Support - when running as an interpreter. (#543)

It's surprising when using a pex in python interpreter mode that `-` is
not recognized as a program coming from stdin as regular python
interpreter would. Add support for this with a test.

The surprise of this missing feature was rightly identified in the
context of pantsbuild/pants#6275.

pex/pex.py

@@ -417,13 +417,17 @@ def _execute(self):
 
   def execute_interpreter(self):
     if sys.argv[1:]:
+      program = sys.argv[1]
       try:
-        with open(sys.argv[1]) as fp:
-          name, content = sys.argv[1], fp.read()
+        if program == '-':
+          content = sys.stdin.read()
+        else:
+          with open(program) as fp:
+            content = fp.read()
       except IOError as e:
-        die("Could not open %s in the environment [%s]: %s" % (sys.argv[1], sys.argv[0], e))
+        die("Could not open %s in the environment [%s]: %s" % (program, sys.argv[0], e))
       sys.argv = sys.argv[1:]
-      self.execute_content(name, content)
+      self.execute_content(program, content)
     else:
       import code
       code.interact()

tests/test_pex.py

@@ -2,6 +2,7 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 import os
+import subprocess
 import sys
 import textwrap
 from contextlib import contextmanager
@@ -342,3 +343,19 @@ def test_activate_interpreter_different_from_current():
           pex._activate()
         except SystemExit as e:
           pytest.fail('PEX activation of %s failed with %s' % (pex, e))
+
+
+def test_execute_interpreter_stdin_program():
+  with temporary_dir() as pex_chroot:
+    pex_builder = PEXBuilder(path=pex_chroot)
+    pex_builder.freeze()
+    process = PEX(pex_chroot).run(args=['-', 'one', 'two'],
+                                  stdout=subprocess.PIPE,
+                                  stderr=subprocess.PIPE,
+                                  stdin=subprocess.PIPE,
+                                  blocking=False)
+    stdout, stderr = process.communicate(input=b'import sys; print(" ".join(sys.argv[1:]))')
+
+    assert 0 == process.returncode
+    assert b'one two\n' == stdout
+    assert b'' == stderr