support empty _in param. closes #427

sh.py

@@ -2119,7 +2119,7 @@ def __init__(self, command, parent_log, cmd, stdin, stdout, stderr,
             # to prevent race conditions
             self.exit_code = None
 
-            self.stdin = stdin or Queue()
+            self.stdin = stdin
 
             # _pipe_queue is used internally to hand off stdout from one process
             # to another.  by default, all stdout from a process gets dumped
@@ -2152,15 +2152,10 @@ def __init__(self, command, parent_log, cmd, stdin, stdout, stderr,
                 attr[3] &= ~termios.ECHO
                 termios.tcsetattr(self._stdin_parent_fd, termios.TCSANOW, attr)
 
-            # we're only going to create a stdin thread iff we have potential
-            # for stdin to come in.  this would be through a stdout callback or
-            # through an object we've passed in for stdin
-            potentially_has_input = callable(stdout) or stdin
-
             # this represents the connection from a Queue object (or whatever
             # we're using to feed STDIN) to the process's STDIN fd
             self._stdin_stream = None
-            if self._stdin_parent_fd and potentially_has_input:
+            if self._stdin_parent_fd:
                 log = self.log.get_child("streamwriter", "stdin")
                 self._stdin_stream =  StreamWriter(log, self._stdin_parent_fd,
                         self.stdin, ca["in_bufsize"], ca["encoding"],
@@ -2651,6 +2646,12 @@ def determine_how_to_read_input(input_obj):
         log_msg = "generator"
         get_chunk = get_iter_chunk_reader(iter(input_obj))
 
+    elif input_obj is None:
+        log_msg = "None"
+        def raise_():
+            raise DoneReadingForever
+        get_chunk = raise_
+
     else:
         try:
             it = iter(input_obj)

test.py

@@ -287,6 +287,17 @@ def test_number_arg(self):
         out = python(py.name, 3).strip()
         self.assertEqual(out, "3")
 
+    def test_empty_stdin_no_hang(self):
+        py = create_tmp_test("""
+import sys
+data = sys.stdin.read()
+sys.stdout.write("no hang")
+""")
+        out = python(py.name, _in="", _timeout=2)
+        self.assertEqual(out, "no hang")
+
+        out = python(py.name, _in=None, _timeout=2)
+        self.assertEqual(out, "no hang")
 
     def test_exit_code(self):
         from sh import ErrorReturnCode