TypeError: unsupported operand type(s) for +: 'NoneType' and 'list' in subprocess.Popen when using shell=True under strict Java SecurityManager #418

Description

When running Jython within a sandboxed JVM environment that utilizes a strict java.lang.SecurityManager (specifically, one that denies java.io.FilePermission for reading system directories like /bin/),
executing commands via the subprocess module with shell=True (or using wrappers like os.popen or os.system) crashes with a cryptic Python TypeError rather than throwing a descriptive OSError or
Java SecurityException.

Environment

• Jython Version: 2.5.x, 2.7.x (including 2.7.3 and current development branch)
• OS: Linux/Unix (where _shell_command resolution relies on /bin/sh)
• JVM State: Running with a SecurityManager that denies read access to standard system executable paths.

Steps to Reproduce

Run a Jython script that attempts to use os.popen() or subprocess.Popen(..., shell=True) inside a JVM where the SecurityManager denies file read access to /bin/sh.

Minimal Jython Script:

  import java.lang.SecurityManager as SecurityManager
  import java.lang.System as System
  import java.security.AccessControlException as AccessControlException

  class MySecurityManager(SecurityManager):
      def checkRead(self, file, context=None):
          if file == "/bin/sh" or file == "cmd.exe" or file.endswith("sh"):
              raise AccessControlException("Access denied: " + file)
      def checkPermission(self, perm, context=None):
          pass

  System.setSecurityManager(MySecurityManager())

import subprocess
# This will crash with a TypeError
subprocess.Popen(["whoami"], shell=True)

**Stack Trace:**
  test_sm.py:16: RuntimeWarning: Unable to determine _shell_command for underlying os: posix
    import subprocess
  Traceback (most recent call last):
    File "test_sm.py", line 17, in <module>
      subprocess.Popen(["whoami"], shell=True)
    File ".../Lib/subprocess$py.class", line 892, in __init__
    File ".../Lib/subprocess$py.class", line 1352, in _execute_child
  TypeError: unsupported operand type(s) for +: 'NoneType' and 'list'

Root Cause Analysis

The bug resides in Lib/subprocess.py and stems from how Jython caches the default system shell executable during module initialization.

1. Initialization (_setup_platform): When the subprocess module is loaded, it attempts to determine the system shell by iterating through known shell paths (e.g., /bin/sh). It validates these paths
   using os.path.exists(executable).
2. SecurityManager Intervention: Because the JVM is sandboxed, os.path.exists("/bin/sh") triggers the SecurityManager, which throws an AccessControlException.
3. Silent Failure: Jython's os.path.exists() implementation catches this Java security exception and simply returns False.
4. Invalid State: Because os.path.exists() returns False for all candidate shells, the global variable _shell_command is never assigned a valid list (like ["/bin/sh", "-c"]) and remains
   initialized to None.
5. The Crash (_execute_child and system): Later, when the user invokes subprocess.Popen(..., shell=True) or os.system(), the code attempts to prepend the shell command to the user's arguments:

   if shell:
       args = _shell_command + args # <--- CRASH HERE

   Because `_shell_command` is `None` and `args` is a list, Python throws the `TypeError`.

Expected Behavior

Instead of a cryptic TypeError about concatenating NoneType and list, Jython should gracefully handle the scenario where a shell cannot be located (whether due to actual absence or SecurityManager
restrictions).