[RFA,01/14] Introduce py-ref.h
Commit Message
>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:
Pedro> I think this should say something about supporting explicitly
Pedro> wrapping a NULL PyObject *, and how to check whether there's
Pedro> a managed object.
[... other review comments...]
Here's a new version of this patch, which I think addresses all the
comments, from both you and others.
Tom
Comments
Hi Tom,
On 11/12/2016 05:31 PM, Tom Tromey wrote:
> +
> + /* Transfer ownership from OTHER. */
> + gdbpy_ref &operator= (gdbpy_ref &&other)
> + {
> + /* Note that this handles self-assignment properly. */
> + std::swap (m_obj, other.m_obj);
Since there's no need to worry about noexcept garantees here
(destructing a gdbpy_ref can't throw), I'd rather that
move would not be implemented as a swap, as that extends the
lifetime of m_obj in OTHER.
I.e., bugs like these will go unnoticed longer:
gdbpy_ref bar = make_whatever (....);
gdbpy_ref foo = make_whatever (....);
if (condition)
{
bar = std::move (foo);
some_function (foo.get ()); // whoops, passed bar.get () instead of NULL and crashing early.
}
I'd be happy with the idiom of implementing move-in-term-of-swap
by first move-constructing, and then swapping that temporary:
gdbpy_ref &operator= (gdbpy_ref &&other)
{
gdbpy_ref (std::move (other)).swap (*this);
return *this;
}
Or perhaps simpler, go with something like what you had in the
first version:
+ /* Transfer ownership from another instance. */
+ gdbpy_reference &operator= (gdbpy_reference &&other)
+ {
+ Py_XDECREF (m_obj);
+ m_obj = other.m_obj;
+ other.m_obj = NULL;
+ return *this;
+ }
+
But add a "if (this != &other)" check. I'd tweak it to be
in terms of reset() while at it:
/* Transfer ownership from another instance. */
gdbpy_reference &operator= (gdbpy_reference &&other)
{
if (this != &other)
{
reset (other.m_obj);
other.m_obj = NULL;
}
return *this;
}
Otherwise LGTM.
Thanks,
Pedro Alves
>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:
Pedro> Since there's no need to worry about noexcept garantees here
Pedro> (destructing a gdbpy_ref can't throw), I'd rather that
Pedro> move would not be implemented as a swap, as that extends the
Pedro> lifetime of m_obj in OTHER.
[...]
Pedro> Or perhaps simpler, go with something like what you had in the
Pedro> first version:
I made this change.
Pedro> But add a "if (this != &other)" check. I'd tweak it to be
Pedro> in terms of reset() while at it:
I also changed the other operator= to use reset(), and I changed the
code to use Py_XINCREF rather than a NULL check.
Tom
On 11/16/2016 11:18 PM, Tom Tromey wrote:
>>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:
>
> Pedro> Since there's no need to worry about noexcept garantees here
> Pedro> (destructing a gdbpy_ref can't throw), I'd rather that
> Pedro> move would not be implemented as a swap, as that extends the
> Pedro> lifetime of m_obj in OTHER.
> [...]
> Pedro> Or perhaps simpler, go with something like what you had in the
> Pedro> first version:
>
> I made this change.
>
> Pedro> But add a "if (this != &other)" check. I'd tweak it to be
> Pedro> in terms of reset() while at it:
>
> I also changed the other operator= to use reset(), and I changed the
> code to use Py_XINCREF rather than a NULL check.
Sounds good.
Thanks,
Pedro Alves
@@ -1,3 +1,7 @@
+2016-11-10 Tom Tromey <tom@tromey.com>
+
+ * python/py-ref.h: New file.
+
2016-11-11 Yao Qi <yao.qi@linaro.org>
* spu-tdep.c (spu_software_single_step): Don't call
new file mode 100644
@@ -0,0 +1,158 @@
+/* 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)
+ {
+ if (m_obj != NULL)
+ Py_INCREF (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)
+ {
+ Py_XDECREF (m_obj);
+ m_obj = other.m_obj;
+ if (m_obj != NULL)
+ Py_INCREF (m_obj);
+ }
+ return *this;
+ }
+
+ /* Transfer ownership from OTHER. */
+ gdbpy_ref &operator= (gdbpy_ref &&other)
+ {
+ /* Note that this handles self-assignment properly. */
+ std::swap (m_obj, other.m_obj);
+ 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 */