Re: [PATCH] ls-files: add pathspec matching for submodules

From: Junio C Hamano <gitster@pobox.com>
Date: Sep 19, 2016, 17:00 UTC
Message-ID: <xmqqvaxrg6zt.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <CAKoko1r6cfv-2HVCJPgGbXyCVe-wdUBS+2nXtaTHO3jshVg8MA@mail.gmail.com>

Brandon Williams <bmwill@google.com> writes:

Show 13 quoted lines

>> OK, so as discussed previously with Heiko and Stefan, the idea is to
>>
>>  - pass the original pathspec as-is,
>>
>>  - when --submodule-prefix is given, a path discovered in a
>>    submodule repository is first prefixed with that string before
>>    getting checked to see if it matches the original pathspec.
>>
>> And this loop is about relaying the original pathspec.
>
> Exactly.  Perhaps I should have made this more clear either with a
> detailed comment or
> more information in the commit msg.

I think you were clear enough.

Don't read everything other people say in their reviews as pointing out issues. Often trying to rephrase what they read in the code in their own words is a good way to make sure the reviewers and the original author are on the same page. The above was one of these cases.

Show 20 quoted lines

>>> +     if (prefix > 0) {
>>> +             if (ps_strncmp(item, pattern, string, prefix))
>>> +                     return WM_NOMATCH;
>>
>> This says: when we have a set prefix that must literally match, and
>> that part does not match what we have, it cannot possibly match.
>>
>> Is that correct?  What do we have in "name" and "item" at this
>> point?  We disable the common-prefix optimization, so we do not have
>> to worry about a pathspec with two elements "sub/dir1/*" and "sub/dir2/*"
>> giving you "sub/dir" as the common prefix, when you are wondering if
>> it is worth descending into "sub/" without knowing what it contains.
>> Is that what guarantees why this part is correct?
>
> I adopted this structure from another part of the code.  The caller
> uses a field in
> the pathspec item which indicates the location of the first wildcard character.
> So the prefix (everything prior to the wildcard char) must match
> literally before
> we drop into a more expensive wildmatch function.

"Another part of the code" is about tree walking, right? Weren't you saying that part of the code may be buggy or something earlier (e.g. pathspec "su?/" vs entry "sub")?

Again, what do we have in "name" and "item" at this point? If we have a submodule at "sub/" and we are checking a pathspec element "sub/dir1/*", what is the non-wildcard part of the pathspec and what is the "string"? Aren't then "sub/dir1/" and "sub/" respectively, which would not pass ps_strncmp() and produce a (false) negative?

I am starting to have a feeling that the best we can do in this function safely is to see if prefix (i.e. the constant part of the pathspec before the first wildcard) is long enough to cover the "name" and if "name" part does not match to the directory boundary, e.g. for this combination

	pathspec = "a/b/sib/c/*"
        name = "a/b/sub/"

we can say with confidence that it is not worth descending into.

When prefix is long enough and "name" and leading part of the prefix matches to the directory boundary, e.g.

	pathspec = "a/b/sub/c/*"
        name = "a/b/sub/"

we can say it is worth descending into.

If these two checks cannot decide, we may have to be pessimistic and say "it may match; we don't know until we descend into it". When prefix is shorter than name, I am not sure if we can devise a set of simple rules, e.g.

	pathspec = "a/**/c/*"
        name = "a/b/sub/"

may match with its ** "b/sub" part and worth descending into, so is

	pathspec = "a/b/*/c/*"
        name = "a/b/sub/"

but not this one:

	pathspec = "a/b/su[c-z]/c/*"
        name = "a/b/sub/"

but this is OK:

	pathspec = "a/b/su[a-z]/c/*"
        name = "a/b/sub/"

So I would think we'd be in the business of counting slashes in the name (called "string" in this function) and the pathspec, while noticing '*' and '**' in the latter, and we may be able to be more precise, but I am not sure how complex the end result would become.