How do I pass all the output of process to a function?

Question

I want to call a command asynchronously, and define a function that handles output from that command. I don't want to simply write output to a buffer.

According to the Elisp info (36.9.2 Process Filter Functions) I need to set a process filter function. I tried this:

(defun my-echo-filter (proc string)
  (message "proc: %s string: %s" proc string))

(let ((my-proc (start-file-process "foo" nil "date")))
  (set-process-filter my-proc #'my-echo-filter)) ; writes the time to *Messages*

However, this makes me nervous. I'm starting the process first, and only afterwards am I attaching the process filter.

If I do anything between creating the process and attaching the process filter, I can miss output.

(let ((my-proc (start-file-process "foo" nil "date")))
  (sleep-for 2)
  (set-process-filter my-proc #'my-echo-filter)) ; Does not write the time!

How do I create a process and ensure that my process filter doesn't miss anything?

Answer 1

As I understand it, this is a non-issue. Emacs is single-threaded, and only processes output from subprocesses when idle. Therefore, there is no time between calling start-file-process and set-process-filter when process output could arrive without being handled.

The info page (elisp) Accepting Output states:

Output from asynchronous subprocesses normally arrives only while Emacs is waiting for some sort of external event, such as elapsed time or terminal input.

For this to become a problem, you would have to separate creating the process and setting the filter function with a call to sit-for, accept-process-output, or some other command that involves user interaction via the event loop.