3 files changed, 32 insertions, 3 deletions

diff --git a/src/ot_accesslist.c b/src/ot_accesslist.c
index 0597e9a..487597e 100644
--- a/src/ot_accesslist.c
+++ b/src/ot_accesslist.c
@@ -353,11 +353,17 @@ static void     *accesslist_adddel_worker(const char *fifoname, ot_accesslist *_
         /* Only if the info hash is not in the adding_to list, create a new head with that entry */
         if (!exactmatch) {
           ot_accesslist *accesslist_new = accesslist_make(*adding_to, (*adding_to)->size + 1);
-          ptrdiff_t      off            = insert_point - (*adding_to)->list;
           if (accesslist_new) {
-            memcpy(accesslist_new->list, (*adding_to)->list, sizeof(ot_hash) * off);
+            ptrdiff_t off   = (*adding_to)->size ? insert_point - (*adding_to)->list : 0;
+            size_t    after = (*adding_to)->size - off;
+
+            /* A reload leaves an empty list with no array at all, and memcpy
+               may not be handed a null pointer even for nothing */
+            if (off)
+              memcpy(accesslist_new->list, (*adding_to)->list, sizeof(ot_hash) * off);
             memcpy(accesslist_new->list + off, info_hash, sizeof(info_hash));
-            memcpy(accesslist_new->list + off + 1, (*adding_to)->list + off, sizeof(ot_hash) * ((*adding_to)->size - off));
+            if (after)
+              memcpy(accesslist_new->list + off + 1, (*adding_to)->list + off, sizeof(ot_hash) * after);
             (*adding_to)->base = time_now_minutes();
             *adding_to = accesslist_new;
           }
diff --git a/tests/integration/harness.py b/tests/integration/harness.py
index e5ec621..f3cd0fa 100644
--- a/tests/integration/harness.py
+++ b/tests/integration/harness.py
@@ -135,6 +135,11 @@ class Tracker:
         self.log.seek(0)
         return self.log.read().decode("utf-8", "replace")
 
+    def reload(self):
+        """SIGHUP makes the accesslist reader re-read its files, which also
+           empties the dynamic add/delete changesets."""
+        os.kill(self.process.pid, signal.SIGHUP)
+
     def stop(self):
         """Returns anything the tracker complained about on the way out.
 
diff --git a/tests/integration/test_dynamic_accesslist.py b/tests/integration/test_dynamic_accesslist.py
index 7fa5fde..2072f5e 100644
--- a/tests/integration/test_dynamic_accesslist.py
+++ b/tests/integration/test_dynamic_accesslist.py
@@ -40,6 +40,24 @@ class DynamicAccesslist(TrackerTestCase):
             time.sleep(0.05)
         return False
 
+    def test_a_hash_added_after_a_reload_is_blocked(self):
+        """A reload leaves the add changeset an empty list rather than an
+        absent one. Adding against it walked a null array; the hash must be
+        blocked and the tracker must stay up."""
+        OTHER = bytes(range(21, 41))
+        self.feed("add.fifo", BLOCKED)
+        self.assertTrue(self.wait_for(BLOCKED, blocked=True), "the first hash was never blocked")
+
+        # The reload empties the add changeset, which unblocks the first hash.
+        # Wait for that before adding the second, so it lands on the empty list
+        # the bug walked, not on the changeset before the reload has cleared it.
+        self.tracker.reload()
+        self.assertTrue(self.wait_for(BLOCKED, blocked=False), "the reload never emptied the changeset")
+
+        self.feed("add.fifo", OTHER)
+        self.assertTrue(self.wait_for(OTHER, blocked=True), "the hash added after the reload was never blocked")
+        # tearDown's stop() surfaces any sanitizer complaint on the way out.
+
     def test_a_hash_can_be_blocked_and_released_while_running(self):
         self.assertNotIn(b"failure reason", self.tracker.announce(BLOCKED))