Introduce py-ref.h

This patch introduces class gdbpy_ref, which is a sort of smart
pointer that owns a single Python reference to a PyObject.  This class
acts a bit like unique_ptr, but also a bit like shared_ptr (in that
copies do what you might expect); I considered going solely with
unique_ptr but it seemed quite strange to have a unique_ptr that
actually manages a shared resource.

Subsequent patches use this new class to simplify logic in the Python
layer.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* python/py-ref.h: New file.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2017-01-10  Tom Tromey  <tom@tromey.com>
+
+	* python/py-ref.h: New file.
+
 2017-01-10  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* cli-out.c (cli_ui_out::do_redirect): Change return type to

gdb/python/py-ref.h

@@ -0,0 +1,159 @@
+/* Python reference-holding class
+
+   Copyright (C) 2016 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDB_PYTHON_REF_H
+#define GDB_PYTHON_REF_H
+
+/* An instance of this class either holds a reference to a PyObject,
+   or is "NULL".  If it holds a reference, then when the object is
+   destroyed, the PyObject is decref'd.
+
+   Normally an instance is constructed using a PyObject*.  This sort
+   of initialization lets this class manage the lifetime of that
+   reference.
+
+   Assignment and copy construction will make a new reference as
+   appropriate.  Assignment from a plain PyObject* is disallowed to
+   avoid confusion about whether this acquires a new reference;
+   instead use the "reset" method -- which, like the PyObject*
+   constructor, transfers ownership.
+*/
+class gdbpy_ref
+{
+ public:
+
+  /* Create a new NULL instance.  */
+  gdbpy_ref ()
+    : m_obj (NULL)
+  {
+  }
+
+  /* Create a new instance.  OBJ is a reference, management of which
+     is now transferred to this class.  */
+  explicit gdbpy_ref (PyObject *obj)
+    : m_obj (obj)
+  {
+  }
+
+  /* Copy another instance.  */
+  gdbpy_ref (const gdbpy_ref &other)
+    : m_obj (other.m_obj)
+  {
+    Py_XINCREF (m_obj);
+  }
+
+  /* Transfer ownership from OTHER.  */
+  gdbpy_ref (gdbpy_ref &&other)
+    : m_obj (other.m_obj)
+  {
+    other.m_obj = NULL;
+  }
+
+  /* Destroy this instance.  */
+  ~gdbpy_ref ()
+  {
+    Py_XDECREF (m_obj);
+  }
+
+  /* Copy another instance.  */
+  gdbpy_ref &operator= (const gdbpy_ref &other)
+  {
+    /* Do nothing on self-assignment.  */
+    if (this != &other)
+      {
+	reset (other.m_obj);
+	Py_XINCREF (m_obj);
+      }
+    return *this;
+  }
+
+  /* Transfer ownership from OTHER.  */
+  gdbpy_ref &operator= (gdbpy_ref &&other)
+  {
+    /* Do nothing on self-assignment.  */
+    if (this != &other)
+      {
+	reset (other.m_obj);
+	other.m_obj = NULL;
+      }
+    return *this;
+  }
+
+  /* Change this instance's referent.  OBJ is a reference, management
+     of which is now transferred to this class.  */
+  void reset (PyObject *obj)
+  {
+    Py_XDECREF (m_obj);
+    m_obj = obj;
+  }
+
+  /* Return this instance's referent.  In Python terms this is a
+     borrowed pointer.  */
+  PyObject *get () const
+  {
+    return m_obj;
+  }
+
+  /* Return this instance's referent, and stop managing this
+     reference.  The caller is now responsible for the ownership of
+     the reference.  */
+  PyObject *release ()
+  {
+    PyObject *result = m_obj;
+
+    m_obj = NULL;
+    return result;
+  }
+
+ private:
+
+  PyObject *m_obj;
+};
+
+inline bool operator== (const gdbpy_ref &self, const gdbpy_ref &other)
+{
+  return self.get () == other.get ();
+}
+
+inline bool operator== (const gdbpy_ref &self, const PyObject *other)
+{
+  return self.get () == other;
+}
+
+inline bool operator== (const PyObject *self, const gdbpy_ref &other)
+{
+  return self == other.get ();
+}
+
+inline bool operator!= (const gdbpy_ref &self, const gdbpy_ref &other)
+{
+  return self.get () != other.get ();
+}
+
+inline bool operator!= (const gdbpy_ref &self, const PyObject *other)
+{
+  return self.get () != other;
+}
+
+inline bool operator!= (const PyObject *self, const gdbpy_ref &other)
+{
+  return self != other.get ();
+}
+
+#endif /* GDB_PYTHON_REF_H */