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

[Andy Wingo <wingo@igalia.com>]

On Mon 16 Mar 2015 18:25, Alexander Smundak <asmundak@google.com> writes:

> 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

Looks great to me :)  Thank you for the consideration!

I might consider naming "set_previous_frame_register" as
"add_saved_register", in anticipation of a possible
add_unavailable_register(), add_unmodified_register(), etc, but that is
just a minor nit.

> 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.

It's true that from user code it barely matters to Python, but Scheme's
monomorphic flavor makes these things more apparent:

  (frame-data-read-register frame "r0")

This doesn't read so well to me -- is it "read-register" on a
"frame-data", or is it "data-read-register" on a "frame" ?  A weak point
but "ephemeral-frame-read-register" avoids the question.

I also think that "ephemeral frame" is easier to document as a concept
than the more generic "frame data", and corresponds better to what is
happening in GDB.  YMMV, though.

As an aside, it seems to me that if we can avoid it, the word "sniffer"
should not enter the documentation or the API.  The Python and Guile
APIs don't just sniff, they do the whole of the unwinding operation, so
it's more clear to call them "unwinders".

Andy