Add a client authentication hook.

KaiGai Kohei, with minor cleanup of the comments by me.

src/backend/libpq/auth.c

@@ -216,6 +216,12 @@ static int	CheckRADIUSAuth(Port *port);
  *----------------------------------------------------------------
  */
 
+/*
+ * This hook allows plugins to get control following client authentication,
+ * but before the user has been informed about the results.  It could be used
+ * to record login events, insert a delay after failed authentication, etc.
+ */
+ClientAuthentication_hook_type ClientAuthentication_hook = NULL;
 
 /*
  * Tell the user the authentication failed, but not (much about) why.
@@ -577,6 +583,9 @@ ClientAuthentication(Port *port)
 			break;
 	}
 
+	if (ClientAuthentication_hook)
+		(*ClientAuthentication_hook)(port, status);
+
 	if (status == STATUS_OK)
 		sendAuthRequest(port, AUTH_REQ_OK);
 	else

src/include/libpq/auth.h

@@ -24,4 +24,8 @@ extern char *pg_krb_realm;
 
 extern void ClientAuthentication(Port *port);
 
+/* Hook for plugins to get control in ClientAuthentication() */
+typedef void (*ClientAuthentication_hook_type)(Port *, int);
+extern PGDLLIMPORT ClientAuthentication_hook_type ClientAuthentication_hook;
+
 #endif   /* AUTH_H */