Track latest completed xid as a FullTransactionId.

The reason for doing so is that a subsequent commit will need that to
avoid wraparound issues. As the subsequent change is large this was
split out for easier review.

The reason this is not a perfect straight-forward change is that we do
not want track 64bit xids in the procarray or the WAL. Therefore we
need to advance lastestCompletedXid in relation to 32 bit xids. The
code for that is now centralized in MaintainLatestCompletedXid*.

Author: Andres Freund
Reviewed-By: Thomas Munro, Robert Haas, David Rowley
Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de

1 files changed, 35 insertions, 2 deletions

diff --git a/src/include/access/transam.h b/src/include/access/transam.h
index 85508300e9a..8db326ad1b5 100644
--- a/src/include/access/transam.h
+++ b/src/include/access/transam.h
@@ -54,6 +54,8 @@
 #define FullTransactionIdFollowsOrEquals(a, b) ((a).value >= (b).value)
 #define FullTransactionIdIsValid(x)		TransactionIdIsValid(XidFromFullTransactionId(x))
 #define InvalidFullTransactionId		FullTransactionIdFromEpochAndXid(0, InvalidTransactionId)
+#define FirstNormalFullTransactionId	FullTransactionIdFromEpochAndXid(0, FirstNormalTransactionId)
+#define FullTransactionIdIsNormal(x)	FullTransactionIdFollowsOrEquals(x, FirstNormalFullTransactionId)
 
 /*
  * A 64 bit value that contains an epoch and a TransactionId.  This is
@@ -102,6 +104,31 @@ FullTransactionIdAdvance(FullTransactionId *dest)
 		dest->value++;
 }
 
+/*
+ * Retreat a FullTransactionId variable, stepping over xids that would appear
+ * to be special only when viewed as 32bit XIDs.
+ */
+static inline void
+FullTransactionIdRetreat(FullTransactionId *dest)
+{
+	dest->value--;
+
+	/*
+	 * In contrast to 32bit XIDs don't step over the "actual" special xids.
+	 * For 64bit xids these can't be reached as part of a wraparound as they
+	 * can in the 32bit case.
+	 */
+	if (FullTransactionIdPrecedes(*dest, FirstNormalFullTransactionId))
+		return;
+
+	/*
+	 * But we do need to step over XIDs that'd appear special only for 32bit
+	 * XIDs.
+	 */
+	while (XidFromFullTransactionId(*dest) < FirstNormalTransactionId)
+		dest->value--;
+}
+
 /* back up a transaction ID variable, handling wraparound correctly */
 #define TransactionIdRetreat(dest)	\
 	do { \
@@ -193,8 +220,8 @@ typedef struct VariableCacheData
 	/*
 	 * These fields are protected by ProcArrayLock.
 	 */
-	TransactionId latestCompletedXid;	/* newest XID that has committed or
-										 * aborted */
+	FullTransactionId latestCompletedXid;	/* newest full XID that has
+											 * committed or aborted */
 
 	/*
 	 * These fields are protected by XactTruncationLock
@@ -244,6 +271,12 @@ extern void AdvanceOldestClogXid(TransactionId oldest_datfrozenxid);
 extern bool ForceTransactionIdLimitUpdate(void);
 extern Oid	GetNewObjectId(void);
 
+#ifdef USE_ASSERT_CHECKING
+extern void AssertTransactionIdInAllowableRange(TransactionId xid);
+#else
+#define AssertTransactionIdInAllowableRange(xid) ((void)true)
+#endif
+
 /*
  * Some frontend programs include this header.  For compilers that emit static
  * inline functions even when they're unused, that leads to unsatisfied