Add support for writing unwinders in Python.

gdb/ChangeLog:

	* Makefile.in (SUBDIR_PYTHON_OBJS): Add py-unwind.o.
	(SUBDIR_PYTHON_SRCS): Add py-unwind.c.
	(py-unwind.o): New recipe.
	* NEWS: mention Python frame unwinding.
	* data-directory/Makefile.in (PYTHON_FILE_LIST): Add
	gdb/unwinder.py and gdb/command/unwinder.py
	* python/lib/gdb/__init__.py (packages): Add frame_unwinders
	list.
	(execute_unwinders): New function.
	* python/lib/gdb/command/unwinders.py: New file.
	* python/lib/gdb/unwinder.py: New file.
	* python/py-objfile.c (objfile_object): Add frame_unwinders field.
	(objfpy_dealloc): Decrement frame_unwinders reference count.
	(objfpy_initialize): Create frame_unwinders list.
	(objfpy_get_frame_unwinders): New function.
	(objfpy_set_frame_unwinders): Ditto.
	(objfile_getset): Add frame_unwinders attribute to Objfile.
	* python/py-progspace.c (pspace_object): Add frame_unwinders field.
	(pspy_dealloc): Decrement frame_unwinders reference count.
	(pspy_initialize): Create frame_unwinders list.
	(pspy_get_frame_unwinders): New function.
	(pspy_set_frame_unwinders): Ditto.
	(pspy_getset): Add frame_unwinders attribute to gdb.Progspace.
	* python/py-unwind.c: New file.
	* python/python-internal.h (pspy_get_name_unwinders): New prototype.
	(objpy_get_frame_unwinders): New prototype.
	(gdbpy_initialize_unwind): New prototype.
	* python/python.c (gdbpy_apply_type_printers): Call
	gdbpy_initialize_unwind.

gdb/doc/ChangeLog:

	* doc/python.texi (Writing a Frame Unwinder in Python): Add
	section.

gdb/testsuite/ChangeLog:

	* gdb.python/py-unwind-maint.c: New file.
	* gdb.python/py-unwind-maint.exp: New test.
	* gdb.python/py-unwind-maint.py: New file.
	* gdb.python/py-unwind.c: New file.
	* gdb.python/py-unwind.exp: New test.
	* gdb.python/py-unwind.py: New test.

gdb/python/lib/gdb/unwinder.py

@@ -0,0 +1,94 @@
+# Copyright (C) 2015 Free Software Foundation, Inc.
+
+# 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/>.
+
+"""Unwinder class and register_unwinder function."""
+
+import gdb
+
+
+class Unwinder(object):
+    """Base class (or a template) for frame unwinders written in Python.
+
+    An unwinder has a single method __call__ and the attributes
+    described below.
+
+    Attributes:
+        name: The name of the unwinder.
+        enabled: A boolean indicating whether the unwinder is enabled.
+    """
+
+    def __init__(self, name):
+        """Constructor.
+
+        Args:
+            name: An identifying name for the unwinder.
+        """
+        self.name = name
+        self.enabled = True
+
+    def __call__(self, pending_frame):
+        """GDB calls this method to unwind a frame.
+
+        Arguments:
+            pending_frame: gdb.PendingFrame instance.
+
+        Returns:
+            gdb.UnwindInfo instance.
+        """
+        raise NotImplementedError("Unwinder __call__.")
+
+
+def register_unwinder(locus, unwinder, replace=False):
+    """Register unwinder in given locus.
+
+    The unwinder is prepended to the locus's unwinders list. Unwinder
+    name should be unique.
+
+    Arguments:
+        locus: Either an objfile, progspace, or None (in which case
+               the unwinder is registered globally).
+        unwinder: An object of a gdb.Unwinder subclass
+        replace: If True, replaces existing unwinder with the same name.
+                 Otherwise, raises exception if unwinder with the same
+                 name already exists.
+
+    Returns:
+        Nothing.
+
+    Raises:
+        RuntimeError: Unwinder name is not unique
+        TypeError: Bad locus type
+    """
+    if locus is None:
+        if gdb.parameter("verbose"):
+            gdb.write("Registering global %s unwinder ...\n" % unwinder.name)
+        locus = gdb
+    elif isinstance(locus, gdb.Objfile) or isinstance(locus, gdb.Progspace):
+        if gdb.parameter("verbose"):
+            gdb.write("Registering %s unwinder for %s ...\n" %
+                      (unwinder.name, locus.filename))
+    else:
+        raise TypeError("locus should be gdb.Objfile or gdb.Progspace or None")
+
+    i = 0
+    for needle in locus.frame_unwinders:
+        if needle.name == unwinder.name:
+            if replace:
+                del locus.frame_unwinders[i]
+            else:
+                raise RuntimeError("Unwinder %s already exists." %
+                                   unwinder.name)
+        i += 1
+    locus.frame_unwinders.insert(0, unwinder)