[RFC] [PATCH] Provide the ability to write the frame unwinder in Python

[Alexander Smundak]

> How about let's meet somewhat halfway.
>
>   * We rename SnifferInfo to EphemeralFrame.
>
>   * Unwinders return UnwindInfo (better name welcome) on success or
>     None/#f otherwise
>
>     * UnwindInfo takes EphemeralFrame as constructor arg
>
>       - the EphemeralFrame must be valid
>
>       - in practice there is only ever one EphemeralFrame alive because
>         unwinding is not recursive
>
>     * UnwindInfo also takes frame ID as positional constructor args
>
>       - setting frame_id is the only thing an unwinder *must* do, so
>         this makes an invariant "if return value is an UnwindInfo, then
>         it is valid and has all necessary info"
>
>     * UnwindInfo has add_saved_register() API (see discussion with Pedro
>       here: http://article.gmane.org/gmane.comp.gdb.patches/105538)
>
>   * After accepting an UnwindInfo as an unwinder return result,
>     py-unwinders.c / scm-frame-unwinders.c marks the UnwindInfo as
>     frozen so that add_saved_register() etc can't alter the state
>
>   * continuation of unwinder call also checks that the ephemeral frame
>     on the unwindinfo is valid
>
> Example of use:
>
>    def unwind(frame):
>        if we_can_handle(frame):
>            var ret = UnwindInfo(frame, fp, pc)
>            ret.add_saved_register(r0)
>            return ret
>
> I will rework the Guile patch along these lines, and then hopefully I am
> done reworking :)

I'd like to propose one improvement on the Python side: UnwinderInfo
is constructed by a frame method instead of an implicit constructor.
I.e., frame.create_frame_with_id(sp, pc) returns UnwindInfo instance
whose ID is the result of calling GDB's frame_id_build(sp, pc),
frame.create_frame_with_id_wild(sp) returns UnwindInfo instance
whose ID is the results of calling frame_id_build_wild(sp), etc.

The example above would then look as follows:
  def unwind(frame):
    if we_can_handle(frame):
      unwind_info = frame.create_frame_with_id(sp, pc)
      unwind_info.set_previous_frame_register("r0", r0)
      unwind_info.set_previous_frame_register(...)
      return unwind_info
    else
      return None

Would this work for Guile?

As to the class of an object passed to a sniffer, how about calling it
FrameData? Note that it's not very important from the user's point of
view as sniffer code does not ever reference it by name.