gpython: Handle program on stdin through just read+exec

Not through interactive console, because it was printing prompts, while
`python <prog.py` does not emit anything:

    $ cat prog.py
    print 'Hello World'

    $ python <prog.py
    Hello World

    $ gpython <prog.py
    Python 2.7.18 (default, Apr 20 2020, 20:30:41)
    [GCC 9.3.0] [GPython 0.0.8] [gevent 20.9.0] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    (InteractiveConsole)
    Hello World

After the patch gpython output is the same as of python:

    $ gpython <prog.py
    Hello World

Test coverage for interactive mode is pending.
We'll add them soon in a follow-up patch after implementing -i.

For _interact - by this patch logic - we should be dropping custom
raw_input, since now _interact is called only when sys.stdin is tty.
But we'll soon be invoking _interact when stdin is not a tty (for
`gpython -i <prog.py`), so leave that logic in place as is.

/reviewed-by @jerome
/reviewed-on !15

gpython/__init__.py

@@ -88,7 +88,7 @@ def pymain(argv, init=None):
 
 
 
-    run = None          # function to run according to -c/-m/file/interactive
+    run = None          # function to run according to -c/-m/file/stdin/interactive
     version = False     # set if `-V`
     warnoptions = []    # collected `-W arg`
     reexec_with = []    # reexecute underlying python with those options (e.g. -O, -S, ...)
@@ -175,13 +175,21 @@ def pymain(argv, init=None):
                      '__package__': None}
                 _execfile(filepath, g)
 
-        # interactive console
+        # interactive console / program on non-tty stdin
         else:
             sys.argv = ['']  if len(argv) == 0  else  argv # e.g. ['-']
             sys.path.insert(0, '')  # cwd
 
+            if sys.stdin.isatty():
                 def run():
                     _interact()
+            else:
+                def run():
+                    import six
+                    prog = sys.stdin.read()
+                    g = {'__name__': '__main__',
+                         '__file__': '<stdin>'}
+                    six.exec_(prog, g)
 
 
     # ---- options processed -> start the interpreter ----

gpython/gpython_test.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2019-2020  Nexedi SA and Contributors.
+# Copyright (C) 2019-2021  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -160,7 +160,7 @@ def test_executable(runtime):
 def test_pymain():
     from golang import b
 
-    # interactive
+    # stdin
     _ = pyout([], stdin=b'import hello\n', cwd=testdata)
     assert _ == b"hello\nworld\n['']\n"
     _ = pyout(['-'], stdin=b'import hello\n', cwd=testdata)
@@ -241,7 +241,7 @@ def test_pymain_syspath():
 
         check_gpy_vs_py(argv, postprocessf=_, **kw)
 
-    check([], stdin=b'import print_syspath', cwd=testprog)  # interactive
+    check([], stdin=b'import print_syspath', cwd=testprog)  # stdin
     check(['-c', 'import print_syspath'], cwd=testprog)     # -c
     check(['-m', 'print_syspath'], cwd=testprog,            # -m
             path0cwd2realpath=(PY2 or is_pypy))