On Sat, Feb 14, 2009 at 5:44 PM, James Reeves <weavejes...@googlemail.com>wrote:
>
> On Feb 14, 5:30 pm, Dan <redalas...@gmail.com> wrote:
> > What about making the file an agent and sending write actions to it?
>
> I don't see how that would solve the problem, unless you're suggesting
> that I have a single agent to handle all reads and writes?
>
> - James
>
Temp files is probably the simplest way to go for "immutable" files. I tried
a temporary agent per file, and it got a little long. To support multiple
writes with interleaved reads and figuring out when to dissoc the agent
would probably make it even longer.
(ns safe-write
(:import (java.io File))
(:use [clojure.contrib.duck-streams :as ds]))
(def writers (ref {}))
(defn noop
"Sends a noop action to an agent and awaits it."
[agt]
(send-off agt #(identity %))
(await agt))
(defn await-writer
"Waits for f's writer agent to finish if there is one and returns it. Else
returns nil."
[f]
(when-let [agt (@writers f)]
(while (not (= @agt :done))
(noop agt))
agt))
(defn exists?
[f]
(.exists (File. f)))
(defn waiting-read
"Reads f, waiting for it to be written if another thread is writing it."
[f]
(await-writer f)
(when (exists? f)
(slurp f)))
(defn create-writer
"Creates an agent to write f and returns it. Returns nil if such an agent
already exists."
[f]
(dosync
(when-not (@writers f)
(let [agt (agent :working)]
(alter writers assoc f agt)
agt))))
(defn safe-write-once
"Writes contents to f, but only if it doesn't exist and no other thread is
writing it."
[f contents]
(when-not (exists? f)
(when-let [writer (create-writer f)]
(send-off writer (fn [_]
;;(Thread/sleep 2000)
(spit f contents)
:done))
(await writer)
(dosync (alter writers dissoc f)))))
(defn do-test
[]
(let [f "/tmp/test4.txt"]
(.delete (File. f))
(.start (Thread. #(safe-write-once f "crino")))
(Thread/sleep 0)
(waiting-read f)))
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