src: Rewrite as proxying writable

This removes the machinery of a Transform stream from the works, acting
solely as a proxy writable. We also provid extended documentation into
the why's of what the module does.

index.js

@@ -1,5 +1,5 @@
 var fs = require('graceful-fs')
-var PassThrough = require('readable-stream').PassThrough
+var Writable = require('readable-stream').Writable
 var util = require('util')
 var MurmurHash3 = require('imurmurhash')
 var iferr = require('iferr')
@@ -17,83 +17,108 @@ function getTmpname (filename) {
   return filename + '.' + murmurhex(__filename, process.pid, ++invocations)
 }
 
+var setImmediate = global.setImmediate || setTimeout
+
 module.exports = WriteStreamAtomic
 
-util.inherits(WriteStreamAtomic, PassThrough)
+// Requirements:
+//   1. Write everything written to the stream to a temp file.
+//   2. If there are no errors:
+//      a. moves the temp file into its final destination
+//      b. emits `finish` & `closed` ONLY after the file is
+//         fully flushed and renamed.
+//   3. If there's an error, removes the temp file.
 
+util.inherits(WriteStreamAtomic, Writable)
 function WriteStreamAtomic (path, options) {
-  if (!options) options = {}
-
   if (!(this instanceof WriteStreamAtomic)) {
     return new WriteStreamAtomic(path, options)
   }
+  Writable.call(this, options)
 
   this.__atomicTarget = path
-  this.__atomicChown = options.chown
   this.__atomicTmp = getTmpname(path)
-  this.__atomicFinished = false
-  this.__atomicMoved = false
-  this.__atomicStream = fs.WriteStream(this.__atomicTmp, options)
-  this.__atomicStream.on('error', handleError.bind(this))
 
-  PassThrough.call(this, options)
-  this.pipe(this.__atomicStream)
-}
+  this.__atomicChown = options && options.chown
 
-function cleanupSync () {
-  try {
-    fs.unlinkSync(this.__atomicTmp)
-  } finally {
-    return
-  }
+  this.__atomicClosed = false
+
+  this.__atomicStream = fs.WriteStream(this.__atomicTmp, options)
+
+  this.__atomicStream.once('open', handleOpen(this))
+  this.__atomicStream.once('close', handleClose(this))
+  this.__atomicStream.once('error', handleError(this))
 }
 
-function handleError (er) {
-  cleanupSync()
-  this.emit('error', er)
+// We have to suppress default finish emitting, because ordinarily it
+// would happen as soon as `end` is called on us and all of the
+// data has been written to our target stream. So we suppress
+// finish from being emitted here, and only emit it after our
+// target stream is closed and we've moved everything around.
+WriteStreamAtomic.prototype.emit = function (event) {
+  if (event === 'finish') return this.__atomicStream.end()
+  return Writable.prototype.emit.apply(this, arguments)
 }
 
-function finish () {
-  if (!this.__atomicFinished) return
-  if (!this.__atomicMoved) return
-  PassThrough.prototype.emit.call(this, 'finish')
-  process.nextTick(function () {
-    this.emit('close')
-  }.bind(this))
+WriteStreamAtomic.prototype._write = function (buffer, encoding, cb) {
+  var flushed = this.__atomicStream.write(buffer, encoding)
+  if (flushed) return cb()
+  this.__atomicStream.once('drain', cb)
 }
 
-WriteStreamAtomic.prototype.emit = function (event) {
-  // We'll emit this ourselves, as we need to hold off on emitting it
-  // until after we've completed putting the final file into place.
-  // To do otherwise creats a race between finish and close ;_;
-  if (event === 'finish') {
-    this.__atomicFinished = true
-    return finish.call(this)
+function handleOpen (writeStream) {
+  return function (fd) {
+    writeStream.emit('open', fd)
   }
-  return PassThrough.prototype.emit.apply(this, arguments)
 }
 
-WriteStreamAtomic.prototype._flush = function (cb) {
-  var writeStream = this
-  if (writeStream.__atomicChown) {
-    var uid = writeStream.__atomicChown.uid
-    var gid = writeStream.__atomicChown.gid
-    return fs.chown(writeStream.__atomicTmp, uid, gid, iferr(cleanup, moveIntoPlace))
-  } else {
-    moveIntoPlace()
+function handleClose (writeStream) {
+  return function () {
+    if (writeStream.__atomicClosed) return
+    writeStream.__atomicClosed = true
+    if (writeStream.__atomicChown) {
+      var uid = writeStream.__atomicChown.uid
+      var gid = writeStream.__atomicChown.gid
+      return fs.chown(writeStream.__atomicTmp, uid, gid, iferr(cleanup, moveIntoPlace))
+    } else {
+      moveIntoPlace()
+    }
   }
   function cleanup (err) {
-    if (!err) return cb()
     fs.unlink(writeStream.__atomicTmp, function () {
       writeStream.emit('error', err)
-      cb()
+      writeStream.emit('close')
     })
   }
   function moveIntoPlace () {
-    fs.rename(writeStream.__atomicTmp, writeStream.__atomicTarget, function (err) {
-      cleanup(err)
-      writeStream.__atomicMoved = true
-      finish.call(writeStream)
+    fs.rename(writeStream.__atomicTmp, writeStream.__atomicTarget, iferr(cleanup, end))
+  }
+  function end () {
+    // We have to use our parent class directly because we suppress `finish`
+    // events fired via our own emit method.
+    Writable.prototype.emit.call(writeStream, 'finish')
+
+    // Delay the close to provide the same temporal separation a physical
+    // file operation would have– that is, the close event is emitted only
+    // after the async close operation completes.
+    setImmediate(function () {
+      writeStream.emit('close')
     })
   }
 }
+
+function handleError (writeStream) {
+  return function (er) {
+    cleanupSync()
+    writeStream.emit('error', er)
+    writeStream.__atomicClosed = true
+    writeStream.emit('close')
+  }
+  function cleanupSync () {
+    try {
+      fs.unlinkSync(writeStream.__atomicTmp)
+    } finally {
+      return
+    }
+  }
+}