Have GetCurrentTransactionStopTimestamp() set xactStopTimestamp if unset

Previously GetCurrentTransactionStopTimestamp() computed a new timestamp
whenever xactStopTimestamp was unset and xactStopTimestamp was only set when a
commit or abort record was written.

An upcoming patch will add additional calls to
GetCurrentTransactionStopTimestamp() from pgstats. To avoid computing
timestamps multiple times, set xactStopTimestamp in
GetCurrentTransactionStopTimestamp() if not already set.

Author: Dave Page <dpage@pgadmin.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Vik Fearing <vik@postgresfriends.org>
Discussion: https://postgr.es/m/20220906155325.an3xesq5o3fq36gt%40awork3.anarazel.de

Author: anarazel

src/backend/access/transam/xact.c

@@ -263,7 +263,10 @@ static bool currentCommandIdUsed;
 /*
  * xactStartTimestamp is the value of transaction_timestamp().
  * stmtStartTimestamp is the value of statement_timestamp().
- * xactStopTimestamp is the time at which we log a commit or abort WAL record.
+ * xactStopTimestamp is the time at which we log a commit / abort WAL record,
+ * or if that was skipped, the time of the first subsequent
+ * GetCurrentTransactionStopTimestamp() call.
+ *
  * These do not change as we enter and exit subtransactions, so we don't
  * keep them inside the TransactionState stack.
  */
@@ -865,15 +868,24 @@ GetCurrentStatementStartTimestamp(void)
 /*
  *	GetCurrentTransactionStopTimestamp
  *
- * We return current time if the transaction stop time hasn't been set
- * (which can happen if we decide we don't need to log an XLOG record).
+ * If the transaction stop time hasn't already been set, which can happen if
+ * we decided we don't need to log an XLOG record, set xactStopTimestamp.
  */
 TimestampTz
 GetCurrentTransactionStopTimestamp(void)
 {
-	if (xactStopTimestamp != 0)
-		return xactStopTimestamp;
-	return GetCurrentTimestamp();
+	TransactionState s PG_USED_FOR_ASSERTS_ONLY = CurrentTransactionState;
+
+	/* should only be called after commit / abort processing */
+	Assert(s->state == TRANS_DEFAULT ||
+		   s->state == TRANS_COMMIT ||
+		   s->state == TRANS_ABORT ||
+		   s->state == TRANS_PREPARE);
+
+	if (xactStopTimestamp == 0)
+		xactStopTimestamp = GetCurrentTimestamp();
+
+	return xactStopTimestamp;
 }
 
 /*
@@ -891,15 +903,6 @@ SetCurrentStatementStartTimestamp(void)
 		Assert(stmtStartTimestamp != 0);
 }
 
-/*
- *	SetCurrentTransactionStopTimestamp
- */
-static inline void
-SetCurrentTransactionStopTimestamp(void)
-{
-	xactStopTimestamp = GetCurrentTimestamp();
-}
-
 /*
  *	GetCurrentTransactionNestLevel
  *
@@ -1396,9 +1399,7 @@ RecordTransactionCommit(void)
 		START_CRIT_SECTION();
 		MyProc->delayChkptFlags |= DELAY_CHKPT_START;
 
-		SetCurrentTransactionStopTimestamp();
-
-		XactLogCommitRecord(xactStopTimestamp,
+		XactLogCommitRecord(GetCurrentTransactionStopTimestamp(),
 							nchildren, children, nrels, rels,
 							ndroppedstats, droppedstats,
 							nmsgs, invalMessages,
@@ -1422,7 +1423,7 @@ RecordTransactionCommit(void)
 		 */
 
 		if (!replorigin || replorigin_session_origin_timestamp == 0)
-			replorigin_session_origin_timestamp = xactStopTimestamp;
+			replorigin_session_origin_timestamp = GetCurrentTransactionStopTimestamp();
 
 		TransactionTreeSetCommitTsData(xid, nchildren, children,
 									   replorigin_session_origin_timestamp,
@@ -1754,8 +1755,7 @@ RecordTransactionAbort(bool isSubXact)
 		xact_time = GetCurrentTimestamp();
 	else
 	{
-		SetCurrentTransactionStopTimestamp();
-		xact_time = xactStopTimestamp;
+		xact_time = GetCurrentTransactionStopTimestamp();
 	}
 
 	XactLogAbortRecord(xact_time,