Subject: Re: [PATCH 07/13] object-filter: common declarations for object filtering

From: Jonathan Tan <[email-masked]>
Date: Wed, 27 Sep 2017 17:05:33 -0700

On Wed, 27 Sep 2017 13:09:42 -0400
Jeff Hostetler <[email-masked]> wrote:

> On 9/26/2017 6:39 PM, Jonathan Tan wrote:
> > On Fri, 22 Sep 2017 20:30:11 +0000
> > Jeff Hostetler <[email-masked]> wrote:
> > 
> >>   Makefile        |   1 +
> >>   object-filter.c | 269 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >>   object-filter.h | 173 ++++++++++++++++++++++++++++++++++++
> >>   3 files changed, 443 insertions(+)
> >>   create mode 100644 object-filter.c
> >>   create mode 100644 object-filter.h
> > 
> > I think these and list-objects-filter-* are multiple levels of
> > indirection too many. Would a single file with a few implementations of
> > filter_object_fn be sufficient?
> 
> I did that in my first draft and I found it confusing.
> 
> Each filter has 3 parts (some filter-specific data structures,
> a filter callback routine, a driver to call the traverse code).
> I found it easier to reason about each filter in isolation.
> And it makes it easier to work on each independently and keep
> their inclusion in separate commits.

I looked at object-filter.{c,h} a bit more. It seems that these files:
 1) define a struct that contains all the options that we would want
 2) supplies a way to populate this struct from code that uses parse-options
 3) supplies a way to populate this struct from code that calculates
    options by hand
 4) supplies a way to populate this struct from "protocol" ("<key>" or
    "<key> <value>" strings)

And the next commit takes the struct that object-filter.{c,h} produces
and actually performs the traversal.

I think this can be significantly simplified, though. Would this work:
 a) Define the object_filter_options struct, but make all fields
    correspond to a single parameter each. Define
    OBJECT_FILTER_OPTIONS_INIT to initialize everything to 0 except for
    large_byte_limit to ULONG_MAX (so that we can detect if something
    else is set to it).
 b) Define one single OPT_PARSE_FILTER macro containing all the
    parameters. We can use the non-callback macros here. That solves 2)
    above.
 c) Define a function that takes in (int *argc, char ***argv) that can
    "massage" it to remove all filter-related arguments, storing them in
    a object_filter_options struct. That solves 3) above. As discussed
    in the API documentation, this means that argument lists of the form
    "--unknown --known" (where "--unknown" takes an argument) are
    processed differently, but then again, rev-list never supported them
    anyway (it required "--unknown=<arg>").
 d) Define a function that converts "<key>" into "--<key>" and "<key>
    <value>" into "--<key>=<value>", and use the existing mechanism.
    That solves 4) above.

This removes the need to maintain the lists of one-per-argument
functions, including the parse_filter_* and opt_parse_filter_* functions
declared in the header file. If we were to add a feature, we wouldn't
need to change anything in the caller, and wouldn't need to hand-edit
object_filter_hand_parse_arg() and object_filter_hand_parse_protocol().