Fix pg_control handling for PostgreSQL 12

"FullTransactionId" is not present in earlier versions, so we'll
have to #ifdef that out for those.

We should probably add more elegant checks to ensure that repmgr
is being executed against the PostgreSQL version it was built against.
In a normal production environment that will be the case as the
matching package will have been installed, so version mismatches
don't usually occur.

Note that while currently most aspects of the repmgr client are
compatible across PostgreSQL versions, the repmgrd code is very
version specific, so version specificity is a given anyway.

controldata.h

@@ -12,6 +12,7 @@
 #include "postgres_fe.h"
 #include "catalog/pg_control.h"
 
+
 #define MAX_VERSION_STRING 24
 /*
  * A simplified representation of pg_control containing only those fields
@@ -55,7 +56,7 @@ typedef struct CheckPoint93
 } CheckPoint93;
 
 
-/* Same for 9.5, 9.6, 10, HEAD */
+/* Same for 9.5, 9.6, 10, 11 */
 typedef struct CheckPoint95
 {
 	XLogRecPtr	redo;			/* next RecPtr available when we began to
@@ -83,6 +84,50 @@ typedef struct CheckPoint95
 } CheckPoint95;
 
 
+#if PG_ACTUAL_VERSION_NUM >= 120000
+/*
+ * Following fields removed in PostgreSQL 12;
+ *
+ *   uint32 nextXidEpoch;
+ *   TransactionId nextXid;
+ *
+ * and replaced by:
+ *
+ *   FullTransactionId nextFullXid;
+ */
+
+typedef struct CheckPoint12
+{
+	XLogRecPtr	redo;			/* next RecPtr available when we began to
+								 * create CheckPoint (i.e. REDO start point) */
+	TimeLineID	ThisTimeLineID; /* current TLI */
+	TimeLineID	PrevTimeLineID; /* previous TLI, if this record begins a new
+								 * timeline (equals ThisTimeLineID otherwise) */
+	bool		fullPageWrites; /* current full_page_writes */
+	FullTransactionId nextFullXid;	/* next free full transaction ID */
+	Oid			nextOid;		/* next free OID */
+	MultiXactId nextMulti;		/* next free MultiXactId */
+	MultiXactOffset nextMultiOffset;	/* next free MultiXact offset */
+	TransactionId oldestXid;	/* cluster-wide minimum datfrozenxid */
+	Oid			oldestXidDB;	/* database with minimum datfrozenxid */
+	MultiXactId oldestMulti;	/* cluster-wide minimum datminmxid */
+	Oid			oldestMultiDB;	/* database with minimum datminmxid */
+	pg_time_t	time;			/* time stamp of checkpoint */
+	TransactionId oldestCommitTsXid;	/* oldest Xid with valid commit
+										 * timestamp */
+	TransactionId newestCommitTsXid;	/* newest Xid with valid commit
+										 * timestamp */
+
+	/*
+	 * Oldest XID still running. This is only needed to initialize hot standby
+	 * mode from an online checkpoint, so we only bother calculating this for
+	 * online checkpoints and only when wal_level is replica. Otherwise it's
+	 * set to InvalidTransactionId.
+	 */
+	TransactionId oldestActiveXid;
+} CheckPoint12;
+#endif
+
 typedef struct ControlFileData93
 {
 	uint64		system_identifier;
@@ -333,19 +378,11 @@ typedef struct ControlFileData11
 } ControlFileData11;
 
 
+#if PG_ACTUAL_VERSION_NUM >= 120000
 /*
  * Following field added in Pg12:
  *
  *   int max_wal_senders;
- *
- * Following fields removed:
- *
- *   uint32 nextXidEpoch;
- *   TransactionId nextXid;
- *
- * and replaced by:
- *
- *   FullTransactionId nextFullXid;
  */
 
 typedef struct ControlFileData12
@@ -359,7 +396,7 @@ typedef struct ControlFileData12
 	pg_time_t	time;			/* time stamp of last pg_control update */
 	XLogRecPtr	checkPoint;		/* last check point record ptr */
 
-	CheckPoint	checkPointCopy; /* copy of last check point record */
+	CheckPoint12	checkPointCopy; /* copy of last check point record */
 
 	XLogRecPtr	unloggedLSN;	/* current fake LSN value, for unlogged rels */
 
@@ -398,6 +435,7 @@ typedef struct ControlFileData12
 
 	uint32		data_checksum_version;
 } ControlFileData12;
+#endif
 
 extern int get_pg_version(const char *data_directory, char *version_string);
 extern DBState get_db_state(const char *data_directory);