bpo-39395: putenv() and unsetenv() always available (GH-18135)

The os.putenv() and os.unsetenv() functions are now always available.

On non-Windows platforms, Python now requires setenv() and unsetenv()
functions to build.

Remove putenv_dict from posixmodule.c: it's not longer needed.

2 files changed, 27 insertions, 24 deletions

diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index de3e560..f59423c 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -111,9 +111,9 @@ process and user.
    to the environment made after this time are not reflected in ``os.environ``,
    except for changes made by modifying ``os.environ`` directly.
 
-   If the platform supports the :func:`putenv` function, this mapping may be used
-   to modify the environment as well as query the environment.  :func:`putenv` will
-   be called automatically when the mapping is modified.
+   This mapping may be used to modify the environment as well as query the
+   environment.  :func:`putenv` will be called automatically when the mapping
+   is modified.
 
    On Unix, keys and values use :func:`sys.getfilesystemencoding` and
    ``'surrogateescape'`` error handler. Use :data:`environb` if you would like
@@ -130,14 +130,10 @@ process and user.
       cause memory leaks.  Refer to the system documentation for
       :c:func:`putenv`.
 
-   If :func:`putenv` is not provided, a modified copy of this mapping  may be
-   passed to the appropriate process-creation functions to cause  child processes
-   to use a modified environment.
-
-   If the platform supports the :func:`unsetenv` function, you can delete items in
-   this mapping to unset environment variables. :func:`unsetenv` will be called
-   automatically when an item is deleted from ``os.environ``, and when
-   one of the :meth:`pop` or :meth:`clear` methods is called.
+   You can delete items in this mapping to unset environment variables.
+   :func:`unsetenv` will be called automatically when an item is deleted from
+   ``os.environ``, and when one of the :meth:`pop` or :meth:`clear` methods is
+   called.
 
 
 .. data:: environb
@@ -439,17 +435,18 @@ process and user.
    changes to the environment affect subprocesses started with :func:`os.system`,
    :func:`popen` or :func:`fork` and :func:`execv`.
 
-   .. availability:: most flavors of Unix, Windows.
+   Assignments to items in ``os.environ`` are automatically translated into
+   corresponding calls to :func:`putenv`; however, calls to :func:`putenv`
+   don't update ``os.environ``, so it is actually preferable to assign to items
+   of ``os.environ``.
 
    .. note::
 
       On some platforms, including FreeBSD and Mac OS X, setting ``environ`` may
-      cause memory leaks. Refer to the system documentation for putenv.
+      cause memory leaks. Refer to the system documentation for :c:func:`putenv`.
 
-   When :func:`putenv` is supported, assignments to items in ``os.environ`` are
-   automatically translated into corresponding calls to :func:`putenv`; however,
-   calls to :func:`putenv` don't update ``os.environ``, so it is actually
-   preferable to assign to items of ``os.environ``.
+   .. versionchanged:: 3.9
+      The function is now always available.
 
 
 .. function:: setegid(egid)
@@ -638,15 +635,13 @@ process and user.
    environment affect subprocesses started with :func:`os.system`, :func:`popen` or
    :func:`fork` and :func:`execv`.
 
-   When :func:`unsetenv` is supported, deletion of items in ``os.environ`` is
-   automatically translated into a corresponding call to :func:`unsetenv`; however,
-   calls to :func:`unsetenv` don't update ``os.environ``, so it is actually
-   preferable to delete items of ``os.environ``.
-
-   .. availability:: most flavors of Unix, Windows.
+   Deletion of items in ``os.environ`` is automatically translated into a
+   corresponding call to :func:`unsetenv`; however, calls to :func:`unsetenv`
+   don't update ``os.environ``, so it is actually preferable to delete items of
+   ``os.environ``.
 
    .. versionchanged:: 3.9
-      The function is now also available on Windows.
+      The function is now always available and is also available on Windows.
 
 
 .. _os-newstreams:
diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
index 751562e..a4c4266 100644
--- a/Doc/whatsnew/3.9.rst
+++ b/Doc/whatsnew/3.9.rst
@@ -227,6 +227,10 @@ descriptors.
 The :func:`os.unsetenv` function is now also available on Windows.
 (Contributed by Victor Stinner in :issue:`39413`.)
 
+The :func:`os.putenv` and :func:`os.unsetenv` functions are now always
+available.
+(Contributed by Victor Stinner in :issue:`39395`.)
+
 poplib
 ------
 
@@ -331,6 +335,10 @@ Build and C API Changes
   Python 3.0, it has been ignored and unused.
   (Contributed by Jeroen Demeyer in :issue:`36974`.)
 
+* On non-Windows platforms, the :c:func:`setenv` and :c:func:`unsetenv`
+  functions are now required to build Python.
+  (Contributed by Victor Stinner in :issue:`39395`.)
+
 
 Deprecated
 ==========