/*
* read_page callback for logical decoding contexts.
*
* Public because it would likely be very helpful for someone writing another
* output method outside walsender, e.g. in a bgworker.
*
* TODO: The walsender has it's own version of this, but it relies on the
* walsender's latch being set whenever WAL is flushed. No such infrastructure
* exists for normal backends, so we have to do a check/sleep/repeat style of
* loop for now.
*/
int
logical_read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
int reqLen, XLogRecPtr targetRecPtr, char *cur_page, TimeLineID *pageTLI)
{
XLogRecPtr flushptr,
loc;
int count;
loc = targetPagePtr + reqLen;
while (1)
{
/*
* TODO: we're going to have to do something more intelligent about
* timelines on standbys. Use readTimeLineHistory() and
* tliOfPointInHistory() to get the proper LSN? For now we'll catch
* that case earlier, but the code and TODO is left in here for when
* that changes.
*/
if (!RecoveryInProgress())
{
*pageTLI = ThisTimeLineID;
flushptr = GetFlushRecPtr();
}
else
flushptr = GetXLogReplayRecPtr(pageTLI);
if (loc <= flushptr)
break;
CHECK_FOR_INTERRUPTS();
pg_usleep(1000L);
}
/* more than one block available */
if (targetPagePtr + XLOG_BLCKSZ <= flushptr)
count = XLOG_BLCKSZ;
/* not enough data there */
else if (targetPagePtr + reqLen > flushptr)
return -1;
/* part of the page available */
else
count = flushptr - targetPagePtr;
XLogRead(cur_page, *pageTLI, targetPagePtr, XLOG_BLCKSZ);
return count;
}
/*
* Read up to MAX_SEND_SIZE bytes of WAL that's been flushed to disk,
* but not yet sent to the client, and buffer it in the libpq output
* buffer.
*
* msgbuf is a work area in which the output message is constructed. It's
* passed in just so we can avoid re-palloc'ing the buffer on each cycle.
* It must be of size 1 + sizeof(WalDataMessageHeader) + MAX_SEND_SIZE.
*
* If there is no unsent WAL remaining, *caughtup is set to true, otherwise
* *caughtup is set to false.
*/
static void
XLogSend(char *msgbuf, bool *caughtup)
{
XLogRecPtr SendRqstPtr;
XLogRecPtr startptr;
XLogRecPtr endptr;
Size nbytes;
WalDataMessageHeader msghdr;
/*
* Attempt to send all data that's already been written out and fsync'd to
* disk. We cannot go further than what's been written out given the
* current implementation of XLogRead(). And in any case it's unsafe to
* send WAL that is not securely down to disk on the master: if the master
* subsequently crashes and restarts, slaves must not have applied any WAL
* that gets lost on the master.
*/
SendRqstPtr = am_cascading_walsender ? GetStandbyFlushRecPtr() : GetFlushRecPtr();
/* Quick exit if nothing to do */
if (XLByteLE(SendRqstPtr, sentPtr))
{
*caughtup = true;
return;
}
/*
* Figure out how much to send in one message. If there's no more than
* MAX_SEND_SIZE bytes to send, send everything. Otherwise send
* MAX_SEND_SIZE bytes, but round back to logfile or page boundary.
*
* The rounding is not only for performance reasons. Walreceiver relies on
* the fact that we never split a WAL record across two messages. Since a
* long WAL record is split at page boundary into continuation records,
* page boundary is always a safe cut-off point. We also assume that
* SendRqstPtr never points to the middle of a WAL record.
*/
startptr = sentPtr;
if (startptr.xrecoff >= XLogFileSize)
{
/*
* crossing a logid boundary, skip the non-existent last log segment
* in previous logical log file.
*/
startptr.xlogid += 1;
startptr.xrecoff = 0;
}
endptr = startptr;
XLByteAdvance(endptr, MAX_SEND_SIZE);
if (endptr.xlogid != startptr.xlogid)
{
/* Don't cross a logfile boundary within one message */
Assert(endptr.xlogid == startptr.xlogid + 1);
endptr.xlogid = startptr.xlogid;
endptr.xrecoff = XLogFileSize;
}
/* if we went beyond SendRqstPtr, back off */
if (XLByteLE(SendRqstPtr, endptr))
{
endptr = SendRqstPtr;
*caughtup = true;
}
else
{
/* round down to page boundary. */
endptr.xrecoff -= (endptr.xrecoff % XLOG_BLCKSZ);
*caughtup = false;
}
nbytes = endptr.xrecoff - startptr.xrecoff;
Assert(nbytes <= MAX_SEND_SIZE);
/*
* OK to read and send the slice.
*/
msgbuf[0] = 'w';
/*
* Read the log directly into the output buffer to avoid extra memcpy
* calls.
*/
XLogRead(msgbuf + 1 + sizeof(WalDataMessageHeader), startptr, nbytes);
/*
* We fill the message header last so that the send timestamp is taken as
* late as possible.
*/
msghdr.dataStart = startptr;
msghdr.walEnd = SendRqstPtr;
msghdr.sendTime = GetCurrentTimestamp();
memcpy(msgbuf + 1, &msghdr, sizeof(WalDataMessageHeader));
pq_putmessage_noblock('d', msgbuf, 1 + sizeof(WalDataMessageHeader) + nbytes);
sentPtr = endptr;
/* Update shared memory status */
{
/* use volatile pointer to prevent code rearrangement */
volatile WalSnd *walsnd = MyWalSnd;
SpinLockAcquire(&walsnd->mutex);
walsnd->sentPtr = sentPtr;
SpinLockRelease(&walsnd->mutex);
}
/* Report progress of XLOG streaming in PS display */
if (update_process_title)
{
char activitymsg[50];
snprintf(activitymsg, sizeof(activitymsg), "streaming %X/%X",
sentPtr.xlogid, sentPtr.xrecoff);
set_ps_display(activitymsg, false);
}
return;
}