gh-135386: Fix "unable to open database file" errors on readonly DB

Lib/dbm/sqlite3.py

@@ -59,19 +59,25 @@ def __init__(self, path, /, *, flag, mode):
 
         # We use the URI format when opening the database.
         uri = _normalize_uri(path)
-        uri = f"{uri}?mode={flag}"
+        if flag == "ro":
+            # Add immutable=1 to allow read-only SQLite access even if wal/shm missing
+            uri = f"{uri}?mode={flag}&immutable=1"
+        else:
+            uri = f"{uri}?mode={flag}"
 
         try:
             self._cx = sqlite3.connect(uri, autocommit=True, uri=True)
         except sqlite3.Error as exc:
             raise error(str(exc))
 
+        self._readonly = (flag == "ro")
         # This is an optimization only; it's ok if it fails.
-        with suppress(sqlite3.OperationalError):
-            self._cx.execute("PRAGMA journal_mode = wal")
+        if not self._readonly:
+            with suppress(sqlite3.OperationalError):
+                self._cx.execute("PRAGMA journal_mode = OFF")
 
-        if flag == "rwc":
-            self._execute(BUILD_TABLE)
+            if flag == "rwc":
+                self._execute(BUILD_TABLE)
 
     def _execute(self, *args, **kwargs):
         if not self._cx:

Lib/test/test_dbm_sqlite3.py

@@ -1,3 +1,4 @@
+import os
 import sys
 import unittest
 from contextlib import closing
@@ -89,6 +90,23 @@ def test_readonly_keys(self):
     def test_readonly_iter(self):
         self.assertEqual([k for k in self.db], [b"key1", b"key2"])
 
+    @unittest.skipIf(sys.platform.startswith("win"), "incompatible with Windows file locking")
+    def test_readonly_open_without_wal_shm(self):
+        wal_path = self.filename + "-wal"
+        shm_path = self.filename + "-shm"
+
+        for suffix in wal_path, shm_path:
+            try:
+                os.remove(suffix)
+            except FileNotFoundError:
+                pass
+
+        os.chmod(self.filename, 0o444)
+
+        with dbm_sqlite3.open(self.filename, "r") as db:
+            self.assertEqual(db[b"key1"], b"value1")
+            self.assertEqual(db[b"key2"], b"value2")
+
 
 class ReadWrite(_SQLiteDbmTests):
 

Misc/NEWS.d/next/Library/2025-06-16-15-00-13.gh-issue-135386.lNrxLc.rst

@@ -0,0 +1 @@
+Fix :exc:`sqlite3.OperationalError` error when using :func:`dbm.open` with a read-only file object.