Merge issue #15833: don't raise an exception if importlib can't write

byte-compiled files.

This fixes a regression introduced by 3.3.

Patch by Charles-François Natali.

Lib/importlib/_bootstrap.py

@@ -1066,17 +1066,17 @@ class SourceFileLoader(FileLoader, SourceLoader):
             except FileExistsError:
                 # Probably another Python process already created the dir.
                 continue
-            except PermissionError:
-                # If can't get proper access, then just forget about writing
-                # the data.
+            except OSError as exc:
+                # Could be a permission error, read-only filesystem: just forget
+                # about writing the data.
+                _verbose_message('could not create {!r}: {!r}', parent, exc)
                 return
         try:
             _write_atomic(path, data, _mode)
             _verbose_message('created {!r}', path)
-        except (PermissionError, FileExistsError):
-            # Don't worry if you can't write bytecode or someone is writing
-            # it at the same time.
-            pass
+        except OSError as exc:
+            # Same as above: just don't write the bytecode.
+            _verbose_message('could not create {!r}: {!r}', path, exc)
 
 
 class SourcelessFileLoader(FileLoader, _LoaderBasics):

Misc/NEWS

@@ -158,6 +158,10 @@ Tests
 Build
 -----
 
+- Issue #15833: Fix a regression in 3.3 that resulted in exceptions being
+  raised if importlib failed to write byte-compiled files.  This affected
+  attempts to build Python out-of-tree from a read-only source directory.
+
 - Issue #15923: Fix a mistake in ``asdl_c.py`` that resulted in a TypeError
   after 2801bf875a24 (see #15801).