abstract historyKeeper's batched reads implementation into a library

historyKeeper.js

@@ -8,6 +8,7 @@ const Crypto = require('crypto');
 const Once = require("./lib/once");
 const Meta = require("./lib/metadata");
 const WriteQueue = require("./lib/write-queue");
+const BatchRead = require("./lib/batch-read");
 
 let Log;
 const now = function () { return (new Date()).getTime(); };
@@ -231,7 +232,7 @@ module.exports.create = function (cfg) {
         as an added bonus:
         if the channel exists but its index does not then it caches the index
     */
-    const indexQueues = {};
+    const batchIndexReads = BatchRead();
     const getIndex = (ctx, channelName, cb) => {
         const chan = ctx.channels[channelName];
         // if there is a channel in memory and it has an index cached, return it
@@ -242,40 +243,14 @@ module.exports.create = function (cfg) {
             });
         }
 
-        // if a call to computeIndex is already in progress for this channel
-        // then add the callback for the latest invocation to the queue
-        // and wait for it to complete
-        if (Array.isArray(indexQueues[channelName])) {
-            indexQueues[channelName].push(cb);
-            return;
-        }
-
-        // otherwise, make a queue for any 'getIndex' calls made before the following 'computeIndex' call completes
-        var queue = indexQueues[channelName] = (indexQueues[channelName] || [cb]);
-
-        computeIndex(channelName, (err, ret) => {
-            if (!Array.isArray(queue)) {
-                // something is very wrong if there's no callback array
-                return void Log.error("E_INDEX_NO_CALLBACK", channelName);
-            }
-
-
-            // clean up the queue that you're about to handle, but keep a local copy
-            delete indexQueues[channelName];
-
-            // this is most likely an unrecoverable filesystem error
-            if (err) {
-                // call back every pending function with the error
-                return void queue.forEach(function (_cb) {
-                    _cb(err);
-                });
-            }
-            // cache the computed result if possible
-            if (chan) { chan.index = ret; }
-
-            // call back every pending function with the result
-            queue.forEach(function (_cb) {
-                _cb(void 0, ret);
+        batchIndexReads(channelName, cb, function (done) {
+            computeIndex(channelName, (err, ret) => {
+                // this is most likely an unrecoverable filesystem error
+                if (err) { return void done(err); }
+                // cache the computed result if possible
+                if (chan) { chan.index = ret; }
+                // return
+                done(void 0, ret);
             });
         });
     };

lib/batch-read.js

@@ -0,0 +1,58 @@
+/*
+
+## Purpose
+
+To avoid running expensive IO or computation concurrently.
+
+If the result of IO or computation is requested while an identical request
+is already in progress, wait until the first one completes and provide its
+result to every routine that requested it.
+
+## Usage
+
+Provide:
+
+1. a named key for the computation or resource,
+2. a callback to handle the result
+3. an implementation which calls back with the result
+
+```
+var batch = Batch();
+
+var read = function (path, cb) {
+    batch(path, cb, function (done) {
+        console.log("reading %s", path);
+        fs.readFile(path, 'utf8', done);
+    });
+};
+
+read('./pewpew.txt', function (err, data) {
+    if (err) { return void console.error(err); }
+    console.log(data);
+});
+
+read('./pewpew.txt', function (err, data) {
+    if (err) { return void console.error(err); }
+    console.log(data);
+});
+```
+
+*/
+
+module.exports = function () {
+    var map = {};
+    return function (id, cb, impl) {
+        if (typeof(cb) !== 'function' || typeof(impl) !== 'function') {
+            throw new Error("expected callback and implementation");
+        }
+        if (map[id]) { return void map[id].push(cb); }
+        map[id] = [cb];
+        impl(function () {
+            var args = Array.prototype.slice.call(arguments);
+            map[id].forEach(function (h) {
+                h.apply(null, args);
+            });
+            delete map[id];
+        });
+    };
+};