Applying a timeout on call-process/shell-command?

Question

In some cases, when delegating work to an external process, it's useful to set a timeout on the command to prevent Emacs from hanging indefinitely.
Unfortunately, the following does not work.

(with-timeout (1 nil)
  (call-process "/usr/bin/bash" nil t nil "-c" "sleep 10"))

The same goes for shell-command.

Is there a way to set a timeout on these synchronous processes?
That is, I want the process to be killed automatically if it doesn't finish within a certain number of seconds. Is that possible?

Answer 1

While call-process is running, emacs will processing events, with-timeout will not work without this:

The timeout is checked whenever Emacs waits for some kind of external event (such as keyboard input, input from subprocesses, or a certain time); if the program loops without waiting in any way, the timeout will not be detected.

You can still use with-timeout with (semi) synchronous processes.

You will actually use a asynchronous process but will synchrously wait while it is running, Emacs will process events when you run sit-for, which you can run for 0 seconds. You can then use the timeout-forms argument of with-timeout to kill the process if it is still running when the timeout fires.

(with-current-buffer (get-buffer-create "*my-proc-buffer*")
  (let ((proc (start-process "myproc" (current-buffer) "bash" "-c" "sleep 4"))) ;; start an async process
    (with-timeout (2 (kill-process proc)) ;; on timeout, kill the process
      (while (process-live-p proc) ;; while process is running
        (sit-for .05)) ;; let emacs read events and run timmers (and check for timeout)
      (message "finished on time!!")))) ;; this will run only if there is no timeout

Answer 2

You can achieve this by just adding a GNU timeout invocation to your shell command, which circumvents needing to know any details about Emacs behavior. For example running:

$ timeout 5 sleep 10

Will return in 5 seconds, not 10 (timeout effectively presses Ctrl-C for you).