Add update-file function

[PavlosMelissinos]

clojure.core/swap! semantics for files

Provide the filename and the (pure) function that will transform the contents of
the file (string in, string out) and update-file will open/close the file for you
and execute the operation.

[PavlosMelissinos]

(posting here for completeness after a discussion on Clojurians slack)

Alternative thread-safe (I think... at least within the JVM process) implementation:

(defn update-file
  "Swaps the contents of file to be:
  (apply f current-contents-of-file args). f should be free of side effects.
  Returns the value that was swapped in."
  [file f & xs]
  (locking file
    (let [old-val (slurp file)
          new-val (apply f old-val xs)]
      (spit file new-val)
      new-val)))

and its test:

(testing "prevent concurrent access to the same file from separate threads"
    (let [file  (fs/file (fs/temp-dir) (str (gensym)))
          file2 (fs/file (fs/temp-dir) (str (gensym)))
          state (atom {})]
      (spit file "")
      (spit file2 "")

      (let [fut1 (future (fs/update-file file (fn [& _xs] (Thread/sleep 50) (pr-str (reset! state (assoc @state :file1-op1-ran true))))))
            fut2 (future (fs/update-file file (fn [& _xs] (pr-str (reset! state (assoc @state :file1-op2-waited (:file1-op1-ran @state)))))))]
        (fs/update-file file2 (fn [& _xs] (pr-str (reset! state (assoc @state :file2-op-waited (:file1-op1-ran @state))))))
        (deref fut1)
        (deref fut2)
        (is (= {:file1-op1-ran true
                :file1-op2-waited true
                :file2-op-waited nil}
               @state)))))

[borkdude]

I think thread-safe is out of scope. Also file can be a string or a file object so you would be locking the string of file object, not the file itself. You can have multiple file objects pointing to the same file, which makes this a bit pointless.