add some apprehension to the sudo exec module

- raise an error if sudo.salt_call is called directly, which would be an
  inductively divergent condition

- return the actual error message (along with some debug logging) when
  the command fails rather than an unhelpful json decode error.

- protect positional args and kwargs to salt-call with a `--` symbol.

- quote the args and kwargs sent to salt-call so they work as expected.

- do not load the sudo exec module if the sudo_user config is unset

salt/modules/sudo.py

@@ -29,7 +29,13 @@ being run on ``sudo_minion``.
 '''
 # Import python libs
 import json
+try:
+    from shlex import quote as _cmd_quote  # pylint: disable=E0611
+except ImportError:
+    from pipes import quote as _cmd_quote
+
 # Import salt libs
+from salt.exceptions import SaltInvocationError
 import salt.utils
 import salt.syspaths
 
@@ -37,7 +43,7 @@ __virtualname__ = 'sudo'
 
 
 def __virtual__():
-    if salt.utils.which('sudo'):
+    if salt.utils.which('sudo') and __opts__.get('sudo_user'):
         return __virtualname__
     return False
 
@@ -58,19 +64,33 @@ def salt_call(runas, fun, *args, **kwargs):
 
         salt '*' test.ping  # is run as saltdev user
     '''
+    if fun == 'sudo.salt_call':
+        __context__['retcode'] = 1
+        raise SaltInvocationError('sudo.salt_call is not designed to be run '
+                                  'directly, but is used by the minion when '
+                                  'the sudo_user config is set.')
+
     cmd = ['sudo',
            '-u', runas,
            'salt-call',
            '--out', 'json',
            '--metadata',
            '-c', salt.syspaths.CONFIG_DIR,
+           '--',
            fun]
     for arg in args:
-        cmd.append(arg)
+        cmd.append(_cmd_quote(arg))
     for key in kwargs:
-        cmd.append('{0}={1}'.format(key, kwargs[key]))
-    cmd_ret = __salt__['cmd.run_all'](cmd, python_shell=False)
-    cmd_meta = json.loads(cmd_ret['stdout'])['local']
-    ret = cmd_meta['return']
-    __context__['retcode'] = cmd_meta.get('retcode', 0)
+        cmd.append(_cmd_quote('{0}={1}'.format(key, kwargs[key])))
+
+    cmd_ret = __salt__['cmd.run_all'](cmd, use_vt=True, python_shell=False)
+
+    if cmd_ret['retcode'] == 0:
+        cmd_meta = json.loads(cmd_ret['stdout'])['local']
+        ret = cmd_meta['return']
+        __context__['retcode'] = cmd_meta.get('retcode', 0)
+    else:
+        ret = cmd_ret['stderr']
+        __context__['retcode'] = cmd_ret['retcode']
+
     return ret