Fix ordering of XIDs in ProcArrayApplyRecoveryInfo

Commit 8431e296 reworked ProcArrayApplyRecoveryInfo to sort XIDs
before adding them to KnownAssignedXids. But the XIDs are sorted using
xidComparator, which compares the XIDs simply as uint32 values, not
logically. KnownAssignedXidsAdd() however expects XIDs in logical order,
and calls TransactionIdFollowsOrEquals() to enforce that. If there are
XIDs for which the two orderings disagree, an error is raised and the
recovery fails/restarts.

Hitting this issue is fairly easy - you just need two transactions, one
started before the 4B limit (e.g. XID 4294967290), the other sometime
after it (e.g. XID 1000). Logically (4294967290 <= 1000) but when
compared using xidComparator we try to add them in the opposite order.
Which makes KnownAssignedXidsAdd() fail with an error like this:

  ERROR: out-of-order XID insertion in KnownAssignedXids

This only happens during replica startup, while processing RUNNING_XACTS
records to build the snapshot. Once we reach STANDBY_SNAPSHOT_READY, we
skip these records. So this does not affect already running replicas,
but if you restart (or create) a replica while there are transactions
with XIDs for which the two orderings disagree, you may hit this.

Long-running transactions and frequent replica restarts increase the
likelihood of hitting this issue. Once the replica gets into this state,
it can't be started (even if the old transactions are terminated).

Fixed by sorting the XIDs logically - this is fine because we're dealing
with normal XIDs (because it's XIDs assigned to backends) and from the
same wraparound epoch (otherwise the backends could not be running at
the same time on the primary node). So there are no problems with the
triangle inequality, which is why xidComparator compares raw values.

Investigation and root cause analysis by Abhijit Menon-Sen. Patch by me.

This issue is present in all releases since 9.4, however releases up to
9.6 are EOL already so backpatch to 10 only.

Reviewed-by: Abhijit Menon-Sen
Reviewed-by: Alvaro Herrera
Backpatch-through: 10
Discussion: https://postgr.es/m/36b8a501-5d73-277c-4972-f58a4dce088a%40enterprisedb.com

src/backend/storage/ipc/procarray.c

@@ -1164,8 +1164,13 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
 		/*
 		 * Sort the array so that we can add them safely into
 		 * KnownAssignedXids.
+		 *
+		 * We have to sort them logically, because in KnownAssignedXidsAdd we
+		 * call TransactionIdFollowsOrEquals and so on. But we know these XIDs
+		 * come from RUNNING_XACTS, which means there are only normal XIDs from
+		 * the same epoch, so this is safe.
 		 */
-		qsort(xids, nxids, sizeof(TransactionId), xidComparator);
+		qsort(xids, nxids, sizeof(TransactionId), xidLogicalComparator);
 
 		/*
 		 * Add the sorted snapshot into KnownAssignedXids.  The running-xacts

src/backend/utils/adt/xid.c

@@ -145,6 +145,32 @@ xidComparator(const void *arg1, const void *arg2)
 	return 0;
 }
 
+/*
+ * xidLogicalComparator
+ *		qsort comparison function for XIDs
+ *
+ * This is used to compare only XIDs from the same epoch (e.g. for backends
+ * running at the same time). So there must be only normal XIDs, so there's
+ * no issue with triangle inequality.
+ */
+int
+xidLogicalComparator(const void *arg1, const void *arg2)
+{
+	TransactionId xid1 = *(const TransactionId *) arg1;
+	TransactionId xid2 = *(const TransactionId *) arg2;
+
+	Assert(TransactionIdIsNormal(xid1));
+	Assert(TransactionIdIsNormal(xid2));
+
+	if (TransactionIdPrecedes(xid1, xid2))
+		return -1;
+
+	if (TransactionIdPrecedes(xid2, xid1))
+		return 1;
+
+	return 0;
+}
+
 Datum
 xid8toxid(PG_FUNCTION_ARGS)
 {

src/include/utils/builtins.h

@@ -88,6 +88,7 @@ extern void text_to_cstring_buffer(const text *src, char *dst, size_t dst_len);
 
 /* xid.c */
 extern int	xidComparator(const void *arg1, const void *arg2);
+extern int	xidLogicalComparator(const void *arg1, const void *arg2);
 
 /* inet_cidr_ntop.c */
 extern char *pg_inet_cidr_ntop(int af, const void *src, int bits,