win: Make auto-updater really work

Apparently that PR was never tested.

atom/browser/api/lib/auto-updater/squirrel-update-win.coffee

@@ -1,57 +1,67 @@
-ChildProcess = require 'child_process'
-fs           = require 'fs'
-path         = require 'path'
+fs      = require 'fs'
+path    = require 'path'
+{spawn} = require 'child_process'
 
-appFolder             = path.dirname process.execPath # i.e. my-app/app-0.1.13/
-rootApplicationFolder = path.resolve appFolder, '..'  # i.e. my-app/
-updateDotExe          = path.join rootApplicationFolder, 'Update.exe'
-exeName               = path.basename process.execPath
+appFolder = path.dirname process.execPath # i.e. my-app/app-0.1.13/
+updateExe = path.resolve appFolder, '..', 'Update.exe' # i.e. my-app/Update.exe
+exeName   = path.basename process.execPath
 
 # Spawn a command and invoke the callback when it completes with an error
 # and the output from standard out.
-spawnUpdate = (args, callback) ->
-  stdout = ''
-
+spawnUpdate = (args, detached, callback) ->
   try
-    spawnedProcess = ChildProcess.spawn(updateDotExe, args)
+    spawnedProcess = spawn updateExe, args, {detached}
   catch error
-    # Spawn can throw an error
-    process.nextTick -> callback?(error, stdout)
+    # Shouldn't happen, but still guard it.
+    process.nextTick -> callback error
     return
 
+  stdout = ''
+  stderr = ''
   spawnedProcess.stdout.on 'data', (data) -> stdout += data
+  spawnedProcess.stderr.on 'data', (data) -> stderr += data
 
-  error = null
-  spawnedProcess.on 'error', (processError) -> error ?= processError
-  spawnedProcess.on 'close', (code, signal) ->
-    error ?= new Error("Command failed: #{signal ? code}") if code isnt 0
-    error?.code ?= code
-    error?.stdout ?= stdout
-    callback?(error, stdout)
+  errorEmitted = false
+  spawnedProcess.on 'error', (error) ->
+    errorEmitted = true
+    callback error
+  spawnedProcess.on 'exit', (code, signal) ->
+    # We may have already emitted an error.
+    return if errorEmitted
 
-processStart = (callback) ->
-  spawnUpdate(['--processStart', exeName], callback)
+    # Process terminated with error.
+    if code isnt 0
+      return callback "Command failed: #{signal ? code}\n#{stderr}"
 
-download = (callback) ->
-  spawnUpdate ['--download', @updateUrl], (error, stdout) ->
+    # Success.
+    callback null, stdout
+
+# Start an instance of the installed app.
+exports.processStart = (callback) ->
+  spawnUpdate ['--processStart', exeName], true, ->
+
+# Download the releases specified by the URL and write new results to stdout.
+exports.download = (updateUrl, callback) ->
+  spawnUpdate ['--download', updateUrl], false, (error, stdout) ->
     return callback(error) if error?
 
     try
       # Last line of output is the JSON details about the releases
-      json   = stdout.trim().split('\n').pop()
+      json = stdout.trim().split('\n').pop()
       update = JSON.parse(json)?.releasesToApply?.pop?()
-    catch error
-      error.stdout = stdout
-      return callback(error)
+    catch
+      return callback "Invalid result:\n#{stdout}"
 
-    callback(null, update)
+    callback null, update
 
-update = (updateUrl, callback) ->
-  spawnUpdate ['--update', updateUrl], callback
+# Update the application to the latest remote version specified by URL.
+exports.update = (updateUrl, callback) ->
+  spawnUpdate ['--update', updateUrl], false, callback
 
 # Is the Update.exe installed with the current application?
 exports.supported = ->
-  fs.accessSync(updateDotExe, fs.R_OK)
-exports.processStart = processStart
-exports.download     = download
-exports.update       = update
+  try
+    fs.accessSync updateExe, fs.R_OK
+    return true
+  catch
+    return false