atomically writing to a socket

Question

I'm using process-send-string to send data to a socket connection, but I'm not satisfied with the effect of this function. Roughly, calling (process-send-string "foo") may end up in sending "bar" and then "foo", as explained below.

As pointed out by Emacs maintainers, the C code of process-send-string calls a function wait_reading_process_output (even before actually writing anything), which may run timers, which in turn may call process-send-string, and no ordering is enforced between such nested calls.

This makes it virtually impossible to implement an RPC protocol which is intended to be used by hooks called at uncontrolled times. So my question is, how could we achieve an atomic, "synchronized" writing primitive for this purpose ?

Answer 1

You could do something like:

(defun my-send-cmd (proc str)
  (if (process-get proc 'my-waiting)
      (process-put proc 'my-pending (append (process-get proc 'my-pending) (list str)))
    (process-put proc 'my-waiting t)
    (process-send-string proc str)))

then in the process-filter, when you get the answer to a command, check `my-pending' and if non-nil, take the first arg, send it to the process, otherwise set my-waiting back to nil. Of course, this assumes that every command will get an answer from the server, and that you can't stream the commands.

This said, the behavior you see should probably count as a bug, or at least as a misfeature, so please report it with M-x report-emacs-bug.