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Modifying a shared file (was: Extending the ecomplete.el data store)

From: Stefan Monnier
Subject: Modifying a shared file (was: Extending the ecomplete.el data store)
Date: Tue, 06 Feb 2018 16:01:59 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

>> I haven't used ecomplete very much so far, but I've noticed some issues
>> which I think are linked to having multiple Emacs sessions use it at the
>> same time.  I haven't investigated enough to be sure, but in any case
>> it's a use case that should be kept in mind.
> Yes, ecomplete should check the modification date and reload the data if
> it has changed.

[ Plus some kind of locking to avoid modifying the file at the
  same time.  ]

I'd welcome some new function

    (defun update-file (file modification)
      "Apply MODIFICATION to FILE."

tho we'd want to avoid re-reading the file if it hasn't been modified
since last time.  So maybe we need to first define

    (cl-defstruct file-contents
      (file nil :type string)
      (reader nil :type function)
      (writer nil :type function)

and then

    (defun file-contents-read (file &optional reader writer)
      "Return a `file-contents` object representing the contents of FILE."
        (insert-file-contents file)
        (make-file-contents :file file :reader reader :writer writer
                            :timestamp ...
                            :val (funcall (or reader #'read))))

    (defun file-contents-update (fc modification)
      "Apply MODIFICATION both to FC and to its file."

where `modification` is a function that takes the old value and returns
a new value (and it might be called several times).

-- Stefan

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