Prevent infinite loop in client when first changed usn != lastUsn

Anki expects the last change in the list to have the server's lastUsn.

Reference: https://github.com/ankitects/anki/blob/cca3fcb2418880d0430a5c5c2e6b81ba260065b7/anki/sync.py#L720

ankisyncd/sync_app.py

@@ -327,6 +327,10 @@ def mediaChanges(self, lastUsn):
             for fname,usn,csum, in self.col.media.db.execute("select fname,usn,csum from media order by usn desc limit ?", server_lastUsn - lastUsn):
                 result.append([fname, usn, csum])
 
+        # anki assumes server_lastUsn == result[-1][1]
+        # ref: anki/sync.py:720 (commit cca3fcb2418880d0430a5c5c2e6b81ba260065b7)
+        result.reverse()
+
         return {'data': result, 'err': ''}
 
     def mediaSanity(self, local=None):

tests/test_media.py

@@ -42,3 +42,18 @@ def test_upgrade(self):
         )
         self.assertEqual(cm.lastUsn(), sm.lastUsn())
         self.assertEqual(list(sm.db.execute("SELECT usn FROM media")), [(161,), (161,)])
+
+    def test_mediaChanges_lastUsn_order(self):
+        col = self.colutils.create_empty_col()
+        col.media = ankisyncd.media.ServerMediaManager(col)
+        mh = ankisyncd.sync_app.SyncMediaHandler(col)
+        mh.col.media.db.execute("""
+            INSERT INTO media (fname, usn, csum)
+            VALUES
+                ('fileA', 101, '53059abba1a72c7aff34a3eaf7fef10ed65541ce'),
+                ('fileB', 100, 'a5ae546046d09559399c80fa7076fb10f1ce4bcd')
+        """)
+
+        # anki assumes mh.col.media.lastUsn() == mh.mediaChanges()['data'][-1][1]
+        # ref: anki/sync.py:720 (commit cca3fcb2418880d0430a5c5c2e6b81ba260065b7)
+        self.assertEqual(mh.mediaChanges(lastUsn=99)['data'][-1][1], mh.col.media.lastUsn())