perf: optimize LineCache to reduce allocations#2903
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Reduce total allocated memory from 442k to 206k bytes (-53.5%) and
objects from 3305 to 1538 (-53.5%) per Rails exception capture.
All changes are internal optimizations with zero behavior changes.
Key optimizations:
- Cache longest_load_path and compute_filename results (class-level,
invalidated on $LOAD_PATH changes)
- Cache backtrace line parsing and Line/Frame object creation (bounded
at 2048 entries)
- Optimize LineCache with Hash#fetch, direct context setting, and
per-(filename, lineno) caching
- Avoid unnecessary allocations: indexed regex captures, match? instead
of =~, byteslice, single-pass iteration in StacktraceBuilder
- RequestInterface: avoid env.dup, cache header name transforms, ASCII
fast-path for encoding
- Scope/BreadcrumbBuffer: shallow dup instead of deep_dup where inner
values are not mutated after duplication
- Hub#add_breadcrumb: hint default nil instead of {} to avoid empty
hash allocation
See sub-PRs for detailed review by risk level:
- #2902 (low risk) — hot path allocation avoidance
- #2903 (low risk) — LineCache optimization
- #2904 (medium risk) — load path and filename caching
- #2905 (needs review) — backtrace parse caching
- #2906 (needs review) — Frame object caching
- #2907 (needs review) — Scope/BreadcrumbBuffer shallow dup
- #2908 (medium risk) — RequestInterface optimizations
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Refactor LineCache internals to reduce memory allocations when reading source context for stacktrace frames: - Use Hash#fetch instead of ||= to avoid double hash lookup in getlines - Remove valid_path?/getline indirection — inline bounds checking via line_at helper - Build pre/post context arrays directly with Array.new instead of creating a single large array and slicing it - Add set_frame_context method that sets frame attributes directly, avoiding the intermediate [pre, context_line, post] array allocation - Cache context results per (filename, lineno) since the same frames repeat across exceptions — avoids recreating identical arrays The public get_file_context API is preserved for custom LineCache implementations.
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✅ Low risk — internal refactor of LineCache
Part of #2901 (reduce memory allocations by ~53%)
Changes
Hash#fetchinstead of||=to avoid double hash lookup ingetlinesvalid_path?/getlineindirection — inline bounds checking vialine_athelperArray.newinstead of creating a single large array and slicing itset_frame_contextmethod that sets frame attributes directly, avoiding the intermediate[pre, context_line, post]array allocation(filename, lineno)since the same frames repeat across exceptions — avoids recreating identical arraysThe public
get_file_contextAPI is preserved for custom LineCache implementations.