PEP 706: Add some notes, remove open issue, fix mistakes & formatting (#3023)

* Add some notes, remove open issue, fix mistakes & formatting

- Remove Open Issue “How far should this be backported?”.
  That'll be up to release managers, no need to put it in the PEP.
- Add a section on adding filters to zipfile. Not *quite* a
  “rejected idea”, but IMO the PEP is good enough without it.
- Add a note on a registration mechanism
- Fix a few obvious mistakes, typos, formatting

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

pep-0706.rst

@@ -343,7 +343,7 @@ New docs will tell users to consider:
 * checking that filenames have expected extensions (discouraging files that
   execute when you “click on them”, or extension-less files like Windows
   special device names),
-* limiting the number of extracted, files total size of extracted data,
+* limiting the number of extracted files, total size of extracted data,
   and size of individual files,
 * checking for files that would be shadowed on case-insensitive filesystems.
 
@@ -385,11 +385,12 @@ tarfile CLI
 -----------
 
 The CLI (``python -m tarfile``) will gain a ``--filter`` option
-that will take the nams of one of the provided default filters.
+that will take the name of one of the provided default filters.
 It won't be possible to specify a custom filter function.
 
 If ``--filter`` is not given, the CLI will use the default filter
-(``'legacy_warning'`` for a deprecation period, then ``'data'``).
+(``'fully_trusted'`` with a deprecation warning now, and ``'data'`` from
+Python 3.14 on).
 
 There will be no short option. (``-f`` would be confusingly similar to
 the filename option of GNU ``tar``.)
@@ -417,7 +418,9 @@ gains a ``filter`` argument, if it ever does).
 
 If ``filter`` is not specified (or left as ``None``), it won't be passed
 on, so extracting a tarball will use the default filter
-(``'legacy_warning'`` for a deprecation period, then ``'data'``).
+(``'fully_trusted'`` with a deprecation warning now, and ``'data'`` from
+Python 3.14 on).
+
 
 Complex filters
 ---------------
@@ -433,6 +436,14 @@ For example, with a hypothetical ``StatefulFilter`` users would write::
 
 A simple ``StatefulFilter`` example will be added to the docs.
 
+.. note::
+
+   The need for stateful filters is a reason against allowing
+   registration of custom filter names in addition to ``'fully_trusted'``,
+   ``'tar'`` and ``'data'``.
+   With such a mechanism, API for (at least) set-up and tear-down would need
+   to be set in stone.
+
 
 Backwards Compatibility
 =======================
@@ -564,10 +575,44 @@ Feature-wise, *tar format* and *UNIX-like filesystem* are essentially
 equivalent, so ``tar`` is a good name.
 
 
-Open Issues
+Possible Further Work
-===========
+=====================
 
-How far should this be backported?
+Adding filters to zipfile and shutil.unpack_archive
+---------------------------------------------------
+
+For consistency, :external+py3.11:mod:`zipfile` and
+:external+py3.11:func:`shutil.unpack_archive` could gain support
+for a ``filter`` argument.
+However, this would require research that this PEP's author can't promise
+for Python 3.12.
+
+Filters for ``zipfile`` would probably not help security.
+Zip is used primarily for cross-platform data bundles, and correspondingly,
+:external+py3.11:meth:`ZipFile.extract <zipfile.ZipFile.extract>`'s defaults
+are already similar to what a ``'data'`` filter would do.
+A ``'fully_trusted'`` filter, which would *newly allow* absolute paths and
+``..`` path components, might not be useful for much except
+a unified ``unpack_archive`` API.
+
+Filters should be useful for use cases other than security, but those
+would usually need custom filter functions, and those would need API that works
+with both :external+py3.11:class:`~tarfile.TarInfo` and
+:external+py3.11:class:`~zipfile.ZipInfo`.
+That is *definitely* out of scope of this PEP.
+
+If only this PEP is implemented and nothing changes for ``zipfile``,
+the effect for callers of ``unpack_archive`` is that the default
+for *tar* files is changing from ``'fully_trusted'`` to
+the more appropriate ``'data'``.
+In the interim period, Python 3.12-3.13 will emit ``DeprecationWarning``.
+That's annoying, but there are several ways to handle it: e.g. add a
+``filter`` argument conditionally, set ``TarFile.extraction_filter``
+globally, or ignore/suppress the warning until Python 3.14.
+
+Also, since many calls to ``unpack_archive`` are likely to be unsafe,
+there's hope that the ``DeprecationWarning`` will often turn out to be
+a helpful hint to review affected code.
 
 
 Thanks