add FileFollower for tail -f functionality

lib/file-ext.js

@@ -3,6 +3,7 @@
 
 var fs = require('fs')
   , ArrayExt = require('./array-ext')
+  , FileFollower = require('./file-follower')
   , LineEmitter = require('./line-emitter')
   , ObjectExt = require('./object-ext')
   , constants = require('constants')
@@ -12,6 +13,7 @@ var fs = require('fs')
 FileExt =
 { eachLine: eachLine
 , exists: exists
+, follow: follow
 , grep: grep
 , home: home
 , readLines: readLines
@@ -58,14 +60,22 @@ function eachLine(f, optionsOrLineFn, endFn) {
 
 function exists(f) {
   try {
-    fs.statSync(f)
-    return true
+    fs.statSync(f);
+    return true;
   } catch (e) {
-    if (e.errno === constants.ENOENT) return false
-    throw e
+    if (e.errno === constants.ENOENT) return false;
+    throw e;
   }
 }
 
+function follow(f, lineFn) {
+  var ff = new FileFollower(f);
+  ff.on('line', lineFn);
+  return {
+    stop: ff.stopFollowing.bind(ff)
+  };
+}
+
 function grep(regex, f, callback) {
   if (!callback) throw new Error('grep requires a callback');
   var results = [];

lib/file-follower.js

@@ -0,0 +1,74 @@
+// batteries
+// Copyright 2010 - 2011 Sami Samhuri <sami@samhuri.net>
+
+var fs = require('fs')
+  , util = require('util')
+  , EventEmitter = require('events').EventEmitter
+  , FileExt = require('./file-ext')
+  ;
+
+module.exports = FileFollower;
+
+// TODO: option to act like tail and only show the last N lines, N >= 0
+function FileFollower(file, options) {
+  options = options || {};
+  var self = this;
+  this.file = file;
+  this.currSize = fs.statSync(file).size;
+  this.prevSize = this.currSize;
+  this.interval = options.interval || 1000;
+  FileExt.eachLine(file,
+    { line: function(line) {
+        self.emit('line', line);
+      }
+    , end: function() {
+        self.startFollowing();
+      }
+    });
+}
+util.inherits(FileFollower, EventEmitter);
+
+FileFollower.prototype.startFollowing = function() {
+  if (this._interval) {
+    console.warn('already following');
+    return;
+  }
+  this.buffer = '';
+  this.fd = fs.openSync(this.file, 'r');
+  this._interval = setInterval(this.checkForLine.bind(this), this.interval);
+};
+
+FileFollower.prototype.stopFollowing = function() {
+  if (!this._interval) {
+    console.warn('not following');
+    return;
+  }
+  delete this.buffer;
+  clearInterval(this._interval);
+  delete this._interval;
+  fs.closeSync(this.fd);
+  delete this.fd;
+};
+
+FileFollower.prototype.checkForLine = function() {
+  this.currSize = fs.statSync(this.file).size;
+  if (this.currSize > this.prevSize) {
+    var n = this.currSize - this.prevSize
+      , buf = new Buffer(n + 1)
+      , self = this
+      ;
+    fs.read(this.fd, buf, 0, n, this.prevSize, function(err, bytesRead, buffer) {
+      if (err) {
+        self.emit('error', err);
+        return;
+      }
+      self.buffer += buf.slice(0, bytesRead);
+      self.prevSize += bytesRead;
+      var i;
+      while ((i = self.buffer.indexOf('\n')) !== -1) {
+        self.emit('line', self.buffer.slice(0, i));
+        self.buffer = self.buffer.slice(i + 1);
+      }
+    });
+  }
+};