status: add a failing test showing a core.untrackedCache bug

The untracked cache gets confused when a directory is swapped out for
a file. It is easiest to reproduce this by swapping out a directory
with a symlink to another directory, and as the tests show the symlink
case is the only case we've found where "git status" will subsequently
report incorrect information, even though it's possible to otherwise
get the untracked cache into a state where its internal data
structures don't reflect reality.

In the symlink case, whatever files are inside the target of the
symlink will be incorrectly shown as untracked. This issue does not
happen if the symlink links to another file, only if it links to
another directory.

A stand-alone testcase for copying into a terminal:

    (
        rm -rf /tmp/testrepo &&
        git init /tmp/testrepo &&
        cd /tmp/testrepo &&
        mkdir x y &&
        touch x/a y/b &&
        git add x/a y/b &&
        git commit -msnap &&
        git rm -rf y &&
        ln -s x y &&
        git add y &&
        git commit -msnap2 &&
        git checkout HEAD~ &&
        git status &&
        git checkout master &&
        sleep 1 &&
        git status &&
        git status
    )

This will incorrectly show y/a as an untracked file. Both the "git
status" call right before "git checkout master" and the "sleep 1"
after the "checkout master" are needed to reproduce this, presumably
due to the untracked cache tracking on the basis of cached whole
seconds from stat(2).

When git gets into this state, a workaround to fix it is to issue a
one-off:

    git -c core.untrackedCache=false status

For the non-symlink case, the bug is that the output of
test-dump-untracked-cache should not include:

   /one/ 0000000000000000000000000000000000000000 recurse valid

It being in the output implies that cached traversal of root includes
the directory "one" which does not exist on disk anymore.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7063-status-untracked-cache.sh

@@ -22,6 +22,12 @@ avoid_racy() {
 	sleep 1
 }
 
+status_is_clean() {
+	>../status.expect &&
+	git status --porcelain >../status.actual &&
+	test_cmp ../status.expect ../status.actual
+}
+
 test_lazy_prereq UNTRACKED_CACHE '
 	{ git update-index --test-untracked-cache; ret=$?; } &&
 	test $ret -ne 1
@@ -683,4 +689,85 @@ test_expect_success 'untracked cache survives a commit' '
 	test_cmp ../before ../after
 '
 
+test_expect_success 'teardown worktree' '
+	cd ..
+'
+
+test_expect_success SYMLINKS 'setup worktree for symlink test' '
+	git init worktree-symlink &&
+	cd worktree-symlink &&
+	git config core.untrackedCache true &&
+	mkdir one two &&
+	touch one/file two/file &&
+	git add one/file two/file &&
+	git commit -m"first commit" &&
+	git rm -rf one &&
+	ln -s two one &&
+	git add one &&
+	git commit -m"second commit"
+'
+
+test_expect_failure SYMLINKS '"status" after symlink replacement should be clean with UC=true' '
+	git checkout HEAD~ &&
+	status_is_clean &&
+	status_is_clean &&
+	git checkout master &&
+	avoid_racy &&
+	status_is_clean &&
+	status_is_clean
+'
+
+test_expect_success SYMLINKS '"status" after symlink replacement should be clean with UC=false' '
+	git config core.untrackedCache false &&
+	git checkout HEAD~ &&
+	status_is_clean &&
+	status_is_clean &&
+	git checkout master &&
+	avoid_racy &&
+	status_is_clean &&
+	status_is_clean
+'
+
+test_expect_success 'setup worktree for non-symlink test' '
+	git init worktree-non-symlink &&
+	cd worktree-non-symlink &&
+	git config core.untrackedCache true &&
+	mkdir one two &&
+	touch one/file two/file &&
+	git add one/file two/file &&
+	git commit -m"first commit" &&
+	git rm -rf one &&
+	cp two/file one &&
+	git add one &&
+	git commit -m"second commit"
+'
+
+test_expect_failure '"status" after file replacement should be clean with UC=true' '
+	git checkout HEAD~ &&
+	status_is_clean &&
+	status_is_clean &&
+	git checkout master &&
+	avoid_racy &&
+	status_is_clean &&
+	test-dump-untracked-cache >../actual &&
+	grep -F "recurse valid" ../actual >../actual.grep &&
+	cat >../expect.grep <<EOF &&
+/ 0000000000000000000000000000000000000000 recurse valid
+/two/ 0000000000000000000000000000000000000000 recurse valid
+EOF
+	status_is_clean &&
+	test_cmp ../expect.grep ../actual.grep
+'
+
+test_expect_success '"status" after file replacement should be clean with UC=false' '
+	git config core.untrackedCache false &&
+	git checkout HEAD~ &&
+	status_is_clean &&
+	status_is_clean &&
+	git checkout master &&
+	avoid_racy &&
+	status_is_clean &&
+	status_is_clean
+'
+
 test_done