bug#64897: 29.0.91; Bug (and patch) in find-dired-with-command

[Warren Lynn <wrn.lynn@gmail.com>]

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On Linux. When I call "find-name-dired" command, which in turn invokes
"find-dired-with-command" function, in a multi-hop tramp remote directory,
an error will occur in side the function, with backtrace (only shown lowest
level):

Debugger entered--Lisp error: (wrong-type-argument processp nil)
  process-mark(nil)
  (move-marker (process-mark proc) (point) (current-buffer))

The issue is around this part of the code:

    ;; Start the find process.
    (shell-command (concat command "&") (current-buffer))
    (let ((proc (get-buffer-process (current-buffer)))) ;; !!!!!!!! proc
could be nil here !!!!!!!!!!
      ;; Initialize the process marker; it is used by the filter.
      (move-marker (process-mark proc) (point) (current-buffer))

After changing the code to:

    ;; Start the find process.
    (let ((proc (shell-command (concat command "&") (current-buffer))))
      ;; Initialize the process marker; it is used by the filter.
      (move-marker (process-mark proc) (point) (current-buffer))

I do not have the error any more.

However, I observe that even with my fix above, the behavior is not
completely right. Sometimes, the (found) file entries appear before the
dired buffer header line. My impression of the code is it does not handle
asynchronous timing in a robust way, and that is more of a problem when the
command is invoked from a remote directory. Probably some serious revamp is
needed here, which is beyond my expertise. Maybe Emacs should include the
"aio" package and rely on that for asynchronous event handling.

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