Run custom hooks after a kernel connection is made.

Introduce new user custom variable `ein:on-kernel-connect-functions' which is an
abnormal hook called after a kernel connection is made. All functions in the
hook are called with the connected kernel as an argument.

This replaces the old, unused `ein:kernel-run-after-start-hook' function.

features/notebook.feature

@@ -112,3 +112,11 @@ Scenario: Test the undo-tree-incompatible logic
   Given new python notebook
   And I call "turn-on-undo-tree-mode"
   Then the value of "undo-tree-mode" is nil
+
+@kernel-on-connect
+Scenario: Test ein:on-kernel-connect-functions abnormal hooks
+  Given I set the kernel connect message
+  Given new python notebook
+  And I wait for the smoke to clear
+  And I switch to buffer "*Messages*"
+  Then I should see "Hello ein."

features/step-definitions/ein-steps.el

@@ -2,6 +2,10 @@
       (lambda ()
         (insert-char 37)))
 
+(When "^I set the kernel connect message$"
+      (lambda ()
+        (add-to-list 'ein:on-kernel-connect-functions #'(lambda (_) (message "Hello ein.")))))
+
 (When "^I type session port \\([0-9]+\\)$"
       (lambda (port)
         (ein:process-refresh-processes)

lisp/ein-kernel.el

@@ -62,6 +62,12 @@
   :type 'list
   :group 'ein)
 
+(defcustom ein:on-kernel-connect-functions nil
+  "Abnormal hook that is run after a websocket connection is made
+to a jupyter kernel. Functions defined here must accept a single
+argument, which is the kernel that was just connected."
+  :type 'list
+  :group 'ein)
 
 
 ;;; Initialization and connection.
@@ -293,7 +299,8 @@ See https://github.com/ipython/ipython/pull/3307"
                             (let* ((websocket (websocket-client-data ws))
                                    (kernel (ein:$websocket-kernel websocket)))
                               (when (ein:kernel-live-p kernel)
-                                (ein:kernel-run-after-start-hook kernel)
+                                (ein:log 'verbose "ein:start-single-websocket: Running on-connect abnormal hooks.")
+                                (run-hook-with-args 'ein:on-kernel-connect-functions kernel)
                                 (when cb
                                   (funcall cb kernel)))
                               (ein:log 'verbose "WS opened: %s" (websocket-url ws))))
@@ -307,10 +314,6 @@ See https://github.com/ipython/ipython/pull/3307"
 (defun ein:kernel-on-connect (kernel content -metadata-not-used-)
   (ein:log 'info "Kernel connect_request_reply received."))
 
-(defun ein:kernel-run-after-start-hook (kernel)
-  (mapc #'ein:funcall-packed
-        (ein:$kernel-after-start-hook kernel)))
-
 (defun ein:kernel-disconnect (kernel)
   "Close websocket connection to running kernel, but do not
 delete the kernel on the server side"