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Arena-escaping values leak 17,800 bytes at exit (xcalloc/xstrdup in arena.c) — pre-existing, and no suite program exercises the shape

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
52/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
c
Domain
compilers

Research direction

Start with src/arena.c, especially xcalloc at line 58 and xstrdup at line 79, then run the preserved arena-escape reproduction with ASAN_OPTIONS=detect_leaks=1. Add an equivalent escape-shape program to the suite corpus before addressing the ownership leak. Done means the corpus program passes the ordinary ASan lane without the reported 17,800-byte leak.

Written by the indexing model from the issue text.

Description

area:memory bug found-by:consumer

A program that lets arena-allocated values escape their arena leaks 17,800 bytes at exit under ASan, and the leak is in src/arena.c's own allocators.

Reproduction

The program is a blind critic's arena-escape probe from the #1183 review (a loop that builds strings inside a scope and returns them outward). Any equivalent shape should do; the exact file is preserved at /tmp/strcrit-r4b/c2_21_arena_escape.eigs.

ASAN_OPTIONS=detect_leaks=1 build/asan/eigenscript c2_21_arena_escape.eigs

Direct leak of 14400 byte(s) in 200 object(s) allocated from:
    #1 xcalloc src/arena.c:58

Indirect leak of 3400 byte(s) in 200 object(s) allocated from:
    #1 xstrdup src/arena.c:79

SUMMARY: AddressSanitizer: 17800 byte(s) leaked in 400 allocation(s).

Exit code 1.

It is PRE-EXISTING — measured, not assumed

Surfaced while reviewing the string cached-length branch (#1183), so the obvious suspicion was that the change caused it. It does not. Both trees were built ASan from source and run on the identical program:

tree result
main @ adf529c, no change 17800 byte(s) leaked in 400 allocation(s)
str-cached-length, with the change 17800 byte(s) leaked in 400 allocation(s)

Byte-identical, allocation-count identical. The branch is not implicated, and it is filed here rather than counted against it.

Why the suite does not see it

The full ASan suite is green at 5280/5280 with a leak tally of 0, on the same build that leaks here. So this shape is absent from the suite corpus — a green suite is evidence about the corpus, not about the runtime. The arena's escape path is the part with no program exercising it.

That is the more useful half of this report: lib/ and the test corpus contain no program that lets a meaningful number of arena values escape, so the arena/heap ownership seam has no standing witness. Related prior art in this area: the arena-vs-heap reference asymmetry class, where an arena list holding heap items is the failure mode.

Suggested next step

Add the escape shape to the suite corpus first, so the leak is visible to the ordinary ASan lane, then fix. A fix without a corpus program that fails beforehand is unverifiable by the repo's own gates.

Dominant language
C
Stars
3
Forks
7
Avg merge
4h 15m
Merged PRs (30d)
106

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