Re: [PATCH v2 2/5] pretty: allow showing specific trailers

[Eric Sunshine <sunshine@sunshineco.com>]

On Mon, Nov 5, 2018 at 3:26 AM Anders Waldenborg <anders@0x63.nu> wrote:
> Eric Sunshine writes:
> > Should the code tolerate a trailing colon? (Genuine question; it's
> > easy to do and would be more user-friendly.)
>
> I would make sense to allow the trailing colon, it is easy enough to
> just strip that away when reading the argument.
>
> However I'm not sure how that would fit together with the possibility to
> later lifting it to a regexp, hard to strip a trailing colon from a
> regexp in a generic way.

Which is a good reason to think about these issues now, before being
set in stone.

> > What happens if 'key=...' is specified multiple times?
>
> My first thought was to simply disallow that. But that seemed hard to
> fit into current model where errors just means don't expand.
>
> I would guess that most useful and intuitive to user would be to handle
> multiple key arguments by showing any of those keys.

Agreed.

> > Thinking further on the last two points, should <val> be a regular expression?
>
> It probably would make sense. I can see how the regexp '^.*-by$' would
> be useful (but glob style matching would suffice in that case).
>
> Also handling multi-matching with an alternation group would be elegant
> %(trailers:key="(A|B)"). Except for the fact that the parser would need to
> understand some kind of quoting, which seems like an major undertaking.

Maybe, maybe not. As long as we're careful not to paint ourselves into
a corner, it might very well be okay to start with the current
implementation of matching the full key as a literal string and
(perhaps much) later introduce regex as an alternate way to specify
the key. For instance, 'key=literal' and 'key=/regex/' can co-exist,
and the extraction of the regex inside /.../ should not be especially
difficult.

> I guess having it as a regular exception would also mean that it needs
> to get some way to cache the re so it is compiled once, and not for each expansion.

Yes, that's something I brought up a few years ago during a GSoC
project; not regex specifically, but that this parsing of the format
is happening repeatedly rather than just once. I had suggested to the
GSoC student that the parsing could be done early, compiling the
format expression into a "machine" which could be applied repeatedly.
It's a larger job, of course, not necessarily something worth tackling
for your current needs.

> > If I understand correctly, this is making a copy of <val> so that it
> > will be NUL-terminated since the code added to trailer.c uses a simple
> > strcasecmp() to match it. Would it make sense to avoid the copy by
> > adding fields 'opts.filter_key' and 'opts.filter_key_len' and using
> > strncasecmp() instead? (Genuine question; not necessarily a request
> > for change.)
>
> I'm also not very happy about that copy.
> Just using strncasecmp would cause it to be prefix match, no?

Well, you could retain full key match by checking for NUL explicitly
with something like this:

    !strncasecmp(tok.buf, opts->filter_key, opts->filter_key_len) &&
        !tok.buf[opts->filter_key_len]