Fix remote_tmp when become with non admin user (#42396)

* Fix tmpdir on non root become

 - also avoid exception if tmpdir and remote_tmp are None
 - give 'None' on deescalation so tempfile will fallback to it's default behaviour
   and use system dirs
 - fix issue with bad tempdir (not existing/not createable/not writeable)
   i.e nobody and ~/.ansible/tmp
 - added tests for blockfile case

* Revert "Temporarily revert c119d54"

This reverts commit 5c614a59a6.

* changes based on PR feedback and changelog fragment

* changes based on the review

* Fix tmpdir when makedirs failed so we just use the system tmp

* Let missing remote_tmp fail

If remote_tmp is missing then there's something more basic wrong in the
communication from the controller to the module-side.  It's better to
be alerted in this case than to silently ignore it.

jborean and I have independently checked what happens if the user sets
ansible_remote_tmp to empty string and !!null and both cases work fine.
(null is turned into a default value controller-side.  empty string
triggers the warning because it is probably not a directory that the
become user is able to use).

lib/ansible/modules/net_tools/basics/get_url.py

@@ -49,7 +49,8 @@ options:
   tmp_dest:
     description:
       - Absolute path of where temporary file is downloaded to.
-      - Defaults to C(TMPDIR), C(TEMP) or C(TMP) env variables or a platform specific value.
+      - When run on Ansible 2.5 or greater, path defaults to ansible's remote_tmp setting
+      - When run on Ansible prior to 2.5, it defaults to C(TMPDIR), C(TEMP) or C(TMP) env variables or a platform specific value.
       - U(https://docs.python.org/2/library/tempfile.html#tempfile.tempdir)
     version_added: '2.1'
   force:
@@ -340,18 +341,17 @@ def url_get(module, url, dest, use_proxy, last_mod_time, force, timeout=10, head
                 module.fail_json(msg="%s is a file but should be a directory." % tmp_dest)
             else:
                 module.fail_json(msg="%s directory does not exist." % tmp_dest)
-
-        fd, tempname = tempfile.mkstemp(dir=tmp_dest)
     else:
-        fd, tempname = tempfile.mkstemp()
+        tmp_dest = module.tmpdir
+
+    fd, tempname = tempfile.mkstemp(dir=tmp_dest)
 
     f = os.fdopen(fd, 'wb')
     try:
         shutil.copyfileobj(rsp, f)
     except Exception as e:
         os.remove(tempname)
-        module.fail_json(msg="failed to create temporary content file: %s" % to_native(e),
-                         exception=traceback.format_exc())
+        module.fail_json(msg="failed to create temporary content file: %s" % to_native(e), exception=traceback.format_exc())
     f.close()
     rsp.close()
     return tempname, info

lib/ansible/modules/net_tools/basics/uri.py

@@ -276,7 +276,7 @@ JSON_CANDIDATES = ('text', 'json', 'javascript')
 
 def write_file(module, url, dest, content):
     # create a tempfile with some test content
-    fd, tmpsrc = tempfile.mkstemp()
+    fd, tmpsrc = tempfile.mkstemp(dir=module.tmpdir)
     f = open(tmpsrc, 'wb')
     try:
         f.write(content)