grid/utils: Improve parsing of instance's python

The path of the python executable used by an instance was parsed from
the shebang of its software release's bin/buildout. But when the path
is too long, the shebang is just /bin/sh and the script starts with
`exec <executable>`.

slapos/grid/utils.py

@@ -37,6 +37,7 @@ import stat
 import sys
 import logging
 import psutil
+import shlex
 import time
 
 if sys.version_info >= (3,):
@@ -221,10 +222,15 @@ def getPythonExecutableFromSoftwarePath(software_path):
   try:
     with open(os.path.join(software_path, 'bin', 'buildout')) as f:
       shebang = f.readline()
+      if shebang.startswith('#!'):
+        executable = shebang[2:].split(None, 1)[0]
+        if executable == '/bin/sh':
+          exec_wrapper = shlex.split(f.readline())
+          if len(exec_wrapper) >= 2 and exec_wrapper[0] == 'exec':
+            return exec_wrapper[1]
+        return executable
   except (IOError, OSError):
     return
-  if shebang.startswith('#!'):
-    return shebang[2:].split(None, 1)[0]
 
 
 def getCleanEnvironment(logger, home_path='/tmp'):

slapos/tests/test_grid_utils.py

@@ -40,6 +40,7 @@ else:
   import subprocess32 as subprocess
 
 import mock
+import six
 import slapos.grid.utils
 
 
@@ -473,3 +474,28 @@ class TestSetRunning(unittest.TestCase):
           tf.name)
       with open(tf.name) as f:
         self.assertEqual(f.read(), str(os.getpid()))
+
+
+@unittest.skipIf(six.PY2, "Python3 only")
+class TestGetPythonExecutableFromBinBuildout(unittest.TestCase):
+  def test_simple_shebang(self):
+    with tempfile.TemporaryDirectory() as d:
+      binpath = os.path.join(d, 'bin')
+      python = os.path.realpath(os.path.join(d, 'python'))
+      os.mkdir(binpath)
+      with open(os.path.join(binpath, 'buildout'), 'w') as f:
+        f.write('#!' + python)
+      self.assertEqual(
+        slapos.grid.utils.getPythonExecutableFromSoftwarePath(d),
+        python)
+
+  def test_exec_wrapper(self):
+    with tempfile.TemporaryDirectory() as d:
+      binpath = os.path.join(d, 'bin')
+      python = os.path.realpath(os.path.join(d, 'python'))
+      os.mkdir(binpath)
+      with open(os.path.join(binpath, 'buildout'), 'w') as f:
+        f.write('#!/bin/sh\n"exec" "%s" "$0" "$@"' % python)
+      self.assertEqual(
+        slapos.grid.utils.getPythonExecutableFromSoftwarePath(d),
+        python)