How to force an org-babel session to reset or initialize?

Question

If I run the following babel shell blocks

#+BEGIN_SRC sh :session one :results output
  ssh staging
#+END_SRC

#+BEGIN_SRC sh :session one :results output
  hostname
#+END_SRC

Org creates a shell buffer called one, runs ssh staging and then after connecting, executes hostname on staging. However, if I tweak the ssh command and run it again it attempts to run it from within session one, from the staging host. If I close the buffer one it resets the session as the next time any command is run with that session name it recreates it. What I haven't been able to find is a method to force a particular babel block to always initialize a new session.

I'm aware that for some languages (elisp in particular) this would not be possible. I suppose I could prepend the session with an elisp block containing (kill-buffer "one"), but would rather this was a header argument if possible. I'm also aware that for this example I could simply combine the two into a single block with no session, but I'm interested in more complicated sequences.

Is there a :prologue command or some other special argument to :session that forces that block to initialize a new shell on every invocation?

Answer 1

Try this

Force an org-babel session to reset or initialize by using an elisp conditional statement to dynamically set the value of the :session header on the first code block.

e.g. :session (if (get-buffer "one") (if (kill-buffer "one") (print "one") (print "one")) (print "one"))

In the above code, get-buffer function checks if buffer named one exists. If true, the kill-buffer function kills the one buffer then returns a value of t or nil which is passed to the if statement. In this specific example, the nested if statement will always return a value of one. Please note, I'm still learning elisp.

Below is the updated example code:

#+BEGIN_SRC sh :session (if (get-buffer "one") (if (kill-buffer "one") (print "one") (print "one")) (print "one")) :results output
  ssh staging
#+END_SRC

#+BEGIN_SRC sh :session one :results output
  hostname
#+END_SRC

Each time the first code block is executed, emacs will ask to kill the one buffer, so you can reset the session when you desire.

EDIT

Oops missed part that you always wanted buffer killed, i.e. no prompting. Execute this block first to temporarily turn off prompting to kill buffer.

#+BEGIN_SRC elisp
  (setq kill-buffer-query-functions
    (remq 'process-kill-buffer-query-function
           kill-buffer-query-functions))
#+END_SRC

Hope that helped!

Note: This code was tested using the following versions of emacs and org-mode.

GNU Emacs 24.4.1 (x86_64-apple-darwin14.0.0, NS apple-appkit-1343.14)
Org-mode version 8.2.10 (8.2.10-29-g89a0ac-elpa)

In conclusion, I found technical inspiration for this answer on the org-scraps website and this kill-buffer example code on stackoverflow.

Answer 2

Inspired by @Melioratus.

Currently, org-babel provides a hook called org-babel-after-execute-hook. I extended the supported arguments of org-babel src block by using it:

(I'm using org-babel for elixir. If you want supports of other languages, extend cond by yourself.)

(add-hook 'org-babel-after-execute-hook 'semacs/ob-args-ext-session-reset)

(defun semacs/ob-args-ext-session-reset()
  (let* ((src-block-info (org-babel-get-src-block-info 'light))
         (language (nth 0 src-block-info))
         (arguments (nth 2 src-block-info))
         (should-reset (member '(:session-reset . "yes") arguments))
         (session (cdr (assoc :session arguments)))
         (session-process
          (cond ((equal language "elixir") (format "*elixir-%s*" session))
                (t nil))))
    (if (and should-reset (get-process session-process))
        (kill-process session-process))))

After evaluating above code, you can write src block like this:

#+begin_src elixir :session-reset yes
  IO.puts("HELLO WORLD")
#+end_src

After evaluating src block, org-babel will cleanup the corresponding session.