[PATCH/RFC] clone: report duplicate entries on case-insensitive filesystems

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

Paths that only differ in case work fine in a case-sensitive
filesystems, but if those repos are cloned in a case-insensitive one,
you'll get problems. The first thing to notice is "git status" will
never be clean with no indication what's exactly is "dirty".

This patch helps the situation a bit by pointing out the problem at
clone time. I have not suggested any way to work around or fix this
problem. But I guess we could probably have a section in
Documentation/ dedicated to this problem and point there instead of
a long advice in this warning.

Another thing we probably should do is catch in "git checkout" too,
not just "git clone" since your linux/unix colleage colleague may
accidentally add some files that your mac/windows machine is not very
happy with. But then there's another problem, once the problem is
known, we probably should stop spamming this warning at every
checkout, but how?

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 builtin/clone.c | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/builtin/clone.c b/builtin/clone.c
index 5c439f1394..32738c2737 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -711,6 +711,33 @@ static void update_head(const struct ref *our, const struct ref *remote,
 	}
 }
 
+static void find_duplicate_icase_entries(struct index_state *istate,
+					 struct string_list *dup)
+{
+	struct string_list list = STRING_LIST_INIT_NODUP;
+	int i;
+
+	for (i = 0; i < istate->cache_nr; i++)
+		string_list_append(&list, istate->cache[i]->name);
+
+	list.cmp = fspathcmp;
+	string_list_sort(&list);
+
+	for (i = 1; i < list.nr; i++) {
+		const char *cur = list.items[i].string;
+		const char *prev = list.items[i - 1].string;
+
+		if (dup->nr &&
+		    !fspathcmp(cur, dup->items[dup->nr - 1].string)) {
+			string_list_append(dup, cur);
+		} else if (!fspathcmp(cur, prev)) {
+			string_list_append(dup, prev);
+			string_list_append(dup, cur);
+		}
+	}
+	string_list_clear(&list, 0);
+}
+
 static int checkout(int submodule_progress)
 {
 	struct object_id oid;
@@ -761,6 +788,20 @@ static int checkout(int submodule_progress)
 	if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
 		die(_("unable to write new index file"));
 
+	if (ignore_case) {
+		struct string_list dup = STRING_LIST_INIT_DUP;
+		int i;
+
+		find_duplicate_icase_entries(&the_index, &dup);
+		if (dup.nr) {
+			warning(_("the following paths in this repository only differ in case and will\n"
+				  "cause problems because you have cloned it on an case-insensitive filesytem:\n"));
+			for (i = 0; i < dup.nr; i++)
+				fprintf(stderr, "\t%s\n", dup.items[i].string);
+		}
+		string_list_clear(&dup, 0);
+	}
+
 	err |= run_hook_le(NULL, "post-checkout", sha1_to_hex(null_sha1),
 			   oid_to_hex(&oid), "1", NULL);
 
-- 
2.18.0.656.gda699b98b3