Support for alternative connectors

The `closql-database' class is abstract now, making it necessary to
define a class that derives from that and also from a connector class.
The choice of which connector to use can be left to the user.  It may
be necessary to define a `closql-db' method for alternative connectors
and possibly other methods as well.

  (cl-case forge-database-connector
    (sqlite
     (defclass forge-database (emacsql-sqlite-connection closql-database)
       ((object-class :initform 'forge-repository))))
    (libsqlite3
     (require (quote emacsql-libsqlite3))
     (with-no-warnings
       (defclass forge-database (emacsql-libsqlite3-connection closql-database)
         ((object-class :initform 'forge-repository))))))

CHANGELOG

@@ -1,4 +1,11 @@
 # -*- mode: org -*-
+* v1.1.0    2021/09/27
+
+- Added support for alternative database connectors.
+  The ~closql-database~ class is abstract now, making it necessary to
+  define a class that derives from that and also from a connector
+  class.
+
 * v1.0.6    2021/06/16
 
 - Adjusted to how unbound slots are represented in Emacs 28.

closql.el

@@ -273,8 +273,9 @@
 
 ;;; Database
 
-(defclass closql-database (emacsql-sqlite-connection)
-  ((object-class :allocation :class)))
+(defclass closql-database ()
+  ((object-class :allocation :class))
+  :abstract t)
 
 (cl-defmethod closql-db ((class (subclass closql-database))
                          &optional variable file debug)
@@ -283,7 +284,8 @@
              (prog1 db (emacsql db [:pragma (= foreign-keys on)]))))
       (let ((db-init (not (and file (file-exists-p file))))
             (db (make-instance class :file file)))
-        (set-process-query-on-exit-flag (oref db process) nil)
+        (when (slot-boundp db 'process)
+          (set-process-query-on-exit-flag (oref db process) nil))
         (when debug
           (emacsql-enable-debugging db))
         (emacsql db [:pragma (= foreign-keys on)])