Re: [PROPOSAL] .gitignore syntax modification

From: Kevin Ballard <kevin@sb.org>
Date: Oct 15, 2010, 11:01 UTC
Message-ID: <3F6772ED-7BCA-4ED0-B089-0E3C4CBDF015@sb.org>
In-Reply-To: <20101013121552.GA29486@do>

On Oct 13, 2010, at 5:15 AM, Nguyen Thai Ngoc Duy wrote:

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> On Tue, Oct 12, 2010 at 07:32:54PM -0700, Kevin Ballard wrote:
>> As soon as I find the time, I'll be working on a patch for this. I
>> only wrote up the proposal because I want to make sure that what I
>> end up implementing is actually something that will be accepted. At
>> this point I'm actually in favor of simply assuming all paths that
>> don't start with / can be matched at any level but I recognize that
>> this is a change to existing functionality, though I personally
>> think that all patterns that are meant to match only at the current
>> level should be prefixed with / anyway. Would such a change to
>> existing behavior be rejected out-of-hand?
> 
> Yes, patterns that only match current level should be prefixed with a
> slash. There are also other cases apart from "current level only" and
> "any level": foo/a* will match only second level, not any level.

Sure, "foo/a*" will match second level only, but that's identical to "/foo/a*". I would have preferred the semantics of .gitignore to behave as if all patterns are prefixed with "**/" unless the pattern starts with "/", but that ship sailed a long time ago and I agree that it would be a bad idea to introduce that behavior now.

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> I was thinking of doing like this. It's not complete (not even build)
> but it shows the idea. I don't think this way it will change existing
> behaviors. Performance is something I haven't thought through.
> 
> Anyway, what do you think? I'm afraid I don't have time to work on
> this. The pathspec unification work still needs to be done.

Got around to glancing at your patch. Looks pretty good, and it does build if you simply define EXC_FLAG_STARSTAR, though there are a few changes that are definitely necessary (a path of "*" will cause this to run off the end of the string while trying to detect "**/"). I'll have some more time next week to take a much closer look though. As for performance, I'm not particularly worried. The only performance change is if EXC_FLAG_STARSTAR is checked, in the worst-case it'll try to apply the pattern once per level of directory nesting. As this is just string twiddling, it's bound to be pretty fast, and I don't think there's any viable alternative to doing this kind of loop anyway. That said, I'd still like to support putting **/ anywhere in the pattern instead of just at the beginning, and 
 possibly even support ** (without the trailing /).

If we do support ** by itself, I wonder if we should special-case having ** as the last path component of the pattern. The possible behavior change we could have is making this only match files and not directories. The use-case here is putting something like "foo/**" in the top-level .gitignore and then a few levels into foo we could put another .gitignore with an inverse pattern in order to un-ignore some deep file (or just "!foo/*/*/bar.c" inside that top-level .gitignore as well). The only way I can think of to achieve this behavior with the current gitignore is something along the lines of

foo/* !foo/bar/ foo/bar/* !foo/bar/baz/ foo/bar/baz/* !foo/bar/baz/bar.c

And even this will only work if you know all the intermediate directories. I cannot think of any way at all right now to ignore everything in a single directory except for one file at least 1 level of nesting deeper if you don't know the names of the intermediate directories. With the proposed special-case we can say

foo/** !foo/*/*/bar.c

and it will behave exactly as specified.

It occurs to me that we could actually tweak this slightly, to say that if a ** is encountered and there are zero slashes in the pattern after it, then it will only match files (with zero or more leading directories). This way you can have a pattern "foo/**.d" which only ignores files with the extension ".d" but will still avoid ignoring directories that end in ".d".

This turned out a bit longer than intended, and slightly more rambling as well, and I apologize for that. I will revisit this again next week.

-Kevin Ballard