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lib/eigen.eigs lexes "\r" as "r": under eigen_run, "a\rb" == "arb" is true (the VM says false)

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
88/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c
Domain
backend, testing

Research direction

Read the escape switches at lines 324-337 and 413-422 in lib/eigen.eigs, then compare them with src/lexer.c:565 and :696. Add suite coverage in the style of tests/test_meta_fstr_host_str.eigs and run those checks for plain strings and f-strings against the VM. Done means both forms decode \r to a carriage return and match the VM.

Written by the indexing model from the issue text.

Description

area:stdlib kind:silent-wrong

Found while root-causing an EigenRegex probe for InauguralSystems/EigenRegex#37, on the v0.46.0 release binary. lib/eigen.eigs is the same blob (bb721a65) on v0.46.0 and on main 5a7b703e.

Repro

import eigen
print of ("a\rb" == "arb")
print of (eigen.eigen_run of "\"a\\rb\" == \"arb\"")
print of (eigen.eigen_run of "f\"a\\rb\" == \"arb\"")

This prints false, true, true and exits 0. The VM decodes \r to a carriage return. The meta-interpreter decodes it to the letter r, in plain strings and in f-strings. The file's header promises evaluator values at parity with the C evaluator.

Root cause

Neither escape switch in lib/eigen.eigs has an r arm, so \r falls into the catch-all buf is buf + esc. The f-string switch is at lines 324-337 and the plain-string switch at lines 413-422. src/lexer.c:565 (f-strings) and :696 (plain strings) decode \r to CR. ouroboros had the same gap and fixed it in src/frontend.eigs:237-246, where the bootstrap fixed point caught it. This copy never got that fix.

Done when

  • Both escape switches in lib/eigen.eigs decode \r to CR, as src/lexer.c:565 and :696 do.
  • A suite test in the style of tests/test_meta_fstr_host_str.eigs compares eigen_run with the VM on both forms: 
    check of ["meta plain string decodes the r escape like the VM", eigen.eigen_run of "\"a\\rb\" == \"arb\"", "a\rb" == "arb"]
check of ["meta f-string decodes the r escape like the VM", eigen.eigen_run of "f\"a\\rb\" == \"arb\"", f"a\rb" == "arb"]
    On v0.46.0 both checks fail, giving true where the VM gives false. With the two missing arms added to a scratch copy of lib/eigen.eigs, both pass.

Out of scope here: unknown escapes (\q, \0, \u212a) silently dropping the backslash in all three lexers is #1641.

Dominant language
C
Stars
3
Forks
7
Avg merge
11h 30m
Merged PRs (30d)
271

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