title: Error in socketConnection(master, port = port, blocking = TRUE, open = "a+b", : cannot open the connection in Stan (from R)
body: <p>I am on a Macbook Pro Retina Display from 2012 running El Capitan with 8 cores according to <code>parallel::detectCores()</code>.  My firewall is off.  I am performing the following call from <code>stan_lmer</code> loaded from the <code>rstanarm</code> package.</p>
  
  <pre><code>hierarchical.model.1 &lt;- stan_lmer(y ~ x1 + (1 | x2) + (1 | x3) +
                              x4 + x5 + x6 + x7 + x8,
                              data = df,
                              cores = NCORES, # NCORES = parallel::detectCores()
                              seed = SEED)
  </code></pre>
  
  <p>The exact error is:</p>
  
  <pre><code>starting worker pid=12968 on localhost:11058 at 23:51:49.107
  Error in socketConnection(master, port = port, blocking = TRUE, open = "a+b",  : 
    cannot open the connection
  Calls: &lt;Anonymous&gt; ... doTryCatch -&gt; recvData -&gt; makeSOCKmaster -&gt; socketConnection
  In addition: Warning message:
  In socketConnection(master, port = port, blocking = TRUE, open = "a+b",  :
    localhost:11058 cannot be opened
  Execution halted
    C-c C-c
  </code></pre>