Move WAL continuation record information to WAL page header.

The continuation record only contained one field, xl_rem_len, so it makes
things simpler to just include it in the WAL page header. This wastes four
bytes on pages that don't begin with a continuation from previos page, plus
four bytes on every page, because of padding.

The motivation of this is to make it easier to calculate how much space a
WAL record needs. Before this patch, it depended on how many page boundaries
the record crosses. The motivation of that, in turn, is to separate the
allocation of space in the WAL from the copying of the record data to the
allocated space. Keeping the calculation of space required simple helps to
keep the critical section of allocating the space from WAL short. But that's
not included in this patch yet.

Bump WAL version number again, as this is an incompatible change.

1 files changed, 14 insertions, 17 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 2f9209f3b8..03feb14551 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -696,7 +696,6 @@ XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
 {
 	XLogCtlInsert *Insert = &XLogCtl->Insert;
 	XLogRecord *record;
-	XLogContRecord *contrecord;
 	XLogRecPtr	RecPtr;
 	XLogRecPtr	WriteRqst;
 	uint32		freespace;
@@ -1085,9 +1084,7 @@ begin:;
 		curridx = Insert->curridx;
 		/* Insert cont-record header */
 		Insert->currpage->xlp_info |= XLP_FIRST_IS_CONTRECORD;
-		contrecord = (XLogContRecord *) Insert->currpos;
-		contrecord->xl_rem_len = write_len;
-		Insert->currpos += SizeOfXLogContRecord;
+		Insert->currpage->xlp_rem_len = write_len;
 		freespace = INSERT_FREESPACE(Insert);
 	}
 
@@ -3941,7 +3938,8 @@ retry:
 	if (total_len > len)
 	{
 		/* Need to reassemble record */
-		XLogContRecord *contrecord;
+		char	   *contrecord;
+		XLogPageHeader pageHeader;
 		XLogRecPtr	pagelsn;
 		uint32		gotlen = len;
 
@@ -3969,30 +3967,30 @@ retry:
 								readOff)));
 				goto next_record_is_invalid;
 			}
-			pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) readBuf);
-			contrecord = (XLogContRecord *) ((char *) readBuf + pageHeaderSize);
-			if (contrecord->xl_rem_len == 0 ||
-				total_len != (contrecord->xl_rem_len + gotlen))
+			pageHeader = (XLogPageHeader) readBuf;
+			pageHeaderSize = XLogPageHeaderSize(pageHeader);
+			contrecord = (char *) readBuf + pageHeaderSize;
+			if (pageHeader->xlp_rem_len == 0 ||
+				total_len != (pageHeader->xlp_rem_len + gotlen))
 			{
 				char fname[MAXFNAMELEN];
 				XLogFileName(fname, curFileTLI, readSegNo);
 				ereport(emode_for_corrupt_record(emode, *RecPtr),
 						(errmsg("invalid contrecord length %u in log segment %s, offset %u",
-								contrecord->xl_rem_len,
+								pageHeader->xlp_rem_len,
 								XLogFileNameP(curFileTLI, readSegNo),
 								readOff)));
 				goto next_record_is_invalid;
 			}
-			len = XLOG_BLCKSZ - pageHeaderSize - SizeOfXLogContRecord;
-			if (contrecord->xl_rem_len > len)
+			len = XLOG_BLCKSZ - pageHeaderSize;
+			if (pageHeader->xlp_rem_len > len)
 			{
-				memcpy(buffer, (char *) contrecord + SizeOfXLogContRecord, len);
+				memcpy(buffer, (char *) contrecord, len);
 				gotlen += len;
 				buffer += len;
 				continue;
 			}
-			memcpy(buffer, (char *) contrecord + SizeOfXLogContRecord,
-				   contrecord->xl_rem_len);
+			memcpy(buffer, (char *) contrecord, pageHeader->xlp_rem_len);
 			break;
 		}
 		if (!RecordIsValid(record, *RecPtr, emode))
@@ -4000,8 +3998,7 @@ retry:
 		pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) readBuf);
 		XLogSegNoOffsetToRecPtr(
 			readSegNo,
-			readOff + pageHeaderSize +
-				MAXALIGN(SizeOfXLogContRecord + contrecord->xl_rem_len),
+			readOff + pageHeaderSize + MAXALIGN(pageHeader->xlp_rem_len),
 			EndRecPtr);
 		ReadRecPtr = *RecPtr;
 		/* needn't worry about XLOG SWITCH, it can't cross page boundaries */