Rename ShmemVariableCache to TransamVariables

The old name was misleading: It's not a cache, the values kept in the
struct are the authoritative source.

Reviewed-by: Tristan Partin, Richard Guo
Discussion: https://www.postgresql.org/message-id/6537d63d-4bb5-46f8-9b5d-73a8ba4720ab@iki.fi

src/include/access/transam.h

@@ -197,7 +197,7 @@ FullTransactionIdAdvance(FullTransactionId *dest)
 #define FirstNormalObjectId		16384
 
 /*
- * VariableCache is a data structure in shared memory that is used to track
+ * TransamVariables is a data structure in shared memory that is used to track
  * OID and XID assignment state.  For largely historical reasons, there is
  * just one struct with different fields that are protected by different
  * LWLocks.
@@ -206,7 +206,7 @@ FullTransactionIdAdvance(FullTransactionId *dest)
  * used just to generate useful messages when xidWarnLimit or xidStopLimit
  * are exceeded.
  */
-typedef struct VariableCacheData
+typedef struct TransamVariablesData
 {
 	/*
 	 * These fields are protected by OidGenLock.
@@ -252,9 +252,7 @@ typedef struct VariableCacheData
 	 */
 	TransactionId oldestClogXid;	/* oldest it's safe to look up in clog */
 
-} VariableCacheData;
-
-typedef VariableCacheData *VariableCache;
+} TransamVariablesData;
 
 
 /* ----------------
@@ -266,7 +264,7 @@ typedef VariableCacheData *VariableCache;
 extern bool TransactionStartedDuringRecovery(void);
 
 /* in transam/varsup.c */
-extern PGDLLIMPORT VariableCache ShmemVariableCache;
+extern PGDLLIMPORT TransamVariablesData *TransamVariables;
 
 /*
  * prototypes for functions in transam/transam.c