Line offset list monotonicity assertion is only checked on the resize slow path
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Assessment
- Difficulty
- 2/5
- Estimated time
- 1-3 hours
- Newbie friendliness
- 78/100
Research direction
Read include/prism/internal/line_offset_list.h and src/line_offset_list.c, focusing on pm_line_offset_list_append and pm_line_offset_list_append_slow. Check how the assertion behaves for both fast and slow appends, then run the relevant line offset list tests. Done means duplicate or out-of-order offsets trigger the assertion in debug builds on either path.
Written by the indexing model from the issue text.
Description
Prism records the byte offset of the start of each source line in parser->line_offsets (pm_line_offset_list_t), which must remain strictly monotonically increasing so that binary search (pm_line_offset_list_line and pm_line_offset_list_line_column) can map byte offsets to 1-based line and column numbers.
Currently, the monotonicity assertion
PRISM_ASSERT(list->size == 0 || cursor > list->offsets[list->size - 1]);
is placed inside pm_line_offset_list_append_slow (src/line_offset_list.c) rather than the inline fast path pm_line_offset_list_append (include/prism/internal/line_offset_list.h). As a result, any duplicate or out-of-order line offset appended while list->size < list->capacity silently succeeds even in debug builds with PRISM_ASSERT enabled.
Reproduction
In #4346, we see on 774aebc1f
$ ruby -Ilib -rprism -e 'p Prism.parse("<<\"\#{0}\n\#{<<0}\\C-\n\#{0}\n\n").source.offsets'
# => [0, 8, 18, 23, 24, 23, 24]
For my mac os prism version, I see an abort:
ruby -rprism -e 'p Prism.parse("<<\"\#{0}\n\#{<<0}\\C-\n\#{0}\n\n").source.offsets'
# => [0, 8, 18, 23, 24, 23, 24]
Assertion failed: (list->size == 0 || newline_offset > list->offsets[list->size - 1]), function pm_newline_list_append, file pm_newline_list.c, line 51.
-e:1: [BUG] Aborted at 0x000000018d7c1650
ruby 4.0.1 (2026-01-13 revision e04267a14b) +PRISM [arm64-darwin25]
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