[Windows] Security model for files permissions - STEP 1 (#6893)

This PR is the first part of #6497 to ease the integration, following the new plan propose by @bmw here: #6497 (comment)

This step 1 refactor existing certbot.compat module into certbot.compat.misc, without any logic changed. Package certbot.compat will host the new modules that constitute the security model for Windows.

* Create the certbot.compat package. Move logic in certbot.compat.misc

* Add doc

* Fix lint

* Correct mypy

* Update client.py

certbot/compat/misc.py

@@ -0,0 +1,165 @@
+"""
+This compat module handles various platform specific calls that do not fall into one
+particular category.
+"""
+import os
+import select
+import sys
+import errno
+import ctypes
+import stat
+
+from certbot import errors
+
+UNPRIVILEGED_SUBCOMMANDS_ALLOWED = [
+    'certificates', 'enhance', 'revoke', 'delete',
+    'register', 'unregister', 'config_changes', 'plugins']
+
+
+def raise_for_non_administrative_windows_rights(subcommand):
+    """
+    On Windows, raise if current shell does not have the administrative rights.
+    Do nothing on Linux.
+
+    :param str subcommand: The subcommand (like 'certonly') passed to the certbot client.
+
+    :raises .errors.Error: If the provided subcommand must be run on a shell with
+        administrative rights, and current shell does not have these rights.
+
+    """
+    # Why not simply try ctypes.windll.shell32.IsUserAnAdmin() and catch AttributeError ?
+    # Because windll exists only on a Windows runtime, and static code analysis engines
+    # do not like at all non existent objects when run from Linux (even if we handle properly
+    # all the cases in the code).
+    # So we access windll only by reflection to trick theses engines.
+    if hasattr(ctypes, 'windll') and subcommand not in UNPRIVILEGED_SUBCOMMANDS_ALLOWED:
+        windll = getattr(ctypes, 'windll')
+        if windll.shell32.IsUserAnAdmin() == 0:
+            raise errors.Error(
+                'Error, "{0}" subcommand must be run on a shell with administrative rights.'
+                .format(subcommand))
+
+
+def os_geteuid():
+    """
+    Get current user uid
+
+    :returns: The current user uid.
+    :rtype: int
+
+    """
+    try:
+        # Linux specific
+        return os.geteuid()
+    except AttributeError:
+        # Windows specific
+        return 0
+
+
+def os_rename(src, dst):
+    """
+    Rename a file to a destination path and handles situations where the destination exists.
+
+    :param str src: The current file path.
+    :param str dst: The new file path.
+    """
+    try:
+        os.rename(src, dst)
+    except OSError as err:
+        # Windows specific, renaming a file on an existing path is not possible.
+        # On Python 3, the best fallback with atomic capabilities we have is os.replace.
+        if err.errno != errno.EEXIST:
+            # Every other error is a legitimate exception.
+            raise
+        if not hasattr(os, 'replace'):  # pragma: no cover
+            # We should never go on this line. Either we are on Linux and os.rename has succeeded,
+            # either we are on Windows, and only Python >= 3.4 is supported where os.replace is
+            # available.
+            raise RuntimeError('Error: tried to run os_rename on Python < 3.3. '
+                               'Certbot supports only Python 3.4 >= on Windows.')
+        getattr(os, 'replace')(src, dst)
+
+
+def readline_with_timeout(timeout, prompt):
+    """
+    Read user input to return the first line entered, or raise after specified timeout.
+
+    :param float timeout: The timeout in seconds given to the user.
+    :param str prompt: The prompt message to display to the user.
+
+    :returns: The first line entered by the user.
+    :rtype: str
+
+    """
+    try:
+        # Linux specific
+        #
+        # Call to select can only be done like this on UNIX
+        rlist, _, _ = select.select([sys.stdin], [], [], timeout)
+        if not rlist:
+            raise errors.Error(
+                "Timed out waiting for answer to prompt '{0}'".format(prompt))
+        return rlist[0].readline()
+    except OSError:
+        # Windows specific
+        #
+        # No way with select to make a timeout to the user input on Windows,
+        # as select only supports socket in this case.
+        # So no timeout on Windows for now.
+        return sys.stdin.readline()
+
+
+def compare_file_modes(mode1, mode2):
+    """Return true if the two modes can be considered as equals for this platform"""
+    if os.name != 'nt':
+        # Linux specific: standard compare
+        return oct(stat.S_IMODE(mode1)) == oct(stat.S_IMODE(mode2))
+    # Windows specific: most of mode bits are ignored on Windows. Only check user R/W rights.
+    return (stat.S_IMODE(mode1) & stat.S_IREAD == stat.S_IMODE(mode2) & stat.S_IREAD
+            and stat.S_IMODE(mode1) & stat.S_IWRITE == stat.S_IMODE(mode2) & stat.S_IWRITE)
+
+
+WINDOWS_DEFAULT_FOLDERS = {
+    'config': 'C:\\Certbot',
+    'work': 'C:\\Certbot\\lib',
+    'logs': 'C:\\Certbot\\log',
+}
+LINUX_DEFAULT_FOLDERS = {
+    'config': '/etc/letsencrypt',
+    'work': '/var/lib/letsencrypt',
+    'logs': '/var/log/letsencrypt',
+}
+
+
+def get_default_folder(folder_type):
+    """
+    Return the relevant default folder for the current OS
+
+    :param str folder_type: The type of folder to retrieve (config, work or logs)
+
+    :returns: The relevant default folder.
+    :rtype: str
+
+    """
+    if os.name != 'nt':
+        # Linux specific
+        return LINUX_DEFAULT_FOLDERS[folder_type]
+    # Windows specific
+    return WINDOWS_DEFAULT_FOLDERS[folder_type]
+
+
+def underscores_for_unsupported_characters_in_path(path):
+    # type: (str) -> str
+    """
+    Replace unsupported characters in path for current OS by underscores.
+    :param str path: the path to normalize
+    :return: the normalized path
+    :rtype: str
+    """
+    if os.name != 'nt':
+        # Linux specific
+        return path
+
+    # Windows specific
+    drive, tail = os.path.splitdrive(path)
+    return drive + tail.replace(':', '_')