/*------------------------------------------------------------------------- * * xlogreader.c * Generic XLog reading facility * * Portions Copyright (c) 2013-2021, PostgreSQL Global Development Group * * IDENTIFICATION * src/backend/access/transam/xlogreader.c * * NOTES * See xlogreader.h for more notes on this facility. * * This file is compiled as both front-end and backend code, so it * may not use ereport, server-defined static variables, etc. *------------------------------------------------------------------------- */ #include "postgres.h" #include #include "access/transam.h" #include "access/xlog_internal.h" #include "access/xlogreader.h" #include "access/xlogrecord.h" #include "catalog/pg_control.h" #include "common/pg_lzcompress.h" #include "replication/origin.h" #ifndef FRONTEND #include "miscadmin.h" #include "pgstat.h" #include "utils/memutils.h" #endif static void report_invalid_record(XLogReaderState *state, const char *fmt,...) pg_attribute_printf(2, 3); static bool allocate_recordbuf(XLogReaderState *state, uint32 reclength); static bool XLogNeedData(XLogReaderState *state, XLogRecPtr pageptr, int reqLen, bool header_inclusive); static void XLogReaderInvalReadState(XLogReaderState *state); static bool ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr, XLogRecPtr PrevRecPtr, XLogRecord *record); static bool ValidXLogRecord(XLogReaderState *state, XLogRecord *record, XLogRecPtr recptr); static void ResetDecoder(XLogReaderState *state); static void WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt, int segsize, const char *waldir); /* size of the buffer allocated for error message. */ #define MAX_ERRORMSG_LEN 1000 /* * Construct a string in state->errormsg_buf explaining what's wrong with * the current record being read. */ static void report_invalid_record(XLogReaderState *state, const char *fmt,...) { va_list args; fmt = _(fmt); va_start(args, fmt); vsnprintf(state->errormsg_buf, MAX_ERRORMSG_LEN, fmt, args); va_end(args); } /* * Allocate and initialize a new XLogReader. * * Returns NULL if the xlogreader couldn't be allocated. */ XLogReaderState * XLogReaderAllocate(int wal_segment_size, const char *waldir, WALSegmentCleanupCB cleanup_cb) { XLogReaderState *state; state = (XLogReaderState *) palloc_extended(sizeof(XLogReaderState), MCXT_ALLOC_NO_OOM | MCXT_ALLOC_ZERO); if (!state) return NULL; /* initialize caller-provided support functions */ state->cleanup_cb = cleanup_cb; state->max_block_id = -1; /* * Permanently allocate readBuf. We do it this way, rather than just * making a static array, for two reasons: (1) no need to waste the * storage in most instantiations of the backend; (2) a static char array * isn't guaranteed to have any particular alignment, whereas * palloc_extended() will provide MAXALIGN'd storage. */ state->readBuf = (char *) palloc_extended(XLOG_BLCKSZ, MCXT_ALLOC_NO_OOM); if (!state->readBuf) { pfree(state); return NULL; } /* Initialize segment info. */ WALOpenSegmentInit(&state->seg, &state->segcxt, wal_segment_size, waldir); /* ReadRecPtr, EndRecPtr, reqLen and readLen initialized to zeroes above */ state->errormsg_buf = palloc_extended(MAX_ERRORMSG_LEN + 1, MCXT_ALLOC_NO_OOM); if (!state->errormsg_buf) { pfree(state->readBuf); pfree(state); return NULL; } state->errormsg_buf[0] = '\0'; /* * Allocate an initial readRecordBuf of minimal size, which can later be * enlarged if necessary. */ if (!allocate_recordbuf(state, 0)) { pfree(state->errormsg_buf); pfree(state->readBuf); pfree(state); return NULL; } return state; } void XLogReaderFree(XLogReaderState *state) { int block_id; if (state->seg.ws_file >= 0) state->cleanup_cb(state); for (block_id = 0; block_id <= XLR_MAX_BLOCK_ID; block_id++) { if (state->blocks[block_id].data) pfree(state->blocks[block_id].data); } if (state->main_data) pfree(state->main_data); pfree(state->errormsg_buf); if (state->readRecordBuf) pfree(state->readRecordBuf); pfree(state->readBuf); pfree(state); } /* * Allocate readRecordBuf to fit a record of at least the given length. * Returns true if successful, false if out of memory. * * readRecordBufSize is set to the new buffer size. * * To avoid useless small increases, round its size to a multiple of * XLOG_BLCKSZ, and make sure it's at least 5*Max(BLCKSZ, XLOG_BLCKSZ) to start * with. (That is enough for all "normal" records, but very large commit or * abort records might need more space.) */ static bool allocate_recordbuf(XLogReaderState *state, uint32 reclength) { uint32 newSize = reclength; newSize += XLOG_BLCKSZ - (newSize % XLOG_BLCKSZ); newSize = Max(newSize, 5 * Max(BLCKSZ, XLOG_BLCKSZ)); #ifndef FRONTEND /* * Note that in much unlucky circumstances, the random data read from a * recycled segment can cause this routine to be called with a size * causing a hard failure at allocation. For a standby, this would cause * the instance to stop suddenly with a hard failure, preventing it to * retry fetching WAL from one of its sources which could allow it to move * on with replay without a manual restart. If the data comes from a past * recycled segment and is still valid, then the allocation may succeed * but record checks are going to fail so this would be short-lived. If * the allocation fails because of a memory shortage, then this is not a * hard failure either per the guarantee given by MCXT_ALLOC_NO_OOM. */ if (!AllocSizeIsValid(newSize)) return false; #endif if (state->readRecordBuf) pfree(state->readRecordBuf); state->readRecordBuf = (char *) palloc_extended(newSize, MCXT_ALLOC_NO_OOM); if (state->readRecordBuf == NULL) { state->readRecordBufSize = 0; return false; } state->readRecordBufSize = newSize; return true; } /* * Initialize the passed segment structs. */ static void WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt, int segsize, const char *waldir) { seg->ws_file = -1; seg->ws_segno = 0; seg->ws_tli = 0; segcxt->ws_segsize = segsize; if (waldir) snprintf(segcxt->ws_dir, MAXPGPATH, "%s", waldir); } /* * Begin reading WAL at 'RecPtr'. * * 'RecPtr' should point to the beginnning of a valid WAL record. Pointing at * the beginning of a page is also OK, if there is a new record right after * the page header, i.e. not a continuation. * * This does not make any attempt to read the WAL yet, and hence cannot fail. * If the starting address is not correct, the first call to XLogReadRecord() * will error out. */ void XLogBeginRead(XLogReaderState *state, XLogRecPtr RecPtr) { Assert(!XLogRecPtrIsInvalid(RecPtr)); ResetDecoder(state); /* Begin at the passed-in record pointer. */ state->EndRecPtr = RecPtr; state->ReadRecPtr = InvalidXLogRecPtr; state->readRecordState = XLREAD_NEXT_RECORD; } /* * Attempt to read an XLOG record. * * XLogBeginRead() or XLogFindNextRecord() must be called before the first call * to XLogReadRecord(). * * This function may return XLREAD_NEED_DATA several times before returning a * result record. The caller shall read in some new data then call this * function again with the same parameters. * * When a record is successfully read, returns XLREAD_SUCCESS with result * record being stored in *record. Otherwise *record is NULL. * * Returns XLREAD_NEED_DATA if more data is needed to finish decoding the * current record. In that case, state->readPagePtr and state->reqLen inform * the desired position and minimum length of data needed. The caller shall * read in the requested data and set state->readBuf to point to a buffer * containing it. The caller must also set state->seg->ws_tli and * state->readLen to indicate the timeline that it was read from, and the * length of data that is now available (which must be >= given reqLen), * respectively. * * If invalid data is encountered, returns XLREAD_FAIL and sets *record to * NULL. *errormsg is set to a string with details of the failure. The * returned pointer (or *errormsg) points to an internal buffer that's valid * until the next call to XLogReadRecord. * * * This function runs a state machine consisting of the following states. * * XLREAD_NEXT_RECORD: * The initial state. If called with a valid XLogRecPtr, try to read a * record at that position. If invalid RecPtr is given try to read a record * just after the last one read. The next state is XLREAD_TOT_LEN. * * XLREAD_TOT_LEN: * Examining record header. Ends after reading record length. * recordRemainLen and recordGotLen are initialized. The next state is * XLREAD_FIRST_FRAGMENT. * * XLREAD_FIRST_FRAGMENT: * Reading the first fragment. Goes to XLREAD_NEXT_RECORD if that's all or * XLREAD_CONTINUATION if we need more data. * XLREAD_CONTINUATION: * Reading continuation of record. If the whole record is now decoded, goes * to XLREAD_NEXT_RECORD. During this state, recordRemainLen indicates how * much is left. * * If invalid data is found in any state, the state machine stays at the * current state. This behavior allows us to continue reading a record * after switching to a different source, during streaming replication. */ XLogReadRecordResult XLogReadRecord(XLogReaderState *state, XLogRecord **record, char **errormsg) { XLogRecord *prec; *record = NULL; /* reset error state */ *errormsg = NULL; state->errormsg_buf[0] = '\0'; switch (state->readRecordState) { case XLREAD_NEXT_RECORD: ResetDecoder(state); if (state->ReadRecPtr != InvalidXLogRecPtr) { /* read the record after the one we just read */ /* * EndRecPtr is pointing to end+1 of the previous WAL record. * If we're at a page boundary, no more records can fit on the * current page. We must skip over the page header, but we * can't do that until we've read in the page, since the * header size is variable. */ state->PrevRecPtr = state->ReadRecPtr; state->ReadRecPtr = state->EndRecPtr; } else { /* * Caller supplied a position to start at. * * In this case, EndRecPtr should already be pointing to a * valid record starting position. */ Assert(XRecOffIsValid(state->EndRecPtr)); state->ReadRecPtr = state->EndRecPtr; /* * We cannot verify the previous-record pointer when we're * seeking to a particular record. Reset PrevRecPtr so that we * won't try doing that. */ state->PrevRecPtr = InvalidXLogRecPtr; state->EndRecPtr = InvalidXLogRecPtr; /* to be tidy */ } state->record_verified = false; state->readRecordState = XLREAD_TOT_LEN; /* fall through */ case XLREAD_TOT_LEN: { uint32 total_len; uint32 pageHeaderSize; XLogRecPtr targetPagePtr; uint32 targetRecOff; XLogPageHeader pageHeader; targetPagePtr = state->ReadRecPtr - (state->ReadRecPtr % XLOG_BLCKSZ); targetRecOff = state->ReadRecPtr % XLOG_BLCKSZ; /* * Check if we have enough data. For the first record in the * page, the requesting length doesn't contain page header. */ if (XLogNeedData(state, targetPagePtr, Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ), targetRecOff != 0)) return XLREAD_NEED_DATA; /* error out if caller supplied bogus page */ if (!state->page_verified) goto err; /* examine page header now. */ pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf); if (targetRecOff == 0) { /* At page start, so skip over page header. */ state->ReadRecPtr += pageHeaderSize; targetRecOff = pageHeaderSize; } else if (targetRecOff < pageHeaderSize) { report_invalid_record(state, "invalid record offset at %X/%X", LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } pageHeader = (XLogPageHeader) state->readBuf; if ((pageHeader->xlp_info & XLP_FIRST_IS_CONTRECORD) && targetRecOff == pageHeaderSize) { report_invalid_record(state, "contrecord is requested by %X/%X", (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr); goto err; } /* XLogNeedData has verified the page header */ Assert(pageHeaderSize <= state->readLen); /* * Read the record length. * * NB: Even though we use an XLogRecord pointer here, the * whole record header might not fit on this page. xl_tot_len * is the first field of the struct, so it must be on this * page (the records are MAXALIGNed), but we cannot access any * other fields until we've verified that we got the whole * header. */ prec = (XLogRecord *) (state->readBuf + state->ReadRecPtr % XLOG_BLCKSZ); total_len = prec->xl_tot_len; /* * If the whole record header is on this page, validate it * immediately. Otherwise do just a basic sanity check on * xl_tot_len, and validate the rest of the header after * reading it from the next page. The xl_tot_len check is * necessary here to ensure that we enter the * XLREAD_CONTINUATION state below; otherwise we might fail to * apply ValidXLogRecordHeader at all. */ if (targetRecOff <= XLOG_BLCKSZ - SizeOfXLogRecord) { if (!ValidXLogRecordHeader(state, state->ReadRecPtr, state->PrevRecPtr, prec)) goto err; state->record_verified = true; } else { /* XXX: more validation should be done here */ if (total_len < SizeOfXLogRecord) { report_invalid_record(state, "invalid record length at %X/%X: wanted %u, got %u", LSN_FORMAT_ARGS(state->ReadRecPtr), (uint32) SizeOfXLogRecord, total_len); goto err; } } /* * Wait for the rest of the record, or the part of the record * that fit on the first page if crossed a page boundary, to * become available. */ state->recordGotLen = 0; state->recordRemainLen = total_len; state->readRecordState = XLREAD_FIRST_FRAGMENT; } /* fall through */ case XLREAD_FIRST_FRAGMENT: { uint32 total_len = state->recordRemainLen; uint32 request_len; uint32 record_len; XLogRecPtr targetPagePtr; uint32 targetRecOff; /* * Wait for the rest of the record on the first page to become * available */ targetPagePtr = state->ReadRecPtr - (state->ReadRecPtr % XLOG_BLCKSZ); targetRecOff = state->ReadRecPtr % XLOG_BLCKSZ; request_len = Min(targetRecOff + total_len, XLOG_BLCKSZ); record_len = request_len - targetRecOff; /* ReadRecPtr contains page header */ Assert(targetRecOff != 0); if (XLogNeedData(state, targetPagePtr, request_len, true)) return XLREAD_NEED_DATA; /* error out if caller supplied bogus page */ if (!state->page_verified) goto err; prec = (XLogRecord *) (state->readBuf + targetRecOff); /* validate record header if not yet */ if (!state->record_verified && record_len >= SizeOfXLogRecord) { if (!ValidXLogRecordHeader(state, state->ReadRecPtr, state->PrevRecPtr, prec)) goto err; state->record_verified = true; } if (total_len == record_len) { /* Record does not cross a page boundary */ Assert(state->record_verified); if (!ValidXLogRecord(state, prec, state->ReadRecPtr)) goto err; state->record_verified = true; /* to be tidy */ /* We already checked the header earlier */ state->EndRecPtr = state->ReadRecPtr + MAXALIGN(record_len); *record = prec; state->readRecordState = XLREAD_NEXT_RECORD; break; } /* * The record continues on the next page. Need to reassemble * record */ Assert(total_len > record_len); /* Enlarge readRecordBuf as needed. */ if (total_len > state->readRecordBufSize && !allocate_recordbuf(state, total_len)) { /* We treat this as a "bogus data" condition */ report_invalid_record(state, "record length %u at %X/%X too long", total_len, LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } /* Copy the first fragment of the record from the first page. */ memcpy(state->readRecordBuf, state->readBuf + targetRecOff, record_len); state->recordGotLen += record_len; state->recordRemainLen -= record_len; /* Calculate pointer to beginning of next page */ state->recordContRecPtr = state->ReadRecPtr + record_len; Assert(state->recordContRecPtr % XLOG_BLCKSZ == 0); state->readRecordState = XLREAD_CONTINUATION; } /* fall through */ case XLREAD_CONTINUATION: { XLogPageHeader pageHeader = NULL; uint32 pageHeaderSize; XLogRecPtr targetPagePtr = InvalidXLogRecPtr; /* * we enter this state only if we haven't read the whole * record. */ Assert(state->recordRemainLen > 0); while (state->recordRemainLen > 0) { char *contdata; uint32 request_len PG_USED_FOR_ASSERTS_ONLY; uint32 record_len; /* Wait for the next page to become available */ targetPagePtr = state->recordContRecPtr; /* this request contains page header */ Assert(targetPagePtr != 0); if (XLogNeedData(state, targetPagePtr, Min(state->recordRemainLen, XLOG_BLCKSZ), false)) return XLREAD_NEED_DATA; if (!state->page_verified) goto err; Assert(SizeOfXLogShortPHD <= state->readLen); /* Check that the continuation on next page looks valid */ pageHeader = (XLogPageHeader) state->readBuf; if (!(pageHeader->xlp_info & XLP_FIRST_IS_CONTRECORD)) { report_invalid_record( state, "there is no contrecord flag at %X/%X reading %X/%X", (uint32) (state->recordContRecPtr >> 32), (uint32) state->recordContRecPtr, (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr); goto err; } /* * Cross-check that xlp_rem_len agrees with how much of * the record we expect there to be left. */ if (pageHeader->xlp_rem_len == 0 || pageHeader->xlp_rem_len != state->recordRemainLen) { report_invalid_record( state, "invalid contrecord length %u at %X/%X reading %X/%X, expected %u", pageHeader->xlp_rem_len, (uint32) (state->recordContRecPtr >> 32), (uint32) state->recordContRecPtr, (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr, state->recordRemainLen); goto err; } /* Append the continuation from this page to the buffer */ pageHeaderSize = XLogPageHeaderSize(pageHeader); /* * XLogNeedData should have ensured that the whole page * header was read */ Assert(pageHeaderSize <= state->readLen); contdata = (char *) state->readBuf + pageHeaderSize; record_len = XLOG_BLCKSZ - pageHeaderSize; if (pageHeader->xlp_rem_len < record_len) record_len = pageHeader->xlp_rem_len; request_len = record_len + pageHeaderSize; /* * XLogNeedData should have ensured all needed data was * read */ Assert(request_len <= state->readLen); memcpy(state->readRecordBuf + state->recordGotLen, (char *) contdata, record_len); state->recordGotLen += record_len; state->recordRemainLen -= record_len; /* If we just reassembled the record header, validate it. */ if (!state->record_verified) { Assert(state->recordGotLen >= SizeOfXLogRecord); if (!ValidXLogRecordHeader(state, state->ReadRecPtr, state->PrevRecPtr, (XLogRecord *) state->readRecordBuf)) goto err; state->record_verified = true; } /* * Calculate pointer to beginning of next page, and * continue */ state->recordContRecPtr += XLOG_BLCKSZ; } /* targetPagePtr is pointing the last-read page here */ prec = (XLogRecord *) state->readRecordBuf; if (!ValidXLogRecord(state, prec, state->ReadRecPtr)) goto err; pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf); state->EndRecPtr = targetPagePtr + pageHeaderSize + MAXALIGN(pageHeader->xlp_rem_len); *record = prec; state->readRecordState = XLREAD_NEXT_RECORD; break; } } /* * Special processing if it's an XLOG SWITCH record */ if ((*record)->xl_rmid == RM_XLOG_ID && ((*record)->xl_info & ~XLR_INFO_MASK) == XLOG_SWITCH) { /* Pretend it extends to end of segment */ state->EndRecPtr += state->segcxt.ws_segsize - 1; state->EndRecPtr -= XLogSegmentOffset(state->EndRecPtr, state->segcxt.ws_segsize); } if (DecodeXLogRecord(state, *record, errormsg)) return XLREAD_SUCCESS; *record = NULL; return XLREAD_FAIL; err: /* * Invalidate the read page. We might read from a different source after * failure. */ XLogReaderInvalReadState(state); if (state->errormsg_buf[0] != '\0') *errormsg = state->errormsg_buf; *record = NULL; return XLREAD_FAIL; } /* * Checks that an xlog page loaded in state->readBuf is including at least * [pageptr, reqLen] and the page is valid. header_inclusive indicates that * reqLen is calculated including page header length. * * Returns false if the buffer already contains the requested data, or found * error. state->page_verified is set to true for the former and false for the * latter. * * Otherwise returns true and requests data loaded onto state->readBuf by * state->readPagePtr and state->readLen. The caller shall call this function * again after filling the buffer at least with that portion of data and set * state->readLen to the length of actually loaded data. * * If header_inclusive is false, corrects reqLen internally by adding the * actual page header length and may request caller for new data. */ static bool XLogNeedData(XLogReaderState *state, XLogRecPtr pageptr, int reqLen, bool header_inclusive) { uint32 targetPageOff; XLogSegNo targetSegNo; uint32 addLen = 0; /* Some data is loaded, but page header is not verified yet. */ if (!state->page_verified && !XLogRecPtrIsInvalid(state->readPagePtr) && state->readLen >= 0) { uint32 pageHeaderSize; /* just loaded new data so needs to verify page header */ /* The caller must have loaded at least page header */ Assert(state->readLen >= SizeOfXLogShortPHD); /* * We have enough data to check the header length. Recheck the loaded * length against the actual header length. */ pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf); /* Request more data if we don't have the full header. */ if (state->readLen < pageHeaderSize) { state->reqLen = pageHeaderSize; return true; } /* Now that we know we have the full header, validate it. */ if (!XLogReaderValidatePageHeader(state, state->readPagePtr, (char *) state->readBuf)) { /* That's bad. Force reading the page again. */ XLogReaderInvalReadState(state); return false; } state->page_verified = true; XLByteToSeg(state->readPagePtr, state->seg.ws_segno, state->segcxt.ws_segsize); } /* * The loaded page may not be the one caller is supposing to read when we * are verifying the first page of new segment. In that case, skip further * verification and immediately load the target page. */ if (state->page_verified && pageptr == state->readPagePtr) { /* * calculate additional length for page header keeping the total * length within the block size. */ if (!header_inclusive) { uint32 pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf); addLen = pageHeaderSize; if (reqLen + pageHeaderSize <= XLOG_BLCKSZ) addLen = pageHeaderSize; else addLen = XLOG_BLCKSZ - reqLen; } /* Return if we already have it. */ if (reqLen + addLen <= state->readLen) return false; } /* Data is not in our buffer, request the caller for it. */ XLByteToSeg(pageptr, targetSegNo, state->segcxt.ws_segsize); targetPageOff = XLogSegmentOffset(pageptr, state->segcxt.ws_segsize); Assert((pageptr % XLOG_BLCKSZ) == 0); /* * Every time we request to load new data of a page to the caller, even if * we looked at a part of it before, we need to do verification on the * next invocation as the caller might now be rereading data from a * different source. */ state->page_verified = false; /* * Whenever switching to a new WAL segment, we read the first page of the * file and validate its header, even if that's not where the target * record is. This is so that we can check the additional identification * info that is present in the first page's "long" header. Don't do this * if the caller requested the first page in the segment. */ if (targetSegNo != state->seg.ws_segno && targetPageOff != 0) { /* * Then we'll see that the targetSegNo now matches the ws_segno, and * will not come back here, but will request the actual target page. */ state->readPagePtr = pageptr - targetPageOff; state->reqLen = XLOG_BLCKSZ; return true; } /* * Request the caller to load the page. We need at least a short page * header so that we can validate it. */ state->readPagePtr = pageptr; state->reqLen = Max(reqLen + addLen, SizeOfXLogShortPHD); return true; } /* * Invalidate the xlogreader's read state to force a re-read. */ static void XLogReaderInvalReadState(XLogReaderState *state) { state->readPagePtr = InvalidXLogRecPtr; } /* * Validate an XLOG record header. * * This is just a convenience subroutine to avoid duplicated code in * XLogReadRecord. It's not intended for use from anywhere else. * * If PrevRecPtr is valid, the xl_prev is is cross-checked with it. */ static bool ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr, XLogRecPtr PrevRecPtr, XLogRecord *record) { if (record->xl_tot_len < SizeOfXLogRecord) { report_invalid_record(state, "invalid record length at %X/%X: wanted %u, got %u", LSN_FORMAT_ARGS(RecPtr), (uint32) SizeOfXLogRecord, record->xl_tot_len); return false; } if (record->xl_rmid > RM_MAX_ID) { report_invalid_record(state, "invalid resource manager ID %u at %X/%X", record->xl_rmid, LSN_FORMAT_ARGS(RecPtr)); return false; } if (PrevRecPtr == InvalidXLogRecPtr) { /* * We can't exactly verify the prev-link, but surely it should be less * than the record's own address. */ if (!(record->xl_prev < RecPtr)) { report_invalid_record(state, "record with incorrect prev-link %X/%X at %X/%X", LSN_FORMAT_ARGS(record->xl_prev), LSN_FORMAT_ARGS(RecPtr)); return false; } } else { /* * Record's prev-link should exactly match our previous location. This * check guards against torn WAL pages where a stale but valid-looking * WAL record starts on a sector boundary. */ if (record->xl_prev != PrevRecPtr) { report_invalid_record(state, "record with incorrect prev-link %X/%X at %X/%X", LSN_FORMAT_ARGS(record->xl_prev), LSN_FORMAT_ARGS(RecPtr)); return false; } } return true; } /* * CRC-check an XLOG record. We do not believe the contents of an XLOG * record (other than to the minimal extent of computing the amount of * data to read in) until we've checked the CRCs. * * We assume all of the record (that is, xl_tot_len bytes) has been read * into memory at *record. Also, ValidXLogRecordHeader() has accepted the * record's header, which means in particular that xl_tot_len is at least * SizeOfXLogRecord. */ static bool ValidXLogRecord(XLogReaderState *state, XLogRecord *record, XLogRecPtr recptr) { pg_crc32c crc; /* Calculate the CRC */ INIT_CRC32C(crc); COMP_CRC32C(crc, ((char *) record) + SizeOfXLogRecord, record->xl_tot_len - SizeOfXLogRecord); /* include the record header last */ COMP_CRC32C(crc, (char *) record, offsetof(XLogRecord, xl_crc)); FIN_CRC32C(crc); if (!EQ_CRC32C(record->xl_crc, crc)) { report_invalid_record(state, "incorrect resource manager data checksum in record at %X/%X", LSN_FORMAT_ARGS(recptr)); return false; } return true; } /* * Validate a page header. * * Check if 'phdr' is valid as the header of the XLog page at position * 'recptr'. */ bool XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr, char *phdr) { XLogRecPtr recaddr; XLogSegNo segno; int32 offset; XLogPageHeader hdr = (XLogPageHeader) phdr; Assert((recptr % XLOG_BLCKSZ) == 0); XLByteToSeg(recptr, segno, state->segcxt.ws_segsize); offset = XLogSegmentOffset(recptr, state->segcxt.ws_segsize); XLogSegNoOffsetToRecPtr(segno, offset, state->segcxt.ws_segsize, recaddr); if (hdr->xlp_magic != XLOG_PAGE_MAGIC) { char fname[MAXFNAMELEN]; XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize); report_invalid_record(state, "invalid magic number %04X in log segment %s, offset %u", hdr->xlp_magic, fname, offset); return false; } if ((hdr->xlp_info & ~XLP_ALL_FLAGS) != 0) { char fname[MAXFNAMELEN]; XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize); report_invalid_record(state, "invalid info bits %04X in log segment %s, offset %u", hdr->xlp_info, fname, offset); return false; } if (hdr->xlp_info & XLP_LONG_HEADER) { XLogLongPageHeader longhdr = (XLogLongPageHeader) hdr; if (state->system_identifier && longhdr->xlp_sysid != state->system_identifier) { report_invalid_record(state, "WAL file is from different database system: WAL file database system identifier is %llu, pg_control database system identifier is %llu", (unsigned long long) longhdr->xlp_sysid, (unsigned long long) state->system_identifier); return false; } else if (longhdr->xlp_seg_size != state->segcxt.ws_segsize) { report_invalid_record(state, "WAL file is from different database system: incorrect segment size in page header"); return false; } else if (longhdr->xlp_xlog_blcksz != XLOG_BLCKSZ) { report_invalid_record(state, "WAL file is from different database system: incorrect XLOG_BLCKSZ in page header"); return false; } } else if (offset == 0) { char fname[MAXFNAMELEN]; XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize); /* hmm, first page of file doesn't have a long header? */ report_invalid_record(state, "invalid info bits %04X in log segment %s, offset %u", hdr->xlp_info, fname, offset); return false; } /* * Check that the address on the page agrees with what we expected. This * check typically fails when an old WAL segment is recycled, and hasn't * yet been overwritten with new data yet. */ if (hdr->xlp_pageaddr != recaddr) { char fname[MAXFNAMELEN]; XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize); report_invalid_record(state, "unexpected pageaddr %X/%X in log segment %s, offset %u", LSN_FORMAT_ARGS(hdr->xlp_pageaddr), fname, offset); return false; } /* * Since child timelines are always assigned a TLI greater than their * immediate parent's TLI, we should never see TLI go backwards across * successive pages of a consistent WAL sequence. * * Sometimes we re-read a segment that's already been (partially) read. So * we only verify TLIs for pages that are later than the last remembered * LSN. */ if (recptr > state->latestPagePtr) { if (hdr->xlp_tli < state->latestPageTLI) { char fname[MAXFNAMELEN]; XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize); report_invalid_record(state, "out-of-sequence timeline ID %u (after %u) in log segment %s, offset %u", hdr->xlp_tli, state->latestPageTLI, fname, offset); return false; } } state->latestPagePtr = recptr; state->latestPageTLI = hdr->xlp_tli; return true; } #ifdef FRONTEND /* * Functions that are currently not needed in the backend, but are better * implemented inside xlogreader.c because of the internal facilities available * here. */ XLogFindNextRecordState * InitXLogFindNextRecord(XLogReaderState *reader_state, XLogRecPtr start_ptr) { XLogFindNextRecordState *state = (XLogFindNextRecordState *) palloc_extended(sizeof(XLogFindNextRecordState), MCXT_ALLOC_NO_OOM | MCXT_ALLOC_ZERO); if (!state) return NULL; state->reader_state = reader_state; state->targetRecPtr = start_ptr; state->currRecPtr = start_ptr; return state; } /* * Find the first record with an lsn >= RecPtr. * * This is different from XLogBeginRead() in that RecPtr doesn't need to point * to a valid record boundary. Useful for checking whether RecPtr is a valid * xlog address for reading, and to find the first valid address after some * address when dumping records for debugging purposes. * * This positions the reader, like XLogBeginRead(), so that the next call to * XLogReadRecord() will read the next valid record. */ bool XLogFindNextRecord(XLogFindNextRecordState *state) { XLogPageHeader header; XLogRecord *record; XLogReadRecordResult result; char *errormsg; Assert(!XLogRecPtrIsInvalid(state->currRecPtr)); /* * skip over potential continuation data, keeping in mind that it may span * multiple pages */ while (true) { XLogRecPtr targetPagePtr; int targetRecOff; uint32 pageHeaderSize; /* * Compute targetRecOff. It should typically be equal or greater than * short page-header since a valid record can't start anywhere before * that, except when caller has explicitly specified the offset that * falls somewhere there or when we are skipping multi-page * continuation record. It doesn't matter though because * XLogNeedData() is prepared to handle that and will read at least * short page-header worth of data */ targetRecOff = state->currRecPtr % XLOG_BLCKSZ; /* scroll back to page boundary */ targetPagePtr = state->currRecPtr - targetRecOff; if (XLogNeedData(state->reader_state, targetPagePtr, targetRecOff, targetRecOff != 0)) return true; if (!state->reader_state->page_verified) goto err; header = (XLogPageHeader) state->reader_state->readBuf; pageHeaderSize = XLogPageHeaderSize(header); /* we should have read the page header */ Assert(state->reader_state->readLen >= pageHeaderSize); /* skip over potential continuation data */ if (header->xlp_info & XLP_FIRST_IS_CONTRECORD) { /* * If the length of the remaining continuation data is more than * what can fit in this page, the continuation record crosses over * this page. Read the next page and try again. xlp_rem_len in the * next page header will contain the remaining length of the * continuation data * * Note that record headers are MAXALIGN'ed */ if (MAXALIGN(header->xlp_rem_len) >= (XLOG_BLCKSZ - pageHeaderSize)) state->currRecPtr = targetPagePtr + XLOG_BLCKSZ; else { /* * The previous continuation record ends in this page. Set * state->currRecPtr to point to the first valid record */ state->currRecPtr = targetPagePtr + pageHeaderSize + MAXALIGN(header->xlp_rem_len); break; } } else { state->currRecPtr = targetPagePtr + pageHeaderSize; break; } } /* * we know now that tmpRecPtr is an address pointing to a valid XLogRecord * because either we're at the first record after the beginning of a page * or we just jumped over the remaining data of a continuation. */ XLogBeginRead(state->reader_state, state->currRecPtr); while ((result = XLogReadRecord(state->reader_state, &record, &errormsg)) != XLREAD_FAIL) { if (result == XLREAD_NEED_DATA) return true; /* past the record we've found, break out */ if (state->targetRecPtr <= state->reader_state->ReadRecPtr) { /* Rewind the reader to the beginning of the last record. */ state->currRecPtr = state->reader_state->ReadRecPtr; XLogBeginRead(state->reader_state, state->currRecPtr); return false; } } err: XLogReaderInvalReadState(state->reader_state); state->currRecPtr = InvalidXLogRecPtr;; return false; } #endif /* FRONTEND */ /* * Helper function to ease writing of routines that read raw WAL data. * If this function is used, caller must supply a segment_open callback and * segment_close callback as that is used here. * * Read 'count' bytes into 'buf', starting at location 'startptr', from WAL * fetched from timeline 'tli'. * * Returns true if succeeded, false if an error occurs, in which case * 'errinfo' receives error details. * * XXX probably this should be improved to suck data directly from the * WAL buffers when possible. */ bool WALRead(XLogReaderState *state, WALSegmentOpenCB segopenfn, WALSegmentCloseCB segclosefn, char *buf, XLogRecPtr startptr, Size count, TimeLineID tli, WALReadError *errinfo) { char *p; XLogRecPtr recptr; Size nbytes; p = buf; recptr = startptr; nbytes = count; while (nbytes > 0) { uint32 startoff; int segbytes; int readbytes; startoff = XLogSegmentOffset(recptr, state->segcxt.ws_segsize); /* * If the data we want is not in a segment we have open, close what we * have (if anything) and open the next one, using the caller's * provided openSegment callback. */ if (state->seg.ws_file < 0 || !XLByteInSeg(recptr, state->seg.ws_segno, state->segcxt.ws_segsize) || tli != state->seg.ws_tli) { XLogSegNo nextSegNo; if (state->seg.ws_file >= 0) segclosefn(state); XLByteToSeg(recptr, nextSegNo, state->segcxt.ws_segsize); segopenfn(state, nextSegNo, &tli); /* This shouldn't happen -- indicates a bug in segment_open */ Assert(state->seg.ws_file >= 0); /* Update the current segment info. */ state->seg.ws_tli = tli; state->seg.ws_segno = nextSegNo; } /* How many bytes are within this segment? */ if (nbytes > (state->segcxt.ws_segsize - startoff)) segbytes = state->segcxt.ws_segsize - startoff; else segbytes = nbytes; #ifndef FRONTEND pgstat_report_wait_start(WAIT_EVENT_WAL_READ); #endif /* Reset errno first; eases reporting non-errno-affecting errors */ errno = 0; readbytes = pg_pread(state->seg.ws_file, p, segbytes, (off_t) startoff); #ifndef FRONTEND pgstat_report_wait_end(); #endif if (readbytes <= 0) { errinfo->wre_errno = errno; errinfo->wre_req = segbytes; errinfo->wre_read = readbytes; errinfo->wre_off = startoff; errinfo->wre_seg = state->seg; return false; } /* Update state for read */ recptr += readbytes; nbytes -= readbytes; p += readbytes; } return true; } /* ---------------------------------------- * Functions for decoding the data and block references in a record. * ---------------------------------------- */ /* private function to reset the state between records */ static void ResetDecoder(XLogReaderState *state) { int block_id; state->decoded_record = NULL; state->main_data_len = 0; for (block_id = 0; block_id <= state->max_block_id; block_id++) { state->blocks[block_id].in_use = false; state->blocks[block_id].has_image = false; state->blocks[block_id].has_data = false; state->blocks[block_id].apply_image = false; } state->max_block_id = -1; } /* * Decode the previously read record. * * On error, a human-readable error message is returned in *errormsg, and * the return value is false. */ bool DecodeXLogRecord(XLogReaderState *state, XLogRecord *record, char **errormsg) { /* * read next _size bytes from record buffer, but check for overrun first. */ #define COPY_HEADER_FIELD(_dst, _size) \ do { \ if (remaining < _size) \ goto shortdata_err; \ memcpy(_dst, ptr, _size); \ ptr += _size; \ remaining -= _size; \ } while(0) char *ptr; uint32 remaining; uint32 datatotal; RelFileNode *rnode = NULL; uint8 block_id; ResetDecoder(state); state->decoded_record = record; state->record_origin = InvalidRepOriginId; state->toplevel_xid = InvalidTransactionId; ptr = (char *) record; ptr += SizeOfXLogRecord; remaining = record->xl_tot_len - SizeOfXLogRecord; /* Decode the headers */ datatotal = 0; while (remaining > datatotal) { COPY_HEADER_FIELD(&block_id, sizeof(uint8)); if (block_id == XLR_BLOCK_ID_DATA_SHORT) { /* XLogRecordDataHeaderShort */ uint8 main_data_len; COPY_HEADER_FIELD(&main_data_len, sizeof(uint8)); state->main_data_len = main_data_len; datatotal += main_data_len; break; /* by convention, the main data fragment is * always last */ } else if (block_id == XLR_BLOCK_ID_DATA_LONG) { /* XLogRecordDataHeaderLong */ uint32 main_data_len; COPY_HEADER_FIELD(&main_data_len, sizeof(uint32)); state->main_data_len = main_data_len; datatotal += main_data_len; break; /* by convention, the main data fragment is * always last */ } else if (block_id == XLR_BLOCK_ID_ORIGIN) { COPY_HEADER_FIELD(&state->record_origin, sizeof(RepOriginId)); } else if (block_id == XLR_BLOCK_ID_TOPLEVEL_XID) { COPY_HEADER_FIELD(&state->toplevel_xid, sizeof(TransactionId)); } else if (block_id <= XLR_MAX_BLOCK_ID) { /* XLogRecordBlockHeader */ DecodedBkpBlock *blk; uint8 fork_flags; if (block_id <= state->max_block_id) { report_invalid_record(state, "out-of-order block_id %u at %X/%X", block_id, LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } state->max_block_id = block_id; blk = &state->blocks[block_id]; blk->in_use = true; blk->apply_image = false; COPY_HEADER_FIELD(&fork_flags, sizeof(uint8)); blk->forknum = fork_flags & BKPBLOCK_FORK_MASK; blk->flags = fork_flags; blk->has_image = ((fork_flags & BKPBLOCK_HAS_IMAGE) != 0); blk->has_data = ((fork_flags & BKPBLOCK_HAS_DATA) != 0); COPY_HEADER_FIELD(&blk->data_len, sizeof(uint16)); /* cross-check that the HAS_DATA flag is set iff data_length > 0 */ if (blk->has_data && blk->data_len == 0) { report_invalid_record(state, "BKPBLOCK_HAS_DATA set, but no data included at %X/%X", LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } if (!blk->has_data && blk->data_len != 0) { report_invalid_record(state, "BKPBLOCK_HAS_DATA not set, but data length is %u at %X/%X", (unsigned int) blk->data_len, LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } datatotal += blk->data_len; if (blk->has_image) { COPY_HEADER_FIELD(&blk->bimg_len, sizeof(uint16)); COPY_HEADER_FIELD(&blk->hole_offset, sizeof(uint16)); COPY_HEADER_FIELD(&blk->bimg_info, sizeof(uint8)); blk->apply_image = ((blk->bimg_info & BKPIMAGE_APPLY) != 0); if (blk->bimg_info & BKPIMAGE_IS_COMPRESSED) { if (blk->bimg_info & BKPIMAGE_HAS_HOLE) COPY_HEADER_FIELD(&blk->hole_length, sizeof(uint16)); else blk->hole_length = 0; } else blk->hole_length = BLCKSZ - blk->bimg_len; datatotal += blk->bimg_len; /* * cross-check that hole_offset > 0, hole_length > 0 and * bimg_len < BLCKSZ if the HAS_HOLE flag is set. */ if ((blk->bimg_info & BKPIMAGE_HAS_HOLE) && (blk->hole_offset == 0 || blk->hole_length == 0 || blk->bimg_len == BLCKSZ)) { report_invalid_record(state, "BKPIMAGE_HAS_HOLE set, but hole offset %u length %u block image length %u at %X/%X", (unsigned int) blk->hole_offset, (unsigned int) blk->hole_length, (unsigned int) blk->bimg_len, LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } /* * cross-check that hole_offset == 0 and hole_length == 0 if * the HAS_HOLE flag is not set. */ if (!(blk->bimg_info & BKPIMAGE_HAS_HOLE) && (blk->hole_offset != 0 || blk->hole_length != 0)) { report_invalid_record(state, "BKPIMAGE_HAS_HOLE not set, but hole offset %u length %u at %X/%X", (unsigned int) blk->hole_offset, (unsigned int) blk->hole_length, LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } /* * cross-check that bimg_len < BLCKSZ if the IS_COMPRESSED * flag is set. */ if ((blk->bimg_info & BKPIMAGE_IS_COMPRESSED) && blk->bimg_len == BLCKSZ) { report_invalid_record(state, "BKPIMAGE_IS_COMPRESSED set, but block image length %u at %X/%X", (unsigned int) blk->bimg_len, LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } /* * cross-check that bimg_len = BLCKSZ if neither HAS_HOLE nor * IS_COMPRESSED flag is set. */ if (!(blk->bimg_info & BKPIMAGE_HAS_HOLE) && !(blk->bimg_info & BKPIMAGE_IS_COMPRESSED) && blk->bimg_len != BLCKSZ) { report_invalid_record(state, "neither BKPIMAGE_HAS_HOLE nor BKPIMAGE_IS_COMPRESSED set, but block image length is %u at %X/%X", (unsigned int) blk->data_len, LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } } if (!(fork_flags & BKPBLOCK_SAME_REL)) { COPY_HEADER_FIELD(&blk->rnode, sizeof(RelFileNode)); rnode = &blk->rnode; } else { if (rnode == NULL) { report_invalid_record(state, "BKPBLOCK_SAME_REL set but no previous rel at %X/%X", LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } blk->rnode = *rnode; } COPY_HEADER_FIELD(&blk->blkno, sizeof(BlockNumber)); } else { report_invalid_record(state, "invalid block_id %u at %X/%X", block_id, LSN_FORMAT_ARGS(state->ReadRecPtr)); goto err; } } if (remaining != datatotal) goto shortdata_err; /* * Ok, we've parsed the fragment headers, and verified that the total * length of the payload in the fragments is equal to the amount of data * left. Copy the data of each fragment to a separate buffer. * * We could just set up pointers into readRecordBuf, but we want to align * the data for the convenience of the callers. Backup images are not * copied, however; they don't need alignment. */ /* block data first */ for (block_id = 0; block_id <= state->max_block_id; block_id++) { DecodedBkpBlock *blk = &state->blocks[block_id]; if (!blk->in_use) continue; Assert(blk->has_image || !blk->apply_image); if (blk->has_image) { blk->bkp_image = ptr; ptr += blk->bimg_len; } if (blk->has_data) { if (!blk->data || blk->data_len > blk->data_bufsz) { if (blk->data) pfree(blk->data); /* * Force the initial request to be BLCKSZ so that we don't * waste time with lots of trips through this stanza as a * result of WAL compression. */ blk->data_bufsz = MAXALIGN(Max(blk->data_len, BLCKSZ)); blk->data = palloc(blk->data_bufsz); } memcpy(blk->data, ptr, blk->data_len); ptr += blk->data_len; } } /* and finally, the main data */ if (state->main_data_len > 0) { if (!state->main_data || state->main_data_len > state->main_data_bufsz) { if (state->main_data) pfree(state->main_data); /* * main_data_bufsz must be MAXALIGN'ed. In many xlog record * types, we omit trailing struct padding on-disk to save a few * bytes; but compilers may generate accesses to the xlog struct * that assume that padding bytes are present. If the palloc * request is not large enough to include such padding bytes then * we'll get valgrind complaints due to otherwise-harmless fetches * of the padding bytes. * * In addition, force the initial request to be reasonably large * so that we don't waste time with lots of trips through this * stanza. BLCKSZ / 2 seems like a good compromise choice. */ state->main_data_bufsz = MAXALIGN(Max(state->main_data_len, BLCKSZ / 2)); state->main_data = palloc(state->main_data_bufsz); } memcpy(state->main_data, ptr, state->main_data_len); ptr += state->main_data_len; } return true; shortdata_err: report_invalid_record(state, "record with invalid length at %X/%X", LSN_FORMAT_ARGS(state->ReadRecPtr)); err: *errormsg = state->errormsg_buf; return false; } /* * Returns information about the block that a block reference refers to. * * If the WAL record contains a block reference with the given ID, *rnode, * *forknum, and *blknum are filled in (if not NULL), and returns true. * Otherwise returns false. */ bool XLogRecGetBlockTag(XLogReaderState *record, uint8 block_id, RelFileNode *rnode, ForkNumber *forknum, BlockNumber *blknum) { DecodedBkpBlock *bkpb; if (!record->blocks[block_id].in_use) return false; bkpb = &record->blocks[block_id]; if (rnode) *rnode = bkpb->rnode; if (forknum) *forknum = bkpb->forknum; if (blknum) *blknum = bkpb->blkno; return true; } /* * Returns the data associated with a block reference, or NULL if there is * no data (e.g. because a full-page image was taken instead). The returned * pointer points to a MAXALIGNed buffer. */ char * XLogRecGetBlockData(XLogReaderState *record, uint8 block_id, Size *len) { DecodedBkpBlock *bkpb; if (!record->blocks[block_id].in_use) return NULL; bkpb = &record->blocks[block_id]; if (!bkpb->has_data) { if (len) *len = 0; return NULL; } else { if (len) *len = bkpb->data_len; return bkpb->data; } } /* * Restore a full-page image from a backup block attached to an XLOG record. * * Returns true if a full-page image is restored. */ bool RestoreBlockImage(XLogReaderState *record, uint8 block_id, char *page) { DecodedBkpBlock *bkpb; char *ptr; PGAlignedBlock tmp; if (!record->blocks[block_id].in_use) return false; if (!record->blocks[block_id].has_image) return false; bkpb = &record->blocks[block_id]; ptr = bkpb->bkp_image; if (bkpb->bimg_info & BKPIMAGE_IS_COMPRESSED) { /* If a backup block image is compressed, decompress it */ if (pglz_decompress(ptr, bkpb->bimg_len, tmp.data, BLCKSZ - bkpb->hole_length, true) < 0) { report_invalid_record(record, "invalid compressed image at %X/%X, block %d", LSN_FORMAT_ARGS(record->ReadRecPtr), block_id); return false; } ptr = tmp.data; } /* generate page, taking into account hole if necessary */ if (bkpb->hole_length == 0) { memcpy(page, ptr, BLCKSZ); } else { memcpy(page, ptr, bkpb->hole_offset); /* must zero-fill the hole */ MemSet(page + bkpb->hole_offset, 0, bkpb->hole_length); memcpy(page + (bkpb->hole_offset + bkpb->hole_length), ptr + bkpb->hole_offset, BLCKSZ - (bkpb->hole_offset + bkpb->hole_length)); } return true; } #ifndef FRONTEND /* * Extract the FullTransactionId from a WAL record. */ FullTransactionId XLogRecGetFullXid(XLogReaderState *record) { TransactionId xid, next_xid; uint32 epoch; /* * This function is only safe during replay, because it depends on the * replay state. See AdvanceNextFullTransactionIdPastXid() for more. */ Assert(AmStartupProcess() || !IsUnderPostmaster); xid = XLogRecGetXid(record); next_xid = XidFromFullTransactionId(ShmemVariableCache->nextXid); epoch = EpochFromFullTransactionId(ShmemVariableCache->nextXid); /* * If xid is numerically greater than next_xid, it has to be from the last * epoch. */ if (unlikely(xid > next_xid)) --epoch; return FullTransactionIdFromEpochAndXid(epoch, xid); } #endif