2 files changed, 20 insertions, 1 deletions
diff --git a/core/module-list.c b/core/module-list.c
index 0d53c60..3b9ca38 100644
--- a/core/module-list.c
+++ b/core/module-list.c
@@ -1147,7 +1147,8 @@ bool pure_list_lookup(const PureList *list, unsigned key_field, PureCell key, un
 size_t pure_list_count(const PureList *list) { return list->count; }
 
 PureList *pure_list_share(PureList *list, PureRetired *retired, uint64_t epoch) {
-    if (!make_tree(list)) return NULL; /* a copy reads a tree */
+    /* A copy reads a tree; an empty list (an event's, between events) stays as it is. */
+    if (list->count && !make_tree(list)) return NULL;
     PureList *copy = malloc(sizeof(*copy));
     if (!copy) return NULL;
     *copy = (PureList){.layout = list->layout, .root = list->root, .count = list->count, .bytes = list->bytes,
diff --git a/core/test-module.c b/core/test-module.c
index f76c871..a509349 100644
--- a/core/test-module.c
+++ b/core/test-module.c
@@ -1483,6 +1483,24 @@ static void arrays_equivalence(void) {
         pure_list_free(t);
     }
     CHECK(forms[0] > 10000 && forms[1] > 10000 && forms[2] > 10000 && conversions > 100);
+    /* A snapshot shares every list: an event's, empty between events, stays in arrays; one with items is a tree for its copy. */
+    {
+        PureList *list = pure_list_new(2, fields);
+        PureRetired retired = {0};
+        CHECK(list && pure_list_arrays(list));
+        PureList *copy = pure_list_share(list, &retired, 1);
+        CHECK(copy && pure_list_count(copy) == 0 && pure_list_form(list) == 1);
+        pure_list_free(copy);
+        for (unsigned i = 0; i < 100; ++i)
+            CHECK(pure_list_insert(list, i, (PureCell[]){{.num = i}, {.size = 0}}, NULL) == PURE_OK);
+        CHECK((copy = pure_list_share(list, &retired, 2)) && pure_list_form(list) == 0 && pure_list_count(copy) == 100);
+        CHECK(pure_list_get(copy, 99, 0).num == 99);
+        pure_list_free(copy);
+        pure_list_clear(list);
+        CHECK(pure_list_form(list) == 1); /* arrays again, the next event */
+        pure_list_free(list);
+        pure_retired_free(&retired);
+    }
     printf("{\"arrays\":{\"steps\":%zu,\"tree\":%zu,\"flat\":%zu,\"dense\":%zu,\"conversions\":%zu}}\n", steps, forms[0], forms[1],
            forms[2], conversions);
 }