clone: report duplicate entries on case-insensitive filesystems

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 exactly is "dirty".

This patch helps the situation a bit by pointing out the problem at
clone time. Even though this patch talks about case sensitivity, the
patch makes no assumption about folding rules by the filesystem. It
simply observes that if an entry has been already checked out at clone
time when we're about to write a new path, some folding rules are
behind this.

In the case that we can't rely on filesystem (via inode number) to do
this check, fall back to fspathcmp() which is not perfect but should
not give false positives.

This patch is tested with vim-colorschemes and Sublime-Gitignore
repositories on a JFS partition with case insensitive support on
Linux.

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

builtin/clone.c

@@ -747,6 +747,7 @@ static int checkout(int submodule_progress)
 	memset(&opts, 0, sizeof opts);
 	opts.update = 1;
 	opts.merge = 1;
+	opts.clone = 1;
 	opts.fn = oneway_merge;
 	opts.verbose_update = (option_verbosity >= 0);
 	opts.src_index = &the_index;

cache.h

@@ -1455,6 +1455,7 @@ struct checkout {
 	unsigned force:1,
 		 quiet:1,
 		 not_new:1,
+		 clone:1,
 		 refresh_cache:1;
 };
 #define CHECKOUT_INIT { NULL, "" }

entry.c

@@ -399,6 +399,34 @@ static int check_path(const char *path, int len, struct stat *st, int skiplen)
 	return lstat(path, st);
 }
+static void mark_colliding_entries(const struct checkout *state,
+				   struct cache_entry *ce, struct stat *st)
+{
+	int i, trust_ino = check_stat;
+
+#if defined(GIT_WINDOWS_NATIVE)
+	trust_ino = 0;
+#endif
+
+	ce->ce_flags |= CE_MATCHED;
+
+	for (i = 0; i < state->istate->cache_nr; i++) {
+		struct cache_entry *dup = state->istate->cache[i];
+
+		if (dup == ce)
+			break;
+
+		if (dup->ce_flags & (CE_MATCHED | CE_VALID | CE_SKIP_WORKTREE))
+			continue;
+
+		if ((trust_ino && dup->ce_stat_data.sd_ino == st->st_ino) ||
+		    (!trust_ino && !fspathcmp(ce->name, dup->name))) {
+			dup->ce_flags |= CE_MATCHED;
+			break;
+		}
+	}
+}
+
 /*
  * Write the contents from ce out to the working tree.
  *
@@ -455,6 +483,9 @@ int checkout_entry(struct cache_entry *ce,
 			return -1;
 		}
+		if (state->clone)
+			mark_colliding_entries(state, ce, &st);
+
 		/*
 		 * We unlink the old file, to get the new one with the
 		 * right permissions (including umask, which is nasty

t/t5601-clone.sh

@@ -624,10 +624,16 @@ test_expect_success 'clone on case-insensitive fs' '
 			git hash-object -w -t tree --stdin) &&
 		c=$(git commit-tree -m bogus $t) &&
 		git update-ref refs/heads/bogus $c &&
-		git clone -b bogus . bogus
+		git clone -b bogus . bogus 2>warning
 	)
 '
+test_expect_success !MINGW,!CYGWIN,CASE_INSENSITIVE_FS 'colliding file detection' '
+	grep X icasefs/warning &&
+	grep x icasefs/warning &&
+	test_i18ngrep "the following paths have collided" icasefs/warning
+'
+
 partial_clone () {
 	       SERVER="$1" &&
 	       URL="$2" &&

unpack-trees.c

@@ -345,6 +345,46 @@ static struct progress *get_progress(struct unpack_trees_options *o)
 	return start_delayed_progress(_("Checking out files"), total);
 }
+static void setup_collided_checkout_detection(struct checkout *state,
+					      struct index_state *index)
+{
+	int i;
+
+	state->clone = 1;
+	for (i = 0; i < index->cache_nr; i++)
+		index->cache[i]->ce_flags &= ~CE_MATCHED;
+}
+
+static void report_collided_checkout(struct index_state *index)
+{
+	struct string_list list = STRING_LIST_INIT_NODUP;
+	int i;
+
+	for (i = 0; i < index->cache_nr; i++) {
+		struct cache_entry *ce = index->cache[i];
+
+		if (!(ce->ce_flags & CE_MATCHED))
+			continue;
+
+		string_list_append(&list, ce->name);
+		ce->ce_flags &= ~CE_MATCHED;
+	}
+
+	list.cmp = fspathcmp;
+	string_list_sort(&list);
+
+	if (list.nr) {
+		warning(_("the following paths have collided (e.g. case-sensitive paths\n"
+			  "on a case-insensitive filesystem) and only one from the same\n"
+			  "colliding group is in the working tree:\n"));
+
+		for (i = 0; i < list.nr; i++)
+			fprintf(stderr, "  '%s'\n", list.items[i].string);
+	}
+
+	string_list_clear(&list, 0);
+}
+
 static int check_updates(struct unpack_trees_options *o)
 {
 	unsigned cnt = 0;
@@ -359,6 +399,9 @@ static int check_updates(struct unpack_trees_options *o)
 	state.refresh_cache = 1;
 	state.istate = index;
+	if (o->clone)
+		setup_collided_checkout_detection(&state, index);
+
 	progress = get_progress(o);
 	if (o->update)
@@ -423,6 +466,10 @@ static int check_updates(struct unpack_trees_options *o)
 	errs |= finish_delayed_checkout(&state);
 	if (o->update)
 		git_attr_set_direction(GIT_ATTR_CHECKIN, NULL);
+
+	if (o->clone)
+		report_collided_checkout(index);
+
 	return errs != 0;
 }

unpack-trees.h

@@ -42,6 +42,7 @@ struct unpack_trees_options {
 	unsigned int reset,
 		     merge,
 		     update,
+		     clone,
 		     index_only,
 		     nontrivial_merge,
 		     trivial_merges_only,