Optimize python tracing debugger performance #3356
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… line events Fixes several performance issues in PEP 669 tracing implementation: Bug fixes: - Module-level breakpoints (func_name='None') were enabling line tracing for all functions in the file, even when the breakpoint was outside the function's line range. Now checks if breakpoint falls within the function's boundaries (co_firstlineno to last line from co_lines()). - CALL event monitoring was never disabled after breakpoints were discovered, causing unnecessary overhead. Now removes CALL monitoring once breakpoint is processed via remove_breakpoint(). - Missing f_back breakpoint tracking prevented proper return event registration when parent frame had breakpoints. Now checks f_back for matching breakpoints. - Frame cache key was added to global_cache_skips even when line tracing was enabled, causing frames with breakpoints to be incorrectly skipped. Now only caches frames that should actually be skipped. - Expensive py_start_callback checks (disposed, thread liveness, path normalization, file type) were performed before cache lookup. Now checks cache first for better performance. - Line cache key wasn't being used to skip already-checked lines with no breakpoints. Now caches negative results at line granularity. - Stack walking on every exception to find top-level frame was O(n) instead of O(1). Added _is_top_level_frame() helper to check frame properties directly without walking the entire stack. Performance impact: Internal benchmark shows step execution time reduced from several seconds to near-instantaneous in files with selective breakpoints (e.g., breakpoint in one function no longer slows down all other functions in the same file).
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Changes Made
See https://gist.github.com/youtux/f21934bda02366ca4eda898e3d92cf52 for a more detailed breakdown.
These bugs and performance fixes have been addressed using Claude Code.
Performance Impact