Re: gitignore + commit with excludes = bug

[Matheus Tavares <matheus.bernardino@usp.br>]

Hi, Paul and Philip

On Tue, May 4, 2021 at 9:55 AM Philip Oakley <philipoakley@iee.email> wrote:
>
> Hi Paul,
>
> On 01/05/2021 19:37, Paul Jackson wrote:
> > Hello,
> >
> > I stumbled upon what I believe is a bug in git.
> > See the following reproduction steps:
> >
> > mkdir test
> > cd test
> > git init
> > echo 1 > ignored
> > echo 1 > not-ignored
> > echo ignored > .gitignore
> > git add -A -- ':!ignored' || echo 'ERROR!!!'
> >
> > In these steps, I ignore the "ignored" file twice - first time in
> > .gitignore, and second time in the "git add" command. I didn't expect
> > this to be a problem, but I'm getting the following error message:
> >
> > The following paths are ignored by one of your .gitignore files:
> > ignored
> >
> > It looks as if git thinks I wanted to include, not exclude "ignored"
> > in "git add".
> I was intrigued by this. The man pages can be hard to decipher, and it
> may be an 'as designed' feature, but still not intuitive
>
> It took a while to find the relevant parts of the man pages.
>
> The `-A` option of `add` is under
> https://git-scm.com/docs/git-add#Documentation/git-add.txt---no-ignore-removal
> which has caveats for whether a pathspec is given.
>
> The `exclude` magic pathspec is under
> https://git-scm.com/docs/gitglossary#Documentation/gitglossary.txt-exclude
> and again has caveats and a double negative regarding whether the
> `exclude` pathspec counts as a path spec.
>
> I _think_ that it is saying that the `exclude` pathspec is ignored for
> the purpose of the `-A` (all) condition for git add.

Hmm, I think the issue is not really related to `-A`. In fact, if we
reproduce Paul's original example without `-A`, we still get the
warning.

The problem seems to be at `dir.c:exclude_matches_pathspec()`, which
creates the list of ignored files that is later used by `git add` to
presented the "The following paths are ignored..." warning.

This function ignores the `exclude` magic, so a path 'x' incorrectly
matches both ':x' and ':!x'. And thus, we end up warning the user about
'x' being ignored even when the user had ran `git add ':!x'`.

I think something like this, might solve the problem:

diff --git a/dir.c b/dir.c
index 3474e67e8f..165ca6a543 100644
--- a/dir.c
+++ b/dir.c
@@ -2042,6 +2042,25 @@ static int exclude_matches_pathspec(const char *path, int pathlen,
 		const struct pathspec_item *item = &pathspec->items[i];
 		int len = item->nowildcard_len;
 
+		if (!(item->magic & PATHSPEC_EXCLUDE))
+			continue;
+
+		if (len == pathlen &&
+		    !ps_strncmp(item, item->match, path, pathlen))
+			return 0;
+		if (len > pathlen &&
+		    item->match[pathlen] == '/' &&
+		    !ps_strncmp(item, item->match, path, pathlen))
+			return 0;
+	}
+
+	for (i = 0; i < pathspec->nr; i++) {
+		const struct pathspec_item *item = &pathspec->items[i];
+		int len = item->nowildcard_len;
+
+		if (item->magic & PATHSPEC_EXCLUDE)
+			continue;
+
 		if (len == pathlen &&
 		    !ps_strncmp(item, item->match, path, pathlen))
 			return 1;
---

I had to split the original loop into two and handle the `exclude`
pathspecs first because we cannot let the original loop return early
when one of the `non-exclude` pathspecs matches the path. Otherwise, we
would still incorrectly warn the user on executions like
`git add ignored ':!ignored'`.

(We might also want to extract the matching part to its own function to
avoid repeating this code on the two loops.)