Add basic infrastructure for 64 bit transaction IDs.

Instead of inferring epoch progress from xids and checkpoints,
introduce a 64 bit FullTransactionId type and use it to track xid
generation.  This fixes an unlikely bug where the epoch is reported
incorrectly if the range of active xids wraps around more than once
between checkpoints.

The only user-visible effect of this commit is to correct the epoch
used by txid_current() and txid_status(), also visible with
pg_controldata, in those rare circumstances.  It also creates some
basic infrastructure so that later patches can use 64 bit
transaction IDs in more places.

The new type is a struct that we pass by value, as a form of strong
typedef.  This prevents the sort of accidental confusion between
TransactionId and FullTransactionId that would be possible if we
were to use a plain old uint64.

Author: Thomas Munro
Reported-by: Amit Kapila
Reviewed-by: Andres Freund, Tom Lane, Heikki Linnakangas
Discussion: https://postgr.es/m/CAA4eK1%2BMv%2Bmb0HFfWM9Srtc6MVe160WFurXV68iAFMcagRZ0dQ%40mail.gmail.com

1 files changed, 48 insertions, 3 deletions

diff --git a/src/include/access/transam.h b/src/include/access/transam.h
index 78997e533e7..6a919084c8f 100644
--- a/src/include/access/transam.h
+++ b/src/include/access/transam.h
@@ -44,6 +44,32 @@
 #define TransactionIdStore(xid, dest)	(*(dest) = (xid))
 #define StoreInvalidTransactionId(dest) (*(dest) = InvalidTransactionId)
 
+#define EpochFromFullTransactionId(x)	((uint32) ((x).value >> 32))
+#define XidFromFullTransactionId(x)		((uint32) (x).value)
+#define U64FromFullTransactionId(x)		((x).value)
+#define FullTransactionIdPrecedes(a, b)	((a).value < (b).value)
+#define FullTransactionIdIsValid(x)		TransactionIdIsValid(XidFromFullTransactionId(x))
+
+/*
+ * A 64 bit value that contains an epoch and a TransactionId.  This is
+ * wrapped in a struct to prevent implicit conversion to/from TransactionId.
+ * Not all values represent valid normal XIDs.
+ */
+typedef struct FullTransactionId
+{
+	uint64		value;
+} FullTransactionId;
+
+static inline FullTransactionId
+FullTransactionIdFromEpochAndXid(uint32 epoch, TransactionId xid)
+{
+	FullTransactionId result;
+
+	result.value = ((uint64) epoch) << 32 | xid;
+
+	return result;
+}
+
 /* advance a transaction ID variable, handling wraparound correctly */
 #define TransactionIdAdvance(dest)	\
 	do { \
@@ -52,6 +78,15 @@
 			(dest) = FirstNormalTransactionId; \
 	} while(0)
 
+/* advance a FullTransactionId variable, stepping over special XIDs */
+static inline void
+FullTransactionIdAdvance(FullTransactionId *dest)
+{
+	dest->value++;
+	while (XidFromFullTransactionId(*dest) < FirstNormalTransactionId)
+		dest->value++;
+}
+
 /* back up a transaction ID variable, handling wraparound correctly */
 #define TransactionIdRetreat(dest)	\
 	do { \
@@ -125,12 +160,12 @@ typedef struct VariableCacheData
 	/*
 	 * These fields are protected by XidGenLock.
 	 */
-	TransactionId nextXid;		/* next XID to assign */
+	FullTransactionId nextFullXid;	/* next full XID to assign */
 
 	TransactionId oldestXid;	/* cluster-wide minimum datfrozenxid */
 	TransactionId xidVacLimit;	/* start forcing autovacuums here */
 	TransactionId xidWarnLimit; /* start complaining here */
-	TransactionId xidStopLimit; /* refuse to advance nextXid beyond here */
+	TransactionId xidStopLimit; /* refuse to advance nextFullXid beyond here */
 	TransactionId xidWrapLimit; /* where the world ends */
 	Oid			oldestXidDB;	/* database with minimum datfrozenxid */
 
@@ -187,11 +222,21 @@ extern XLogRecPtr TransactionIdGetCommitLSN(TransactionId xid);
 
 /* in transam/varsup.c */
 extern TransactionId GetNewTransactionId(bool isSubXact);
-extern TransactionId ReadNewTransactionId(void);
+extern void AdvanceNextFullTransactionIdPastXid(TransactionId xid);
+extern FullTransactionId ReadNextFullTransactionId(void);
 extern void SetTransactionIdLimit(TransactionId oldest_datfrozenxid,
 					  Oid oldest_datoid);
 extern void AdvanceOldestClogXid(TransactionId oldest_datfrozenxid);
 extern bool ForceTransactionIdLimitUpdate(void);
 extern Oid	GetNewObjectId(void);
 
+/*
+ * For callers that just need the XID part of the next transaction ID.
+ */
+static inline TransactionId
+ReadNewTransactionId(void)
+{
+	return XidFromFullTransactionId(ReadNextFullTransactionId());
+}
+
 #endif							/* TRAMSAM_H */