@@ -97,8 +97,9 @@ def _update(self):
         try:
             f = _io.open(self._dirfile, 'r', encoding="Latin-1")
         except OSError:
-            pass
+            self._modified = not self._readonly
         else:
+            self._modified = False
             with f:
                 for line in f:
                     line = line.rstrip()
@@ -113,7 +114,7 @@ def _commit(self):
         # CAUTION:  It's vital that _commit() succeed, and _commit() can
         # be called from __del__().  Therefore we must never reference a
         # global in this routine.
-        if self._index is None:
+        if self._index is None or not self._modified:
             return  # nothing to do
 
         try:
@@ -197,6 +198,7 @@ def __setitem__(self, key, val):
         elif not isinstance(val, (bytes, bytearray)):
             raise TypeError("values must be bytes or strings")
         self._verify_open()
+        self._modified = True
         if key not in self._index:
             self._addkey(key, self._addval(val))
         else:
@@ -229,6 +231,7 @@ def __delitem__(self, key):
         if isinstance(key, str):
             key = key.encode('utf-8')
         self._verify_open()
+        self._modified = True
         # The blocks used by the associated value are lost.
         del self._index[key]
         # XXX It's unclear why we do a _commit() here (the code always
 
@@ -360,9 +360,9 @@ def _rmtree_inner(path):
                     mode = 0
                 if stat.S_ISDIR(mode):
                     _waitfor(_rmtree_inner, fullname, waitall=True)
-                    _force_run(path, os.rmdir, fullname)
+                    _force_run(fullname, os.rmdir, fullname)
                 else:
-                    _force_run(path, os.unlink, fullname)
+                    _force_run(fullname, os.unlink, fullname)
         _waitfor(_rmtree_inner, path, waitall=True)
         _waitfor(lambda p: _force_run(p, os.rmdir, p), path)
 else:
 
@@ -5,6 +5,7 @@
 import io
 import operator
 import os
+import stat
 import unittest
 import warnings
 import dbm.dumb as dumbdbm
@@ -259,6 +260,21 @@ def test_invalid_flag(self):
                 f = dumbdbm.open(_fname, flag)
             f.close()
 
+    @unittest.skipUnless(hasattr(os, 'chmod'), 'test needs os.chmod()')
+    def test_readonly_files(self):
+        with support.temp_dir() as dir:
+            fname = os.path.join(dir, 'db')
+            with dumbdbm.open(fname, 'n') as f:
+                self.assertEqual(list(f.keys()), [])
+                for key in self._dict:
+                    f[key] = self._dict[key]
+            os.chmod(fname + ".dir", stat.S_IRUSR)
+            os.chmod(fname + ".dat", stat.S_IRUSR)
+            os.chmod(dir, stat.S_IRUSR|stat.S_IXUSR)
+            with dumbdbm.open(fname, 'r') as f:
+                self.assertEqual(sorted(f.keys()), sorted(self._dict))
+                f.close()  # don't write
+
     def tearDown(self):
         _delete_files()
 
 
@@ -13,6 +13,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #28847: dbm.dumb now supports reading read-only files and no longer
+  writes the index file when it is not changed.
+
 
 What's New in Python 3.6.0 release candidate 1
 ==============================================