nova.network.linux_net refactor

The nova.network.linux_net module that is responsible for a number
of networking aspects of Nova is quite large and not very flexible
in terms of code maintainance and portability, so it could have a
refactor.

Blueprint: linux-net-refactor
Change-Id: I3c98445349bc82f63913bff90d2baf868c3f7007

specs/liberty/approved/linux_net_refactor.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+==========================================
+nova.network.linux_net refactor
+==========================================
+
+https://blueprints.launchpad.net/nova/+spec/linux-net-refactor
+
+The nova.network.linux_net module that is responsible for a number
+of networking aspects of Nova is quite large and not very flexible
+in terms of code maintainance and portability, so it could have a refactor.
+
+In addition to this, improving flexibility of the linux_net.py module
+will be a huge step forward towards FreeBSD host support, see the mailing
+list discussion for details [1].
+
+Problem description
+===================
+
+Currently, nova.network.linux_net is responsible for a number of different
+things:
+
+* Network interface management (the code to manipulate bridges,
+  adding/removing interfaces etc)
+* Firewall/Iptables management
+* dnsmasq management
+
+The linux_net.py file is almost 2k lines long, that's not critically large,
+but could be smaller.
+
+Some parts of it is flexible and allows overriding classes, for example
+linuxnet_interface_driver. Some parts are not, for example IptablesManager.
+Even for classes that allow overriding, there are consumers that use it
+directly, for example virt.libvirt.vif uses LinuxBridgeInterfaceDriver
+directly.
+
+Also, there are some relatively similar blocks that do the same thing and
+could be grouped into functions, for example, LinuxBridgeInterfaceDriver and
+NeutronLinuxBridgeInterfaceDriver use almost identical code for bridge
+creation.
+
+It would be good to improve code maintainability, readability and portability
+by refactoring the linux_net.py module.
+
+Use Cases
+----------
+
+The only type of audience that would benefit from this change is developers
+who work with linux_net.py and ones who are looking into extending Nova
+with new mechanisms for interface management, firewalling and related.
+
+Project Priority
+-----------------
+
+None
+
+Proposed change
+===============
+
+The proposed appoach would be:
+
+Currently the usage of linux_net in libvirt.vif looks this way:
+
+ * The following methods are used:
+   - create_tap_dev()
+   - create_ovs_vif_port()
+   - create_ivs_vif_port()
+   - device_exists()
+   - delete_net_dev()
+
+ * Usage of linux_net.LinuxBridgeInterfaceDriver methods:
+   - ensure_vlan_bridge()
+   - ensure_bridge()
+
+
+One could notice this interface is pretty complex and actually
+it is responsible for two things at the same time:
+
+ * Providing a Nova network API logic
+ * Providing helpers for OS-level network device management
+
+In order to make it more portable the proposal is to split out
+the OS-level helpers into its own entity and allow custom
+implementations for specific platform.
+
+For example, it would look this way::
+
+
+        """
+        nova.network.netdev module
+        """
+
+        def get_driver():
+            "Method returning platfrom specific implementation"
+            if our_os == "Linux":
+                return LinuxNetDevDriver
+            else
+                # not implemented
+
+        # network device helpers
+        def create_bridge(brname):
+            return get_driver().create_bridge(brname)
+
+        # other methods go here
+
+        """
+        nova.network.netdev.driver
+        """
+
+        class NetDevDriver(object):
+            """A class that defines an interface for
+            OS-level network device manipulation"""
+
+            def create_bridge(self, brname):
+                raise NotImplementedError
+
+            # other methods go here
+
+
+        """
+        nova.netowrk.netdev.linux
+        """
+
+        class LinuxNetDevDriver(NetDevDriver):
+            """A class that implements NetDevDriver
+            interface for Linux"""
+
+            def create_bridge(self, brname):
+                # Linux impl goes here
+
+            # other methods
+
+
+The plan is:
+
+ - Move out helper functions from linux_net to netdev
+ - Convert consumers of these helper functions from linux_net
+   to use the new netdev helpers
+ - Drop the old implementation of helpers from linux_net
+ - Move out Iptables related classes to its own module firewall
+   and allow to override with actual class to be used so it was
+   possible to use other firewall packages
+ - Move out dnsmasq related code to its own module dhcp
+
+Alternatives
+------------
+
+None
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+As the new option will be introduced that allows to specify
+the firewall class to use, deployers will be able to integrate
+third party firewalling packages. As this option will default
+to IptablesManager, there would be no changes to current deployments.
+
+Developer impact
+----------------
+
+Developers will have a more readable, maintainable and extendible linux_net.py
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  novel
+
+Work Items
+----------
+
+* Refactor the interface management code
+* Split out the firewalling code
+* Split out the dnsmasq management code
+
+Dependencies
+============
+
+None
+
+Testing
+=======
+
+Unit tests will be updated accordingly.
+
+Documentation Impact
+====================
+
+None
+
+References
+==========
+
+[1]: http://lists.openstack.org/pipermail/openstack-dev/2015-June/066342.html
+
+History
+=======
+
+.. list-table:: Revisions
+   :header-rows: 1
+
+   * - Release Name
+     - Description
+   * - Liberty
+     - Introduced