util/lru: replace container/list with a custom ring implementation

pre-generics container/list is quite unpleasant to use, and the pointer
manipulation operations for an LRU are simple enough to implement directly
now that we have generic types.

With this change, the LRU uses a ring (aka circularly linked list) rather
than a simple doubly-linked list as its internals, because the ring makes
list manipulation edge cases more regular: the only remaining edge case is
the transition between 0 and 1 elements, rather than also having to deal
specially with manipulating the first and last members of the list.

While the primary purpose was improved readability of the code, as it
turns out removing the indirection through an interface box also speeds
up the LRU:

       │ before.txt  │              after.txt              │
       │   sec/op    │   sec/op     vs base                │
LRU-32   67.05n ± 2%   59.73n ± 2%  -10.90% (p=0.000 n=20)

       │ before.txt │             after.txt              │
       │    B/op    │    B/op     vs base                │
LRU-32   21.00 ± 0%   10.00 ± 0%  -52.38% (p=0.000 n=20)

       │ before.txt │           after.txt            │
       │ allocs/op  │ allocs/op   vs base            │
LRU-32   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=20) ¹

Updates #cleanup

Signed-off-by: David Anderson <danderson@tailscale.com>

0 files changed, 0 insertions, 0 deletions