Invalidate asset URLs when prefix or digest settings change

[OskarEichler]

Summary

Include the asset prefix and digest setting in the existing asset-URL cache dependency. Reusing a Sprockets cache after either setting changes must not retain old URLs in compiled CSS.

Reproduction

Use require "sprockets/railtie"; env in the focused examples is a Sprockets environment with Rails::Context included, digest assets enabled and prefix /assets.

require "sprockets/railtie"
require "tmpdir"
Dir.mktmpdir do |dir|
  File.write(File.join(dir, "main.css"), "p { background: url(icon.png); }")
  File.write(File.join(dir, "icon.png"), "image bytes")
  cache = {}
  [["/old", true], ["/new", true], ["/new", false]].each do |prefix, digest|
    env = Sprockets::Environment.new(dir)
    env.cache = cache
    env.append_path(dir)
    env.context_class.include(Sprockets::Rails::Context)
    env.context_class.assets_prefix = prefix
    env.context_class.digest_assets = digest
    env.context_class.config = ActiveSupport::OrderedOptions.new
    env.register_postprocessor "text/css", Sprockets::Rails::AssetUrlProcessor
    puts env["main.css"].to_s
  end
end
# Before: all three outputs retain /old/icon-<digest>.png.
# After: /old/icon-<digest>.png, /new/icon-<digest>.png, /new/icon.png.

Verification

• Unmodified existing rake test: 151 tests / 796 assertions, zero failures/errors/skips on Ruby 4.0.6 + Rails 8.1.3.1 + Sprockets 4.4.1; Ruby 3.2.11 with the same dependencies; and Ruby 3.2.11 + Rails 7.2.3.1 + Sprockets 3.7.5. Baselines pass too.
• 48 focused external checks pass per configuration; original behavior reproduced before applying the patch.
• MT_CPU=1 serializes Rails isolation tests that share fixture directories. External verification Gemfiles pin these combinations because the repository root Gemfile combines current Rails with Rack 2 and Sprockets 3.
• Package remains 3.5.2 with unchanged requirements/file list. Ruby syntax and diff whitespace checked; the consumer RuboCop configuration retains the same 16 pre-existing offenses.

Compatibility and limitations

No public API signature or dependency changes. Breaking changes: none intended; the incorrect behavior described above changes. Older supported Ruby versions and Linux CI have not been run locally. No test files were added or edited under the consumer repository's no-test-edit constraint; regression evidence is supplied here and through external focused checks. Prepared with AI assistance and verified with the commands above.