[PATCH 1/2] dir.c: fix bug in 'nd/exclusion-regression-fix' topic

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

The topic in question introduces "sticky" path list for this purpose:
given a path 'abc/def', if 'abc' already matches a pattern X, it's added
to X's sticky path list. When we need to check if 'abc/def' matches
pattern X and see that 'abc' is already in the list, we conclude right
away that 'abc/def' also matches X.

The short reason for sticky path list is to workaround limitations of
matching code (*) that will return "not match" when we compare 'abc/def'
and pattern X.

The bug is in this code. Not only it does "when we need to check if
'abc/def' matches...", it does an extra thing: if 'foo/bar' is _not_ in
the list, return 'not matched' by bypassing all matching code with the
"continue;" statement. It should let the remaining code decide match
status instead.

This bug affects both .gitignore and sparse checkout, but it's reported
as a sparse checkout bug, so let's examine how it happens. The
sparse-checkout pattern has two rules

    /*
    !one/hideme

and the worktree has three tracked files, one/hideme, one/showme and
two/showme. What happens is this

* check "one", it matches the first pattern -> positive -> keep
  examining.

*1* "thanks" to 'nd/exclusion-regression-fix' we detect this pair of
  patterns, so we put "one" in the sticky list of pattern "/*"

* enter "one", check "one/hideme", it matches the second pattern
  first (we search from bottom up) -> negative -> excluded

* check "one/showme", it does not match the second pattern.

*2* We then check it against the first pattern and notice the sticky list
  that includes "one", so we decide right away that "one/showme" is
  included.

* leave "one", check "two" which does not match the second pattern.

*3* then we check "two" against the first pattern and notice that this
   pattern has a non-empty sticky list, which contains "one", not "two".
   This bug kicks in and bypasses the true matching logic for pattern
   "/*". As a result, we exclude "two/showme".

One may notice that the order of these steps matter. If *3* occurs
before *1*, then the sticky list at that moment is empty and the bug
does not kick in. Sparse checkout always examines entries in
alphabetical order, so "abc/showme" would be examined before "one" and
not hit this bug!

The last remark is important when we move to .gitignore. We receive the
list of entries with readdir() and cannot control the order of
entries. Which means we can't write a test for .gitignore that will
reliably fail without this fix. Which is why this patch only adds a test
for sparse checkout, even though the same above steps happen to
.gitignore.

(*) which will be fixed later and described in detail then. For this
commit, it's sufficient to see the following link because the
explanation is long:

http://article.gmane.org/gmane.comp.version-control.git/288479

Reported-by: Durham Goode <durham@fb.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 dir.c                                |  1 -
 t/t1011-read-tree-sparse-checkout.sh | 20 ++++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/dir.c b/dir.c
index 69e0be6..77f38a5 100644
--- a/dir.c
+++ b/dir.c
@@ -1027,7 +1027,6 @@ static struct exclude *last_exclude_matching_from_list(const char *pathname,
 				exc = x;
 				break;
 			}
-			continue;
 		}
 
 		if (x->flags & EXC_FLAG_MUSTBEDIR) {
diff --git a/t/t1011-read-tree-sparse-checkout.sh b/t/t1011-read-tree-sparse-checkout.sh
index 0c74bee..ecc5e93 100755
--- a/t/t1011-read-tree-sparse-checkout.sh
+++ b/t/t1011-read-tree-sparse-checkout.sh
@@ -274,4 +274,24 @@ test_expect_success 'checkout with --ignore-skip-worktree-bits' '
 	git diff --exit-code HEAD
 '
 
+test_expect_success 'sparse checkout and dir.c sticky bits' '
+	git init sticky &&
+	(
+		cd sticky &&
+		mkdir one two &&
+		touch one/hideme one/showme two/showme &&
+		git add . &&
+		git commit -m initial &&
+		cat >.git/info/sparse-checkout <<-\EOF &&
+		/*
+		!one/hideme
+		EOF
+		git config core.sparsecheckout true &&
+		git checkout &&
+		test_path_is_missing one/hideme &&
+		test_path_is_file    one/showme &&
+		test_path_is_file    two/showme
+	)
+'
+
 test_done
-- 
2.8.0.rc0.210.gd302cd2