Fix sourcemap lookup when line numbers are enabled#81
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When calling sites line numbers are reported (StartProfiling called with CpuProfilingMode::kCallerLineNumbers), sometimes locations point to the part of a code line before the first word. In that case, SourceMapConsumer.originalPositionFor will locate with a binary search the first generated position less or equal to the input. Since the input location does not point to a generated location and is at the start of a line, the binary search will return the last generated location from the previous line. But since the found location is on a different line from the input location, originalPositionFor will return null. Changes done to fix this: * do a first search with LEAST_UPPER_BOUND instead of GREATEST_LOWER_BOUND: binary search will search for the first element greater or equal to the input location. As a result, this will work for locations pointing to blank parts at the start of a line. Still it will not work for locations pointing at the end of a line, but this case does not seem to occur, therefore it makes sense to start with LEAST_UPPER_BOUND * if previous search fails, do a second lookup with GREATEST_LOWER_BOUND
Overall package sizeSelf size: 1.18 MB Dependency sizes
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Qard
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nsavoire
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When calling sites line numbers are reported (StartProfiling called with CpuProfilingMode::kCallerLineNumbers), sometimes locations point to the part of a code line before the first word. In that case, SourceMapConsumer.originalPositionFor will locate with a binary search the first generated position less or equal to the input. Since the input location does not point to a generated location and is at the start of a line, the binary search will return the last generated location from the previous line. But since the found location is on a different line from the input location, originalPositionFor will return null. Changes done to fix this: * do a first search with LEAST_UPPER_BOUND instead of GREATEST_LOWER_BOUND: binary search will search for the first element greater or equal to the input location. As a result, this will work for locations pointing to blank parts at the start of a line. Still it will not work for locations pointing at the end of a line, but this case does not seem to occur, therefore it makes sense to start with LEAST_UPPER_BOUND * if previous search fails, do a second lookup with GREATEST_LOWER_BOUND
nsavoire
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When calling sites line numbers are reported (StartProfiling called with CpuProfilingMode::kCallerLineNumbers), sometimes locations point to the part of a code line before the first word. In that case, SourceMapConsumer.originalPositionFor will locate with a binary search the first generated position less or equal to the input. Since the input location does not point to a generated location and is at the start of a line, the binary search will return the last generated location from the previous line. But since the found location is on a different line from the input location, originalPositionFor will return null. Changes done to fix this: * do a first search with LEAST_UPPER_BOUND instead of GREATEST_LOWER_BOUND: binary search will search for the first element greater or equal to the input location. As a result, this will work for locations pointing to blank parts at the start of a line. Still it will not work for locations pointing at the end of a line, but this case does not seem to occur, therefore it makes sense to start with LEAST_UPPER_BOUND * if previous search fails, do a second lookup with GREATEST_LOWER_BOUND
nsavoire
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Mar 22, 2023
When calling sites line numbers are reported (StartProfiling called with CpuProfilingMode::kCallerLineNumbers), sometimes locations point to the part of a code line before the first word. In that case, SourceMapConsumer.originalPositionFor will locate with a binary search the first generated position less or equal to the input. Since the input location does not point to a generated location and is at the start of a line, the binary search will return the last generated location from the previous line. But since the found location is on a different line from the input location, originalPositionFor will return null. Changes done to fix this: * do a first search with LEAST_UPPER_BOUND instead of GREATEST_LOWER_BOUND: binary search will search for the first element greater or equal to the input location. As a result, this will work for locations pointing to blank parts at the start of a line. Still it will not work for locations pointing at the end of a line, but this case does not seem to occur, therefore it makes sense to start with LEAST_UPPER_BOUND * if previous search fails, do a second lookup with GREATEST_LOWER_BOUND
nsavoire
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szegedi
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Mar 13, 2026
Add tests for SourceMapper.mappingInfo with a synthetic webpack-style single-line bundle to document the known limitation introduced by #248. Background ---------- PR #81 changed originalPositionFor to always try LEAST_UPPER_BOUND first (then fall back to GREATEST_LOWER_BOUND). This was reverted in #106 because it broke webpack source maps: for real non-zero columns LEAST_UPPER_BOUND finds the *next* mapping (≥ column) rather than the one at the column, returning wrong function names. PR #248 fixed that regression by using LEAST_UPPER_BOUND only when column === 0, and GREATEST_LOWER_BOUND otherwise. This correctly handles Node.js ≥ 25 where V8's LineTick struct carries real column numbers. Residual limitation (Node.js < 25) ----------------------------------- On Node.js < 25, the LineTick struct has no column field. The C++ layer therefore always emits column=0 for every LineTick sample. With column=0, the sourcemapper uses LEAST_UPPER_BOUND, which finds the *first* mapping on the line. In a webpack bundle (all output on one line) every function maps to the same first source function in the map. This is not a regression vs. the pre-#248 state: before #248, those functions were simply unmapped (column=0 + GREATEST_LOWER_BOUND → nothing ≤ 0 → null → falls back to generated name/file). Both outcomes are imperfect; #248 trades "unmapped" for "mapped to first function", which may or may not be preferable depending on the use case. The two new tests pin both behaviours explicitly so any future change to this logic is immediately visible. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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When calling sites line numbers are reported (StartProfiling called with CpuProfilingMode::kCallerLineNumbers), sometimes locations point to the part of a code line before the first word. In that case, SourceMapConsumer.originalPositionFor will locate with a binary search the first generated position less or equal to the input. Since the input location does not point to a generated location and is at the start of a line, the binary search will return the last generated location from the previous line. But since the found location is on a different line from the input location, originalPositionFor will return null.
Changes done to fix this: