Fix write-after-close of file descriptor in ScriptInterpreterPython

[zturner]

Summary

When we acquire the GIL with Locker, we save off the current stdio handles, redirect them, and when we release the GIL we put the old handles back. This triggers a potential write-after-close, as follows:

In ScriptInterpreterPython::ExecuteOneLine, we set up a pipe, lock the gil, kick off a thread to do some work, then close the write end of the pipe. However, the write end of the pipe in this case is the same thing that we set Python's stdout and stderr handles to, and we haven't yet restored them until ExecuteOneLine ends, due to the scope in which the RAII object is declared.

This has always been fine, but only as a coincidence. In Python 3 it seems that when you set stderr to a new file, it writes to the *original* file first (probably to flush it). In the case of ExecuteOneLine, that file descriptor has been intentionally closed by caller as a means to break the pipe and join the other thread it was communicating with, so we get an error.

The fix here is simple: Just acquire the GIL in a tighter scope so that stdin, stdout, and stderr are reset to their original values before trying to join the read thread.

It doesn't *seem* like we need to hold the GIL any longer than this since no Python calls are happening outside of the new smaller scope I've introduced, but let me know if you can think of anything wrong with this.

[zturner]

Greg, Enrico, anyone have a chance to take a look at this? I think it's probably just a rubber stamp but rather err on the side of caution.

[clayborg]

Looks good to me. Wait for Enrico to OK as well.

[granata.enrico]

Looks reasonable

source/Plugins/ScriptInterpreter/Python/PythonDataObjects.h

 class PythonFile : public PythonObject
 {
   public:
+    PythonFile();
     PythonFile(File &file, const char *mode);
     PythonFile(const char *path, const char *mode);
     PythonFile(PyRefType type, PyObject *o);

source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp

     return result;
 }
 
+PythonFile::PythonFile()
+    : PythonObject()
+{
+}
+
 PythonFile::PythonFile(File &file, const char *mode)
 {
     Reset(file, mode);

source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.h

     bool
     GetEmbeddedInterpreterModuleObjects ();
 
-    PythonObject m_saved_stdin;
-    PythonObject m_saved_stdout;
-    PythonObject m_saved_stderr;
+    PythonFile m_saved_stdin;
+    PythonFile m_saved_stdout;
+    PythonFile m_saved_stderr;
     PythonObject m_main_module;
     PythonObject m_lldb_module;
     PythonDictionary m_session_dict;

source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

                 // Flush the file before giving it to python to avoid interleaved output.
                 in_file.Flush();
 
-                m_saved_stdin = sys_module_dict.GetItemForKey(PythonString("stdin"));
+                m_saved_stdin = sys_module_dict.GetItemForKey(PythonString("stdin")).AsType<PythonFile>();
                 // This call can deadlock your process if the file is locked
                 PythonFile new_file(in_file, "r");
                 sys_module_dict.SetItemForKey (PythonString("stdin"), new_file);

             // Flush the file before giving it to python to avoid interleaved output.
             out_file.Flush();
 
-            m_saved_stdout = sys_module_dict.GetItemForKey(PythonString("stdout"));
+            m_saved_stdout = sys_module_dict.GetItemForKey(PythonString("stdout")).AsType<PythonFile>();
 
             PythonFile new_file(out_file, "w");
             sys_module_dict.SetItemForKey (PythonString("stdout"), new_file);

             // Flush the file before giving it to python to avoid interleaved output.
             err_file.Flush();
 
-            m_saved_stderr = sys_module_dict.GetItemForKey(PythonString("stderr"));
+            m_saved_stderr = sys_module_dict.GetItemForKey(PythonString("stderr")).AsType<PythonFile>();
 
             PythonFile new_file(err_file, "w");
             sys_module_dict.SetItemForKey (PythonString("stderr"), new_file);

         FILE *in_file = input_file_sp->GetFile().GetStream();
         FILE *out_file = output_file_sp->GetFile().GetStream();
         FILE *err_file = error_file_sp->GetFile().GetStream();
-        Locker locker(this,
-                      ScriptInterpreterPython::Locker::AcquireLock |
-                      ScriptInterpreterPython::Locker::InitSession |
-                      (options.GetSetLLDBGlobals() ? ScriptInterpreterPython::Locker::InitGlobals : 0) |
-                      ((result && result->GetInteractive()) ? 0: Locker::NoSTDIN),
-                      ScriptInterpreterPython::Locker::FreeAcquiredLock |
-                      ScriptInterpreterPython::Locker::TearDownSession,
-                      in_file,
-                      out_file,
-                      err_file);
-        
         bool success = false;
-        
-        // Find the correct script interpreter dictionary in the main module.
-        PythonDictionary &session_dict = GetSessionDictionary ();
-        if (session_dict.IsValid())
         {
-            if (GetEmbeddedInterpreterModuleObjects ())
+            Locker locker(this,
+                          ScriptInterpreterPython::Locker::AcquireLock |
+                          ScriptInterpreterPython::Locker::InitSession |
+                          (options.GetSetLLDBGlobals() ? ScriptInterpreterPython::Locker::InitGlobals : 0) |
+                          ((result && result->GetInteractive()) ? 0: Locker::NoSTDIN),
+                          ScriptInterpreterPython::Locker::FreeAcquiredLock |
+                          ScriptInterpreterPython::Locker::TearDownSession,
+                          in_file,
+                          out_file,
+                          err_file);
+        
+            // Find the correct script interpreter dictionary in the main module.
+            PythonDictionary &session_dict = GetSessionDictionary ();
+            if (session_dict.IsValid())
             {
-                if (PyCallable_Check(m_run_one_line_function.get()))
+                if (GetEmbeddedInterpreterModuleObjects ())
                 {
-                    PythonObject pargs(PyRefType::Owned, Py_BuildValue("(Os)", session_dict.get(), command));
-                    if (pargs.IsValid())
+                    if (PyCallable_Check(m_run_one_line_function.get()))
                     {
-                        PythonObject return_value(PyRefType::Owned,
-                            PyObject_CallObject(m_run_one_line_function.get(), pargs.get()));
-                        if (return_value.IsValid())
-                            success = true;
-                        else if (options.GetMaskoutErrors() && PyErr_Occurred ())
+                        PythonObject pargs(PyRefType::Owned, Py_BuildValue("(Os)", session_dict.get(), command));
+                        if (pargs.IsValid())
                         {
-                            PyErr_Print();
-                            PyErr_Clear();
+                            PythonObject return_value(PyRefType::Owned,
+                                PyObject_CallObject(m_run_one_line_function.get(), pargs.get()));
+                            if (return_value.IsValid())
+                                success = true;
+                            else if (options.GetMaskoutErrors() && PyErr_Occurred ())
+                            {
+                                PyErr_Print();
+                                PyErr_Clear();
+                            }
                         }
                     }
                 }
             }
-        }
 
             // Flush our output and error file handles
             ::fflush (out_file);
             if (out_file != err_file)
                 ::fflush (err_file);
+        }
         
         if (join_read_thread)
         {