Preserve existing source maps when adding line counts

[OskarEichler]

Summary

The default source-map preprocessor copies an existing map before adding line-count metadata, preserving caller-owned and frozen maps.

Only the top-level existing map is copied. This preprocessor only writes x_sprockets_linecount; nested arrays/maps are not edited or deeply copied. Nil/empty map construction, line counts, existing metadata and generated output remain unchanged. Verified frozen/mutable maps, index maps, existing line counts, empty content, newline/tail/CRLF/Unicode input.

Reproduction

With require "sprockets"; require "tmpdir":

env = Sprockets::Environment.new("/tmp")
map = {"version"=>3, "file"=>"entry.js", "sources"=>["entry.js"],
       "names"=>[], "mappings"=>"AAAA"}.freeze
input = {data:"var a=1;\n", metadata:{map:map}, filename:"/tmp/entry.js",
         load_path:"/tmp", environment:env}
result = Sprockets::Preprocessors::DefaultSourceMap.new.call(input)
p result[:map]["x_sprockets_linecount"] # 1; previously FrozenError.
p map.key?("x_sprockets_linecount") # false; caller map remains unchanged.

Verification

• Isolated patch: unchanged bundle exec rake test and bundle exec rake test_isolated each pass 913 tests /4,109 assertions /zero failures/errors /four skips on Ruby 3.2.11 and 4.0.6.
• 252 focused external checks pass on each Ruby. Baseline reproduces the defect. All Ruby commands use rbenv.
• Full RuboCop: 133 files, zero offenses. Changed Ruby syntax passes both Rubies; git diff --check passes.
• Gem build/content comparison preserves all 84 package paths, version 4.4.1, Ruby >=2.5, and dependency metadata; only README, CHANGELOG and relevant runtime files differ.

Compatibility and limitations

Breaking changes: no intended change to valid supported behavior. Incorrect cache reuse/input mutation/resource retention above changes. No new dependency or version upgrade.

Based on installed release/current upstream main 49d9f20c00ac7d9a7ee57db058961edcccd86699. This is upstream-only; no application fork pin or deployment. Older Ruby/Linux/JRuby verification is left to upstream CI, not claimed locally. Existing tests were not edited or added under the requesting repository's no-test-edit policy; focused reproductions ran externally. Prepared with AI assistance. This is not an exhaustive Sprockets review.