Interprocess communication is not really working in spite of async-test-5

[TobiasZawada]

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There exists a test case async-test-5 for async-send.

I modified that test case a bit to make debugging easier:

(defun async-test-5 ()
  (interactive)
  (message "Starting async-test-5...")
  (let ((proc
         (async-start
          ;; What to do in the child process
          (lambda ()
	    (with-temp-buffer
	      (insert (format "Value of `async-callback': %S\n" async-callback))
	      (async-send :hello "world")
	      ;; wait for messages
	      (while (let* ((msg (async-receive))
			    (str (plist-get msg :goodbye)))
		       (insert (format "Child got message: %s\n" str))
		       (sleep-for 1)
		       (null (string-equal str "goodbye"))))
	      (buffer-string)))

          ;; What to do when it finishes
          (lambda (result)
            (if (async-message-p result)
                (message "Got hello from child process: %s"
                         (plist-get result :hello))
              (message "Async process done, result: %s"
                       result))))))
    (setq async-message-from-child nil)
    (async-send proc :goodbye "everyone")
    (sleep-for 1)
    (async-send proc :goodbye "just for you")
    (sleep-for 1)
    (async-send proc :goodbye "goodbye"))
  (message "Starting async-test-5...done"))

Running async-test-5 interactively outputs following message:

Async process done, result: Value of `async-callback': nil
Child got message: everyone
Child got message: just for you
Child got message: goodbye

As one sees one can send messages from the mother process to the child but sending messages from the child to the mother process (the (async-send :hello "world") thing) fails.

async-send uses the value of async-callback in the child and that variable is never set in the child. The output string Value of 'async-callback': nil proves that.

Note that this text is copied from a question on emacs.stackexchange.com.