$synchronize works with promises too.

src/fibers-utils.coffee

@@ -6,6 +6,10 @@ $fiber = require 'fibers'
 
 #=====================================================================
 
+$isPromise = (obj) -> obj? and $_.isFunction obj.then
+
+#=====================================================================
+
 # Makes a function running in its own fiber.
 $fiberize = (fn) ->
   (args...) ->
@@ -29,17 +33,16 @@ $synchronize = (fn, ctx) ->
         fiber.throwInto error
       else
         fiber.run result
-    fn.apply ctx, args
+    result = fn.apply ctx, args
 
-    $fiber.yield()
-
-# TODO: remove promises ASAP.
-$waitForPromise = (promise) ->
-  fiber = $fiber.current
-  promise.then(
-    (value) -> fiber.run value
-    (error) -> fiber.throwInto error
+    # A promise can only be detected once the function has been
+    # called.
+    if $isPromise result
+      result.then(
+        (result) -> fiber.run result
+        (error) -> fiber.throwInto result
       )
+
     $fiber.yield()
 
 #=====================================================================
@@ -48,5 +51,4 @@ module.exports = {
   $fiberize
   $sleep
   $synchronize
-  $waitForPromise
 }

src/main.coffee

@@ -25,7 +25,7 @@ $Session = require './session'
 $XO = require './xo'
 
 # Helpers for dealing with fibers.
-{$fiberize, $waitForPromise} = require './fibers-utils'
+{$fiberize, $synchronize} = require './fibers-utils'
 
 # HTTP/HTTPS server which can listen on multiple ports.
 $WebServer = require './web-server'
@@ -59,10 +59,11 @@ $handleJsonRpcCall = (api, session, encodedRequest) ->
     return formatError $API.err.INVALID_REQUEST
 
   # Executes the requested method on the API.
+  exec = $synchronize 'exec', api
   try
     JSON.stringify {
       jsonrpc: '2.0'
-      result: $waitForPromise (api.exec session, request)
+      result: exec session, request
       id: request.id
     }
   catch error

src/xo.coffee

@@ -28,12 +28,18 @@ $Model = require './model'
 $XAPI = require './xapi'
 
 # Helpers for dealing with fibers.
-{$fiberize, $synchronize, $waitForPromise} = require './fibers-utils'
+{$fiberize, $synchronize} = require './fibers-utils'
 
 #=====================================================================
 
+$hash = $synchronize 'hash', $hashy
+
+$needsRehash = $synchronize 'needsRehash', $hashy
+
 $randomBytes = $synchronize 'randomBytes', $crypto
 
+$verifyHash = $synchronize 'verify', $hashy
+
 #=====================================================================
 # Models and collections.
 
@@ -67,16 +73,16 @@ class $User extends $Model
   validate: -> # TODO
 
   setPassword: (password) ->
-    @set 'password', $waitForPromise ($hashy.hash password)
+    @set 'password', $hash password
 
   # Checks the password and updates the hash if necessary.
   checkPassword: (password) ->
     hash = @get 'hash'
 
-    unless $waitForPromise ($hashy.verify password, hash)
+    unless $verifyHash password, hash
       return false
 
-    if $waitForPromise ($hashy.needsRehash hash)
+    if $needsRehash hash
       @setPassword password
 
     true
@@ -261,7 +267,8 @@ class $XO
           throw error unless error[0] is 'SESSION_NOT_REGISTERED'
 
     # Connects to existing servers.
-    connect server for server in $waitForPromise @servers.get()
+    getServers = $synchronize 'get', @servers
+    connect server for server in getServers()
 
     # Automatically connects to new servers.
     @servers.on 'add', (servers) ->