Re: [PATCH] attr: do not mark queried macros as unset

[Duy Nguyen <pclouds@gmail.com>]

On Sat, Jan 19, 2019 at 4:35 AM Jeff King <peff@peff.net> wrote:
>
> On Fri, Jan 18, 2019 at 11:58:01AM -0500, Jeff King wrote:
>
> > Now, on to the actual bug. The simplest reproduction is:
> >
> >   (echo "[attr]foo bar"; echo "* foo") >.gitattributes
> >   git check-attr foo file
>
> Actually, even simpler is to just "binary", which is pre-defined as a
> macro. :)
>
> > which should report "foo" as set. This bisects to 60a12722ac (attr:
> > remove maybe-real, maybe-macro from git_attr, 2017-01-27), and it seems
> > like an unintentional regression there. I haven't yet poked into that
> > commit to see what the fix will look like.
>
> So here's the fix I came up with. +cc Duy, as this is really tangled
> with his older 06a604e670.
>
> -- >8 --
> Subject: [PATCH] attr: do not mark queried macros as unset
>
> Since 60a12722ac (attr: remove maybe-real, maybe-macro from git_attr,
> 2017-01-27), we will always mark an attribute macro (e.g., "binary")
> that is specifically queried for as "unspecified", even though listing
> _all_ attributes would display it at set. E.g.:
>
>   $ echo "* binary" >.gitattributes
>
>   $ git check-attr -a file
>   file: binary: set
>   file: diff: unset
>   file: merge: unset
>   file: text: unset
>
>   $ git check-attr binary file
>   file: binary: unspecified
>
> The problem stems from an incorrect conversion of the optimization from
> 06a604e670 (attr: avoid heavy work when we know the specified attr is
> not defined, 2014-12-28). There we tried in collect_some_attrs() to
> avoid even looking at the attr_stack when the user has asked for "foo"
> and we know that "foo" did not ever appear in any .gitattributes file.
>
> It used a flag "maybe_real" in each attribute struct, where "real" meant
> that the attribute appeared in an actual file (we have to make this
> distinction because we also create an attribute struct for any names
> that are being queried). But as explained in that commit message, the
> meaning of "real" was tangled with some special cases around macros.
>
> When 06a604e670 later refactored the macro code, it dropped maybe_real
> entirely. This missed the fact that "maybe_real" could be unset for two
> reasons: because of a macro, or because it was never found during
> parsing. This had two results:
>
>   - the optimization in collect_some_attrs() ceased doing anything
>     meaningful, since it no longer kept track of "was it found during
>     parsing"
>
>   - worse, it actually kicked in when the caller _did_ ask about a macro
>     by name, causing us to mark it as unspecified
>
> It should be possible to salvage this optimization, but let's start with
> just removing the remnants. It hasn't been doing anything (except
> creating bugs) since 60a12722ac, and nobody seems to have noticed the
> performance regression. It's more important to fix the correctness
> problem clearly first.

But muh optimization!!! You're right of course, correctness comes
first. I did try to look at this code but it's been a while and I'm
afraid I don't have anything valuable to say. I'll dig in more in the
next couple days.
-- 
Duy