Fix infinite recursion when ~/.pantsrc PEX_PYTHON points at a symlink

When ~/.pexrc sets PEX_PYTHON to a symlink, pex goes into an ~infinite loop.  On each iteration, it checks whether target python == current python, and if not, it reexecs as target python.  It previously called os.path.realpath on current but not on target, so for a symlink, they would never be equal.

Testing Done:
https://travis-ci.org/pantsbuild/pex/builds/91981334

Bugs closed: 182

Reviewed at https://rbcommons.com/s/twitter/r/3149/

pex/pex_bootstrapper.py

@@ -82,8 +82,7 @@ def maybe_reexec_pex():
   target = find_in_path(target_python)
   if not target:
     die('Failed to find interpreter specified by PEX_PYTHON: %s' % target)
-  current = os.path.realpath(sys.executable)
-  if os.path.exists(target) and target != current:
+  if os.path.exists(target) and os.path.realpath(target) != os.path.realpath(sys.executable):
     TRACER.log('Detected PEX_PYTHON, re-exec to %s' % target)
     ENV.delete('PEX_PYTHON')
     os.execve(target, [target_python] + sys.argv, ENV.copy())

tests/test_integration.py

@@ -2,8 +2,9 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 import os
+import sys
 
-from twitter.common.contextutil import temporary_file
+from twitter.common.contextutil import environment_as, temporary_dir, temporary_file
 
 from pex.testing import run_simple_pex_test
 
@@ -30,3 +31,17 @@ def test_pex_interpreter():
     so, rc = run_simple_pex_test("", args=(fp.name,), coverage=True, env=env)
     assert so == b'Hello world\n'
     assert rc == 0
+
+
+def test_pex_python_symlink():
+  with temporary_dir() as td:
+    with environment_as(HOME=td):
+      symlink_path = os.path.join(td, 'python-symlink')
+      os.symlink(sys.executable, symlink_path)
+      pexrc_path = os.path.join(td, '.pexrc')
+      with open(pexrc_path, 'w') as pexrc:
+        pexrc.write("PEX_PYTHON=%s" % symlink_path)
+
+      body = "print('Hello')"
+      _, rc = run_simple_pex_test(body, coverage=True)
+      assert rc == 0