bpo-30661: Improve docs for tarfile pax change and effect on shutil (GH-12635)

The shutil archive creation helpers use the default tarfile format,
so that API is also switching to use `pax` by default.

Doc/library/shutil.rst

@@ -559,6 +559,10 @@ provided.  They rely on the :mod:`zipfile` and :mod:`tarfile` modules.
 
    The *verbose* argument is unused and deprecated.
 
+   .. versionchanged:: 3.8
+      The modern pax (POSIX.1-2001) format is now used instead of
+      the legacy GNU format for archives created with ``format="tar"``.
+
 
 .. function:: get_archive_formats()
 
@@ -568,7 +572,7 @@ provided.  They rely on the :mod:`zipfile` and :mod:`tarfile` modules.
    By default :mod:`shutil` provides these formats:
 
    - *zip*: ZIP file (if the :mod:`zlib` module is available).
-   - *tar*: uncompressed tar file.
+   - *tar*: Uncompressed tar file. Uses POSIX.1-2001 pax format for new archives.
    - *gztar*: gzip'ed tar-file (if the :mod:`zlib` module is available).
    - *bztar*: bzip2'ed tar-file (if the :mod:`bz2` module is available).
    - *xztar*: xz'ed tar-file (if the :mod:`lzma` module is available).

Doc/library/tarfile.rst

@@ -813,8 +813,8 @@ Supported tar formats
 There are three tar formats that can be created with the :mod:`tarfile` module:
 
 * The POSIX.1-1988 ustar format (:const:`USTAR_FORMAT`). It supports filenames
-  up to a length of at best 256 characters and linknames up to 100 characters. The
-  maximum file size is 8 GiB. This is an old and limited but widely
+  up to a length of at best 256 characters and linknames up to 100 characters.
+  The maximum file size is 8 GiB. This is an old and limited but widely
   supported format.
 
 * The GNU tar format (:const:`GNU_FORMAT`). It supports long filenames and
@@ -826,14 +826,15 @@ There are three tar formats that can be created with the :mod:`tarfile` module:
   format with virtually no limits. It supports long filenames and linknames, large
   files and stores pathnames in a portable way. Modern tar implementations,
   including GNU tar, bsdtar/libarchive and star, fully support extended *pax*
-  features; some older or unmaintained libraries may not, but should treat
+  features; some old or unmaintained libraries may not, but should treat
   *pax* archives as if they were in the universally-supported *ustar* format.
+  It is the current default format for new archives.
 
-  The *pax* format is an extension to the existing *ustar* format. It uses extra
-  headers for information that cannot be stored otherwise. There are two flavours
-  of pax headers: Extended headers only affect the subsequent file header, global
-  headers are valid for the complete archive and affect all following files. All
-  the data in a pax header is encoded in *UTF-8* for portability reasons.
+  It extends the existing *ustar* format with extra headers for information
+  that cannot be stored otherwise. There are two flavours of pax headers:
+  Extended headers only affect the subsequent file header, global
+  headers are valid for the complete archive and affect all following files.
+  All the data in a pax header is encoded in *UTF-8* for portability reasons.
 
 There are some more variants of the tar format which can be read, but not
 created:

Doc/whatsnew/3.8.rst

@@ -300,6 +300,11 @@ shutil
 :func:`shutil.copytree` now accepts a new ``dirs_exist_ok`` keyword argument.
 (Contributed by Josh Bronson in :issue:`20849`.)
 
+:func:`shutil.make_archive` now defaults to the modern pax (POSIX.1-2001)
+format for new archives to improve portability and standards conformance,
+inherited from the corresponding change to the :mod:`tarfile` module.
+(Contributed by C.A.M. Gerlach in :issue:`30661`.)
+
 
 ssl
 ---