[PyROOT][9632] Fix iteration on std::vector<char>

As part of the implementation of std.vector.__iter__, the vector's
data() method is called. In the case of a vector<char>, data()
returns a char*, which is converted by cppyy to a Python string.
The issue is that the conversion char*->str expects that the
sequence of characters to convert is null-terminated, e.g. in
Python3 the following function is used for the conversion:
https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_FromString

In the case of the data pointed by the char* returned by data(),
there is no guarantee that the sequence of characters will be
null-terminated, and this results in unpredictable and erroneous
behaviour.

The fix of this commit adds a pythonization for vector<char>::data()
that temporarily adds a null character to the vector before calling
the actual data(), so that the conversion is done correctly and
the returned Python string contains the characters of the vector.
This also fixes the vector<char> iteration, which relies on data().

Author: etejedor

bindings/pyroot/pythonizations/python/ROOT/_pythonization/_stl_vector.py

@@ -1,4 +1,4 @@
-# Author: Stefan Wunsch CERN  08/2018
+# Author: Stefan Wunsch, Enric Tejedor CERN  08/2018
 
 ################################################################################
 # Copyright (C) 1995-2018, Rene Brun and Fons Rademakers.                      #
@@ -11,6 +11,17 @@
 from . import pythonization
 from ._rvec import add_array_interface_property
 
+def _data_vec_char(self):
+    # vector<char>::data() returns char*.
+    # Cppyy attemps to convert char* into Python string, but if the
+    # character sequence is not null-terminated the conversion fails.
+    # This is likely to happen with the result of vector<char>::data().
+    # For the conversion char* -> str to succeed when calling data(),
+    # temporarily append a null character to the vector<char>.
+    self.push_back('\0')
+    d = self._original_data()
+    self.pop_back()
+    return d
 
 @pythonization("vector<", ns="std", is_prefix=True)
 def pythonize_stl_vector(klass, name):
@@ -21,3 +32,8 @@ def pythonize_stl_vector(klass, name):
     # Add numpy array interface
     # NOTE: The pythonization is reused from ROOT::VecOps::RVec
     add_array_interface_property(klass, name)
+
+    # Inject custom vector<char>::data()
+    if klass.value_type == 'char':
+        klass._original_data = klass.data
+        klass.data = _data_vec_char