[interface] Provide intf_reinit() to reinitialise nullified interfaces

Provide an abstraction intf_reinit() to restore the descriptor of a
previously nullified interface.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/interface.c

  * blocked during shutdown.
  */
 void intf_restart ( struct interface *intf, int rc ) {
-	struct interface_descriptor *desc = intf->desc;
 
 	/* Shut down the interface */
 	intf_shutdown ( intf, rc );
 	 * infinite loop as the intf_close() operations on each side
 	 * of the link call each other recursively.
 	 */
-	intf->desc = desc;
+	intf_reinit ( intf );
 }
 
 /**

src/include/ipxe/interface.h

 	struct refcnt *refcnt;
 	/** Interface descriptor */
 	struct interface_descriptor *desc;
+	/** Original interface descriptor
+	 *
+	 * Used by intf_reinit().
+	 */
+	struct interface_descriptor *original;
 };
 
 extern void intf_plug ( struct interface *intf, struct interface *dest );
 	intf->dest = &null_intf;
 	intf->refcnt = refcnt;
 	intf->desc = desc;
+	intf->original = desc;
 }
 
 /**
 		.dest = &null_intf,		\
 		.refcnt = NULL,			\
 		.desc = &(descriptor),		\
+		.original = &(descriptor),	\
 	}
 
 /**
  */
 #define INTF_INTF_DBG( intf, dest ) INTF_DBG ( intf ), INTF_DBG ( dest )
 
+/**
+ * Reinitialise an object interface
+ *
+ * @v intf		Object interface
+ */
+static inline void intf_reinit ( struct interface *intf ) {
+
+	/* Restore original interface descriptor */
+	intf->desc = intf->original;
+}
+
 #endif /* _IPXE_INTERFACE_H */

src/net/tcp/httpcore.c

 	/* Restart content decoding interfaces (which may be attached
 	 * to the same object).
 	 */
-	intf_nullify ( &http->content );
-	intf_nullify ( &http->transfer );
+	intf_nullify ( &http->transfer ); /* avoid potential loops */
 	intf_restart ( &http->content, http->response.rc );
 	intf_restart ( &http->transfer, http->response.rc );
-	http->content.desc = &http_content_desc;
-	http->transfer.desc = &http_transfer_desc;
 	intf_plug_plug ( &http->transfer, &http->content );
 	http->len = 0;
 	assert ( http->remaining == 0 );