore invalid xmin (can't actually happen with current walreceiver) */
if (!TransactionIdIsNormal(reply.xmin))
return;
/*
* Check that the provided xmin/epoch are sane, that is, not in the future
* and not so far back as to be already wrapped around. Ignore if not.
*
* Epoch of nextXid should be same as standby, or if the counter has
* wrapped, then one greater than standby.
*/
GetNextXidAndEpoch(&nextXid, &nextEpoch);
if (reply.xmin <= nextXid)
{
if (reply.epoch != nextEpoch)
return;
}
else
{
if (reply.epoch + 1 != nextEpoch)
return;
}
if (!TransactionIdPrecedesOrEquals(reply.xmin, nextXid))
return; /* epoch OK, but it's wrapped around */
/*
* Set the WalSender's xmin equal to the standby's requested xmin, so that
* the xmin will be taken into account by GetOldestXmin. This will hold
* back the removal of dead rows and thereby prevent the generation of
* cleanup conflicts on the standby server.
*
* There is a small window for a race condition here: although we just
* checked that reply.xmin precedes nextXid, the nextXid could have gotten
* advanced between our fetching it and applying the xmin below, perhaps
* far enough to make reply.xmin wrap around. In that case the xmin we
* set here would be "in the future" and have no effect. No point in
* worrying about this since it's too late to save the desired data
* anyway. Assuming that the standby sends us an increasing sequence of
* xmins, this could only happen during the first reply cycle, else our
* own xmin would prevent nextXid from advancing so far.
*
* We don't bother taking the ProcArrayLock here. Setting the xmin field
* is assumed atomic, and there's no real need to prevent a concurrent
* GetOldestXmin. (If we're moving our xmin forward, this is obviously
* safe, and if we're moving it backwards, well, the data is at risk
* already since a VACUUM could have just finished calling GetOldestXmin.)
*/
MyPgXact->xmin = reply.xmin;
}
/*
* Send hot standby feedback message to primary, plus the current time,
* in case they don't have a watch.
*/
static void
XLogWalRcvSendHSFeedback(void)
{
char buf[sizeof(StandbyHSFeedbackMessage) + 1];
TimestampTz now;
TransactionId nextXid;
uint32 nextEpoch;
TransactionId xmin;
/*
* If the user doesn't want status to be reported to the master, be sure
* to exit before doing anything at all.
*/
if (wal_receiver_status_interval <= 0 || !hot_standby_feedback)
return;
/* Get current timestamp. */
now = GetCurrentTimestamp();
/*
* Send feedback at most once per wal_receiver_status_interval.
*/
if (!TimestampDifferenceExceeds(feedback_message.sendTime, now,
wal_receiver_status_interval * 1000))
return;
/*
* If Hot Standby is not yet active there is nothing to send. Check this
* after the interval has expired to reduce number of calls.
*/
if (!HotStandbyActive())
return;
/*
* Make the expensive call to get the oldest xmin once we are certain
* everything else has been checked.
*/
xmin = GetOldestXmin(true, false);
/*
* Get epoch and adjust if nextXid and oldestXmin are different sides of
* the epoch boundary.
*/
GetNextXidAndEpoch(&nextXid, &nextEpoch);
if (nextXid < xmin)
nextEpoch--;
/*
* Always send feedback message.
*/
feedback_message.sendTime = now;
feedback_message.xmin = xmin;
feedback_message.epoch = nextEpoch;
elog(DEBUG2, "sending hot standby feedback xmin %u epoch %u",
feedback_message.xmin,
feedback_message.epoch);
/* Prepend with the message type and send it. */
buf[0] = 'h';
memcpy(&buf[1], &feedback_message, sizeof(StandbyHSFeedbackMessage));
walrcv_send(buf, sizeof(StandbyHSFeedbackMessage) + 1);
}