picopackage/lib/picopackage/cache.rb

# frozen_string_literal: true

require "fileutils"

module Picopackage
  # Content-addressed store of upstream payloads, kept so that updates have a
  # merge base to work from.
  #
  # A three-way merge needs the common ancestor: the payload exactly as upstream
  # last sent it. Metadata already records that payload's checksum in
  # `base_checksum`, so the checksum doubles as the cache key and a package
  # carries the pointer to its own ancestor.
  class Cache
    def self.default_root
      ENV["PICOPACKAGE_CACHE"] ||
        File.join(ENV["XDG_CACHE_HOME"] || File.join(Dir.home, ".cache"), "picopackage")
    end

    attr_reader :root

    def initialize(root: self.class.default_root)
      @root = root
    end

    # Returns the checksum the payload was stored under, for recording as a
    # `base_checksum`.
    def store(payload)
      Payload.checksum(payload).tap do |checksum|
        path = path_for(checksum)
        next if File.exist?(path)

        FileUtils.mkdir_p(File.dirname(path))
        # Written via a sibling tempfile and renamed so a concurrent reader
        # never sees a half-written ancestor.
        tmp = "#{path}.#{Process.pid}.tmp"
        File.write(tmp, payload)
        File.rename(tmp, path)
      end
    end

    def fetch(checksum)
      path = path_for(checksum)
      File.read(path) if path && File.exist?(path)
    end

    def include?(checksum) = !fetch(checksum).nil?

    private

    # "sha256:abcdef…" => <root>/sha256/ab/cdef…
    def path_for(checksum)
      return nil if checksum.nil? || checksum.empty?

      algorithm, digest = checksum.split(":", 2)
      return nil if digest.nil? || digest.length < 3 || !digest.match?(/\A[a-f0-9]+\z/i)

      File.join(root, algorithm, digest[0, 2], digest[2..])
    end
  end
end