Add XLogRecGetFullTransactionId() function.

In order to be able to work with 64 bit transaction IDs without increasing
the size of the WAL, infer the epoch.  In general we can't do that safely,
but during replay we can because we know that the next XID can't advance
concurrently (it's only advanced by replaying WAL records).

src/backend/access/transam/xlogreader.c

@@ -26,6 +26,7 @@
 #include "replication/origin.h"
 
 #ifndef FRONTEND
+#include "miscadmin.h"
 #include "utils/memutils.h"
 #endif
 
@@ -1460,3 +1461,37 @@ RestoreBlockImage(XLogReaderState *record, uint8 block_id, char *page)
 
 	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->nextFullXid);
+	epoch = EpochFromFullTransactionId(ShmemVariableCache->nextFullXid);
+
+	/*
+	 * 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

src/include/access/xlogreader.h

@@ -25,6 +25,10 @@
 #ifndef XLOGREADER_H
 #define XLOGREADER_H
 
+#ifndef FRONTEND
+#include "access/transam.h"
+#endif
+
 #include "access/xlogrecord.h"
 #include "storage/smgr.h"
 
@@ -242,6 +246,10 @@ extern bool DecodeXLogRecord(XLogReaderState *state, XLogRecord *record,
 #define XLogRecBlockImageApply(decoder, block_id) \
 	((decoder)->blocks[block_id].apply_image)
 
+#ifndef FRONTEND
+extern FullTransactionId XLogRecGetFullXid(XLogReaderState *record);
+#endif
+
 extern bool RestoreBlockImage(XLogReaderState *recoder, uint8 block_id, char *dst);
 extern char *XLogRecGetBlockData(XLogReaderState *record, uint8 block_id, Size *len);
 extern bool XLogRecGetBlockTag(XLogReaderState *record, uint8 block_id,