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Rewrite summary

NOTE: The public API of getmac is unchanged as part of this rewrite. get_mac_address() is still the primary way of getting a MAC address, it's just the "under the hood" internals that have changed completely.

Motivation

The current system of finding a MAC is, to put it bluntly, throw commands at the wall, see if they stick, and promptly forget what stuck for next time. While this has worked up till now, it's a hack built on hacks and has needed a rewrite for a while. It's prone to false-positives (multiple nasty bugs were caused by this), is quite slow ("misses" can take seconds to return!), extremely difficult to test (and thus aforementioned bugs were missed), and is generally a unreadable pile of spaghetti to anyone except me.

Rewrite

The rewrite is built from the ground up as a class-based modular architecture. Each "method" (a way of getting a MAC) is implemented as a subclass of the Method base class. The methods define what platforms they apply to (platforms, e.g. platforms = {"windows", "wsl"}), the type of method (method_type, e.g. method_type = "ip4") and other attributes, such as if they make a network request as part of the check (network_request).

There are two functions that are implemented by Method subclasses: test() and get(arg). The test() functions implements a fast test for the feasibility of the method, e.g. checking if the /proc/net/arp file exists for the ArpFile method. The get(arg) functions implements the actual functionality of looking up the MAC, e.g. in the case of ArpFile, parsing the contents of /proc/net/arp.

When get_mac_address() is called for the first time for a particular method type (e.g. "iface"), a cache is initialized for that method type (in initialize_method_cache()):

  1. Create a list of all methods
  2. Remove any that don't apply to this method type (e.g. "iface" or "ip4")
  3. Remove any that don't apply to the current platform (e.g. "windows")
  4. If method type is ip, ip4 or ip6, and network_request=False, remove any methods that result in network traffic. These will have the attribute Method.network_request = True.
  5. Test all methods by calling test() and remove any that fail (return False)
  6. Store any methods that remain in the cache for this method type ("iface")

The first of the methods in the cache is used to fulfill the get_mac_address() via a call to get(arg) on the method. If there's a critical failure during the get(), then the method is marked as unusable, removed from the cache, and the next method in the cache is used instead. Some methods can't be tested reliably without starting a process, which is expensive, so instead we fail them on first attempt. Calling a single method addresses the old system's issue of trying every method until there was a success, which led to "misses" (no MAC available for whatever was requested) taking several seconds (or longer in extreme cases).

Benefits

The new system has a number of benefits

  • Reduction of false-positives and false-negatives by improving method selection accuracy (platform, validity, etc.)
  • Significantly faster overall
  • "Misses" have the same performance as "Hits"
  • Easier to test, since each method can be tested directly via it's class
  • Easier to type annotate and analyze with mypy
  • Easier to read, improving reviewability and ease of contributing for newcomers
  • Extensible! Custom methods can be defined and added at runtime (which is perfect if you have some particular edge case but aren't able to open-source it).