read-cache: touch shared index files when used

When a split-index file is created, let's update the mtime of the
shared index file that the split-index file is referencing.

In a following commit we will make shared index file expire
depending on their mtime, so updating the mtime makes sure that
the shared index file will not be deleted soon.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -1682,6 +1682,19 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	die("index file corrupt");
 }
 
+/*
+ * Signal that the shared index is used by updating its mtime.
+ *
+ * This way, shared index can be removed if they have not been used
+ * for some time.
+ */
+static void freshen_shared_index(char *base_sha1_hex, int warn)
+{
+	const char *shared_index = git_path("sharedindex.%s", base_sha1_hex);
+	if (!check_and_freshen_file(shared_index, 1) && warn)
+		warning("could not freshen shared index '%s'", shared_index);
+}
+
 int read_index_from(struct index_state *istate, const char *path)
 {
 	struct split_index *split_index;
@@ -2253,6 +2266,7 @@ static int too_many_not_shared_entries(struct index_state *istate)
 int write_locked_index(struct index_state *istate, struct lock_file *lock,
 		       unsigned flags)
 {
+	int new_shared_index, ret;
 	struct split_index *si = istate->split_index;
 
 	if (!si || alternate_index_output ||
@@ -2269,13 +2283,22 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 	}
 	if (too_many_not_shared_entries(istate))
 		istate->cache_changed |= SPLIT_INDEX_ORDERED;
-	if (istate->cache_changed & SPLIT_INDEX_ORDERED) {
-		int ret = write_shared_index(istate, lock, flags);
+
+	new_shared_index = istate->cache_changed & SPLIT_INDEX_ORDERED;
+
+	if (new_shared_index) {
+		ret = write_shared_index(istate, lock, flags);
 		if (ret)
 			return ret;
 	}
 
-	return write_split_index(istate, lock, flags);
+	ret = write_split_index(istate, lock, flags);
+
+	/* Freshen the shared index only if the split-index was written */
+	if (!ret && !new_shared_index)
+		freshen_shared_index(sha1_to_hex(si->base_sha1), 1);
+
+	return ret;
 }
 
 /*