Add a qp-trie data structure

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A qp-trie is a kind of radix tree that is particularly well-suited to
DNS servers. I invented the qp-trie in 2015, based on Dan Bernstein's
crit-bit trees and Phil Bagwell's HAMT. https://dotat.at/prog/qp/

This code incorporates some new ideas that I prototyped using
NLnet Labs NSD in 2020 (optimizations for DNS names as keys)
and 2021 (custom allocator and garbage collector).
https://dotat.at/cgi/git/nsd.git

The BIND version of my qp-trie code has a number of improvements
compared to the prototype developed for NSD.

  * The main omission in the prototype was the very sketchy outline of
    how locking might work. Now the locking has been implemented,
    using a reader/writer lock and a mutex. However, it is designed to
    benefit from liburcu if that is available.

  * The prototype was designed for two-version concurrency, one
    version for readers and one for the writer. The new code supports
    multiversion concurrency, to provide a basis for BIND's dbversion
    machinery, so that updates are not blocked by long-running zone
    transfers.

  * There are now two kinds of transaction that modify the trie: an
    `update` aims to support many very small zones without wasting
    memory; a `write` avoids unnecessary allocation to help the
    performance of many small changes to the cache.

  * There is also a single-threaded interface for situations where
    concurrent access is not necessary.

  * The API makes better use of types to make it more clear which
    operations are permitted when.

  * The lookup table used to convert a DNS name to a qp-trie key is
    now initialized by a run-time constructor instead of a programmer
    using copy-and-paste. Key conversion is more flexible, so the
    qp-trie can be used with keys other than DNS names.

  * There has been much refactoring and re-arranging things to improve
    the terminology and order of presentation in the code, and the
    internal documentation has been moved from a comment into a file
    of its own.

Some of the required functionality has been stripped out, to be
brought back later after the basics are known to work.

  * Garbage collector performance statistics are missing.

  * Fancy searches are missing, such as longest match and
    nearest match.

  * Iteration is missing.

  * Search for update is missing, for cases where the caller needs to
    know if the value object is mutable or not.

...

This commit is contained in:

Tony Finch

2022-05-09 14:31:35 +01:00

parent 7975b785fd

commit 6b9ddbd1ce

7 changed files with 3638 additions and 21 deletions

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