Here is the updated revision of the patch, synced with upstream and with new API
(the input to a sniffer is still named SnifferInfo, and the output is
named UnwindInfo).

gdb/ChangeLog:
2015-02-30  Sasha Smundak  <asmundak@google.com>


        * 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 sniffers.py.
        * doc/python.texi (Writing a Frame Unwinder in Python): Add
        section.
        * python/lib/gdb/__init__.py (packages): Add frame_sniffers list.
        * python/lib/gdb/command/sniffers.py: New file, implements GDB
        commands to list/enable/disable Python sniffers.
        * python/lib/gdb/function/sniffers.py: New file, implements
        execute_sniffers function.
        * python/lib/gdb/sniffer.py: New file, contains Sniffer class and
        register_sniffer function.
        * python/py-objfile.c (objfile_object): Add frame_sniffers field.
        (objfpy_dealloc): Decrement frame_sniffers reference count.
        (objfpy_initialize): Create frame_sniffers list.
        (objfpy_get_frame_sniffers): Implement Objfile.frame_sniffers
        getter.
        (objfpy_set_frame_sniffers): Implement Objfile.frame_sniffers
        setter.
        (objfile_getset): Add frame_sniffers attribute to Objfile.
        * python/py-progspace.c (pspace_object): Add frame_sniffers field.
        (pspy_dealloc): Decrement frame_sniffers reference count.
        (pspy_initialize): Create frame_sniffers list.
        (pspy_get_frame_sniffers): Implement gdb.Progspace.frame_sniffers
        getter.
        (pspy_set_frame_sniffers): Implement gdb.Progspace.frame_sniffers
        setter.
        (pspy_getset): Add frame_sniffers attribute to gdb.Progspace.
        * python/py-unwind.c: New file, implements Python frame sniffers
        interface.
        * python/python-internal.h (pspy_get_name_sniffers): New prototype.
        (objpy_get_frame_sniffers): New prototype.
        (gdbpy_initialize_unwind): New prototype.
        * python/python.c (gdbpy_apply_type_printers): Call
        gdbpy_initialize_unwind.

gdb/testsuite/ChangeLog:
2014-02-30  Sasha Smundak  <asmundak@google.com>

        * gdb.python/py-unwind-maint.c: Test program for py-unwind-maint.
        * gdb.python/py-unwind-maint.exp: Tests sniffer-related GDB
        commands.
        * gdb.python/py-unwind-maint.py: Pythons sniffers for the test.
        * gdb.python/py-unwind.c: Test program for the py-unwind test.
        * gdb.python/py-unwind.exp: Python frame sniffers test.
        * gdb.python/py-unwind.py: Frame sniffer in Python tested by
        py-unwind test.