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WIP-ONGOING-REFACTORIZATION.rst

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Ongoing Refactors

This captures ongoing refactoring projects in the codebase. This is intended as documentation for developers involved in the refactoring, but also for other developers who may interact with the code being refactored in the meantime.

cloudinit.net -> cloudinit.distros.networking Hierarchy

cloudinit.net was imported from the curtin codebase as a chunk, and then modified enough that it integrated with the rest of the cloud-init codebase. Over the ~4 years since, the fact that it is not fully integrated into the Distro hierarchy has caused several issues.

The common pattern of these problems is that the commands used for networking are different across distributions and operating systems. This has lead to cloudinit.net developing its own "distro determination" logic: get_interfaces_by_mac is probably the clearest example of this. Currently, these differences are primarily split along Linux/BSD lines. However, it would be short-sighted to only refactor in a way that captures this difference: we can anticipate that differences will develop between Linux-based distros in future, or there may already be differences in tooling that we currently work around in less obvious ways.

The high-level plan is to introduce a hierarchy of networking classes in cloudinit.distros.networking, which each Distro subclass will reference. These will capture the differences between networking on our various distros, while still allowing easy reuse of code between distros that share functionality (e.g. most of the Linux networking behaviour). Distro objects will instantiate the networking classes at self.networking, so callers will call distro.networking.<func> instead of cloudinit.net.<func>; this will necessitate access to an instantiated Distro object.

An implementation note: there may be external consumers of the cloudinit.net module. We don't consider this a public API, so we will be removing it as part of this refactor. However, we will ensure that the new API is complete from its introduction, so that any such consumers can move over to it wholesale. (Note, however, that this new API is still not considered public or stable, and may not replicate the existing API exactly.)

In more detail:

  • The root of this hierarchy will be the cloudinit.distros.networking.Networking class. This class will have a corresponding method for every cloudinit.net function that we identify to be involved in refactoring. Initially, these methods' implementations will simply call the corresponding cloudinit.net function. (This gives us the complete API from day one, for existing consumers.)
  • As the biggest differentiator in behaviour, the next layer of the hierarchy will be two subclasses: LinuxNetworking and BSDNetworking. These will be introduced in the initial PR.
  • When a difference in behaviour for a particular distro is identified, a new Networking subclass will be created. This new class should generally subclass either LinuxNetworking or BSDNetworking.
  • To be clear: Networking subclasses will only be created when needed, we will not create a full hierarchy of per-Distro subclasses up-front.
  • Each Distro class will have a class variable (cls.networking_cls) which points at the appropriate networking class (initially this will be either LinuxNetworking or BSDNetworking).
  • When Distro classes are instantiated, they will instantiate cls.networking_cls and store the instance at self.networking. (This will be implemented in cloudinit.distros.Distro.__init__.)
  • A helper function will be added which will determine the appropriate Distro subclass for the current system, instantiate it and return its networking attribute. (This is the entry point for existing consumers to migrate to.)
  • Callers of refactored functions will change from calling cloudinit.net.<func> to distro.networking.<func>, where distro is an instance of the appropriate Distro class for this system. (This will require making such an instance available to callers, which will constitute a large part of the work in this project.)

After the initial structure is in place, the work in this refactor will consist of replacing the cloudinit.net.some_func call in each cloudinit.distros.networking.Networking method with the actual implementation. This can be done incrementally, one function at a time:

  • pick an unmigrated cloudinit.distros.networking.Networking method
  • find it in the the list of bugs tagged net-refactor and assign yourself to it (see :ref:`Managing Work/Tracking Progress` below for more details)
  • refactor all of its callers to call the distro.networking.<func> method on Distro instead of the cloudinit.net.<func> function. (This is likely to be the most time-consuming step, as it may require plumbing Distro objects through to places that previously have not consumed them.)
  • refactor its implementation from cloudinit.net into the Networking hierarchy (e.g. if it has an if/else on BSD, this is the time to put the implementations in their respective subclasses)
    • if part of the method contains distro-independent logic, then you may need to create new methods to capture this distro-specific logic; we don't want to replicate common logic in different Networking subclasses
    • if after the refactor, the method on the root Networking class no longer has any implementation, it should be converted to an abstractmethod
  • ensure that the new implementation has unit tests (either by moving existing tests, or by writing new ones)
  • ensure that the new implementation has a docstring
  • add any appropriate type annotations
    • note that we must follow the constraints described in the "Type Annotations" section above, so you may not be able to write complete annotations
    • we have type aliases defined in cloudinit.distros.networking which should be used when applicable
  • finally, remove it (and any other now-unused functions) from cloudinit.net (to avoid having two parallel implementations)

cloudinit.net Functions/Classes

The functions/classes that need refactoring break down into some broad categories:

  • helpers for accessing /sys (that should not be on the top-level Networking class as they are Linux-specific):
    • get_sys_class_path
    • sys_dev_path
    • read_sys_net
    • read_sys_net_safe
    • read_sys_net_int
  • those that directly access /sys (via helpers) and should (IMO) be included in the API of the Networking class:
    • generate_fallback_config
      • the config_driver parameter is used and passed as a boolean, so we can change the default value to False (instead of None)
    • get_ib_interface_hwaddr
    • get_interface_mac
    • interface_has_own_mac
    • is_bond
    • is_bridge
    • is_physical
    • is_renamed
    • is_up
    • is_vlan
    • wait_for_physdevs
  • those that directly access /sys (via helpers) but may be Linux-specific concepts or names:
    • get_master
    • device_devid
    • device_driver
  • those that directly use ip:
    • _get_current_rename_info
      • this has non-distro-specific logic so should potentially be refactored to use helpers on self instead of ip directly (rather than being wholesale reimplemented in each of BSDNetworking or LinuxNetworking)
      • we can also remove the check_downable argument, it's never specified so is always True
    • _rename_interfaces
      • this has several internal helper functions which use ip directly, and it calls _get_current_rename_info. That said, there appears to be a lot of non-distro-specific logic that could live in a function on Networking, so this will require some careful refactoring to avoid duplicating that logic in each of BSDNetworking and LinuxNetworking.
      • only the renames and current_info parameters are ever passed in (and current_info only by tests), so we can remove the others from the definition
    • EphemeralIPv4Network
      • this is another case where it mixes distro-specific and non-specific functionality. Specifically, __init__, __enter__ and __exit__ are non-specific, and the remaining methods are distro-specific.
      • when refactoring this, the need to track cleanup_cmds likely means that the distro-specific behaviour cannot be captured only in the Networking class. See this comment in PR #363 for more thoughts.
  • those that implicitly use /sys via their call dependencies:
    • master_is_bridge_or_bond
      • appends to get_master return value, which is a /sys path
    • extract_physdevs
      • calls device_driver and device_devid in both _version_* impls
    • apply_network_config_names
      • calls extract_physdevs
      • there is already a Distro.apply_network_config_names which in the default implementation calls this function; this and its BSD subclass implementations should be refactored at the same time
      • the strict_present and strict_busy parameters are never passed, nor are they used in the function definition, so they can be removed
    • get_interfaces
      • calls device_driver, device_devid amongst others
    • get_ib_hwaddrs_by_interface
      • calls get_interfaces
  • those that may fall into the above categories, but whose use is only related to netfailover (which relies on a Linux-specific network driver, so is unlikely to be relevant elsewhere without a substantial refactor; these probably only need implementing in LinuxNetworking):
    • get_dev_features
    • has_netfail_standby_feature
      • calls get_dev_features
    • is_netfailover
    • is_netfail_master
      • this is called from generate_fallback_config
    • is_netfail_primary
    • is_netfail_standby
    • N.B. all of these take an optional driver argument which is used to pass around a value to avoid having to look it up by calling device_driver every time. This is something of a leaky abstraction, and is better served by caching on device_driver or storing the cached value on self, so we can drop the parameter from the new API.
  • those that use /sys (via helpers) and have non-exhaustive BSD logic:
    • get_devicelist
  • those that already have separate Linux/BSD implementations:
    • find_fallback_nic
    • get_interfaces_by_mac
  • those that have no OS-specific functionality (so do not need to be refactored):
    • ParserError
    • RendererNotFoundError
    • is_ip_address
    • is_ipv4_address
    • natural_sort_key

Note that the functions in cloudinit.net use inconsistent parameter names for "string that contains a device name"; we can standardise on devname (the most common one) in the refactor.

Managing Work/Tracking Progress

To ensure that we won't have multiple people working on the same part of the refactor at the same time, there is a bug for each function. You can see the current status by looking at the list of bugs tagged net-refactor.

When you're working on refactoring a particular method, ensure that you have assigned yourself to the corresponding bug, to avoid duplicate work.

Generally, when considering what to pick up to refactor, it is best to start with functions in cloudinit.net which are not called by anything else in cloudinit.net. This allows you to focus only on refactoring that function and its callsites, rather than having to update the other cloudinit.net function also.

References