When trying to write new bytecode, importlib was not catching the IOError

thrown if the file happened to be read-only to keep the failure silent.

Fixes issue #7187. Thanks, Dave Malcolm for the report and analysis of the
problem.

Lib/importlib/_bootstrap.py

@@ -526,9 +526,9 @@ def write_bytecode(self, name, data):
         bytecode_path = self.bytecode_path(name)
         if not bytecode_path:
             bytecode_path = self._base_path + _suffix_list(imp.PY_COMPILED)[0]
-        file = _io.FileIO(bytecode_path, 'w')  # Assuming bytes.
         try:
-            with _closing(file) as bytecode_file:
+            # Assuming bytes.
+            with _closing(_io.FileIO(bytecode_path, 'w')) as bytecode_file:
                 bytecode_file.write(data)
                 return True
         except IOError as exc:

Lib/importlib/test/source/test_file_loader.py

@@ -6,6 +6,7 @@
 import imp
 import os
 import py_compile
+import stat
 import sys
 import unittest
 
@@ -121,6 +122,10 @@ class BadBytecodeTest(unittest.TestCase):
     But if the marshal data is bad, even if the magic number and timestamp
     work, a ValueError is raised and the source is not used [bad marshal].
 
+    The case of not being able to write out the bytecode must also be handled
+    as it's possible it was made read-only. In that instance the attempt to
+    write the bytecode should fail silently [bytecode read-only].
+
     """
 
     def import_(self, file, module_name):
@@ -159,6 +164,7 @@ def test_bad_bytecode(self):
                 self.assertEqual(bytecode_file.read(4), source_timestamp)
 
     # [bad marshal]
+    @source_util.writes_bytecode_files
     def test_bad_marshal(self):
         with source_util.create_modules('_temp') as mapping:
             bytecode_path = source_util.bytecode_path(mapping['_temp'])
@@ -172,6 +178,26 @@ def test_bad_marshal(self):
                 self.import_(mapping['_temp'], '_temp')
             self.assertTrue('_temp' not in sys.modules)
 
+    # [bytecode read-only]
+    @source_util.writes_bytecode_files
+    def test_read_only_bytecode(self):
+        with source_util.create_modules('_temp') as mapping:
+            # Create bytecode that will need to be re-created.
+            py_compile.compile(mapping['_temp'])
+            bytecode_path = source_util.bytecode_path(mapping['_temp'])
+            with open(bytecode_path, 'r+b') as bytecode_file:
+                bytecode_file.seek(0)
+                bytecode_file.write(b'\x00\x00\x00\x00')
+            # Make the bytecode read-only.
+            os.chmod(bytecode_path,
+                        stat.S_IRUSR | stat.S_IRGRP | stat.S_IROTH)
+            try:
+                # Should not raise IOError!
+                self.import_(mapping['_temp'], '_temp')
+            finally:
+                # Make writable for eventual clean-up.
+                os.chmod(bytecode_path, stat.S_IWUSR)
+
 
 def test_main():
     from test.support import run_unittest

Misc/ACKS

@@ -472,6 +472,7 @@ Nick Maclaren
 Don MacMillen
 Steve Majewski
 Grzegorz Makarewicz
+Dave Malcolm
 Ken Manheimer
 Vladimir Marangozov
 David Marek

Misc/NEWS

@@ -123,6 +123,9 @@ C-API
 Library
 -------
 
+- Issue #7187: Importlib would not silence the IOError raised when trying to
+  write new bytecode when it was made read-only.
+
 - Issue #7264: Fix a possible deadlock when deallocating thread-local objects
   which are part of a reference cycle.