Add client capabilities, enable messages

The API version the client sends can now be used to check what the client
expects or is capable of.

All version tests IPA does will be be named and listed in one module,
ipalib.capabilities, which includes a function to test a specific capability
against an API version.
Similarly to Python's __future__ module, capabilities.py also serves as
documentation of backwards-incompatible changes to the API.

The first capability to be defined is "messages". Recent enough clients can
accept a list of warnings or other info under the "messages" key in the
result dict.

If a JSON client does not send the API version, it is assumed this is a testing
client (e.g. curl from the command line). Such a client "has" all capabilities,
but it will always receive a warning mentioning that forward compatibility
is not guaranteed.
If a XML client does not send the API version, it is assumed it uses the API
version before capabilities were introduced. (This is to keep backwards
compatibility with clients containing bug https://fedorahosted.org/freeipa/ticket/3294)

Whenever a capability is added, the API version must be incremented.
To ensure that, capabilities are written to API.txt and checked by
`makeapi --validate`.

Design page: http://freeipa.org/page/V3/Messages
Ticket: https://fedorahosted.org/freeipa/ticket/2732

1 files changed, 50 insertions, 0 deletions

diff --git a/ipalib/capabilities.py b/ipalib/capabilities.py
new file mode 100644
index 000000000..751b93e2b
--- /dev/null
+++ b/ipalib/capabilities.py
@@ -0,0 +1,50 @@
+# Authors:
+#   Petr Viktorin <pviktori@redhat.com>
+#
+# Copyright (C) 2012  Red Hat
+# see file 'COPYING' for use and warranty information
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+"""List of, and utilities for working with, client capabilities by API version
+
+The API version is given in ipapython.version.API_VERSION.
+
+This module defines a dict, ``capabilities``, that maps feature names to API
+versions they were introduced in.
+"""
+
+from distutils import version
+
+VERSION_WITHOUT_CAPABILITIES = u'2.51'
+
+capabilities = dict(
+    # messages: Server output may include an extra key, "messages", that
+    # contains a list of warnings and other messages.
+    # http://freeipa.org/page/V3/Messages
+    messages=u'2.52',
+
+)
+
+
+def client_has_capability(client_version, capability):
+    """Determine whether the client has the given capability
+
+    :param capability: Name of the capability to test
+    :param client_version: The API version string reported by the client
+    """
+
+    version_tuple = version.LooseVersion(client_version)
+
+    return version_tuple >= version.LooseVersion(capabilities[capability])