Change call function to Popen

The call function can deadlock if the child process prints larger output. Use Popen with the communicate() method with you need pipes. If you switch to Popen, you'll get cleaner debugging.

fetch-macOS.py

@@ -87,8 +87,8 @@ class ReplicationError(Exception):
 
 
 def cmd_exists(cmd):
-    return subprocess.call("type " + cmd, shell=True,
+    proc subprocess.Popen("type " + cmd, shell=True,
-                           stdout=subprocess.PIPE, stderr=subprocess.PIPE) == 0
+                           stdout=subprocess.PIPE, stderr=subprocess.PIPE)
 
 
 def replicate_url(full_url,