Avoid recreating cache files removed during a read

[OskarEichler]

Summary

A file cache read may finish after another process evicts that entry; updating its access time does not recreate an empty cache file.

File.utime does not recreate files. Only ENOENT from that operation is ignored. The already-read value is returned. General cross-process atomicity and unrelated Windows atomic-write issue #136/#773 are not addressed.

Reproduction

With require "sprockets"; require "tmpdir":

Dir.mktmpdir do |dir|
  store = Sprockets::Cache::FileStore.new(dir)
  store.set("entry", "content")
  store.define_singleton_method(:safe_open) do |path, &block|
    value = super(path, &block)
    File.unlink(path) # model eviction after reading
    value
  end
  p store.get("entry") # "content"
  p File.exist?(File.join(dir, "entry.cache")) # false; previously an empty file
end

Verification

• Original behavior reproduced on release 4.4.1/current main (49d9f20c00ac7d9a7ee57db058961edcccd86699).
• This isolated patch passes unchanged bundle exec rake test: 913 tests / 4,109 assertions / zero failures/errors / four skips on Ruby 3.2.11 and 4.0.6. 25 focused external checks pass per Ruby.
• Combined six cache fixes also pass the full suite and rake test_isolated on both Rubies, plus 396 focused checks. Normal skips cover Windows-only, legacy-Ruby and two Sass compatibility cases. Non-terminal stdin additionally skips the Unix device check (4,107 assertions).
• Full RuboCop passes 133 files. Gem build preserves 84 package paths, version 4.4.1, Ruby >=2.5 and dependency requirements. All runtime files pass Ruby 3.2/4.0 syntax checks; diff whitespace checked.
• No test files added or edited under the consumer repository's no-test-edit policy. Reproductions and additional checks were run externally.

Compatibility and limitations

Breaking changes: none intended. The incorrect behavior above changes. No public signature, dependency, serialized cache format or version changes. Older supported Rubies, JRuby and Linux are not claimed locally verified. This is a focused cache correction, not a completed review of all Sprockets behavior. Prepared with AI assistance; results above are from actual local runs.