allow any CA system thread to attach to a ca context

src/ca/oldCAC.cpp

@@ -20,6 +20,8 @@
 
 #include "oldAccess.h"
 
+extern epicsThreadPrivateId caClientContextId;
+
 oldCAC::oldCAC ( bool enablePreemptiveCallback ) :
     clientCtx ( * new cac ( *this, enablePreemptiveCallback ) ),
     ca_exception_func ( 0 ), ca_exception_arg ( 0 ), 
@@ -199,8 +201,8 @@ void oldCAC::show ( unsigned level ) const
 
 void oldCAC::attachToClientCtx ()
 {
-    int status = ca_attach_context ( reinterpret_cast<struct ca_client_context *>(this) );
-    SEVCHK ( status, "error in virtual attach to client context" );
+    assert ( ! epicsThreadPrivateGet ( caClientContextId ) );
+    epicsThreadPrivateSet ( caClientContextId, this );
 }