Segfault when parsing a List (Str, Dec)

[ageron]

I'm trying to add the rest-api exercise to the roc track on exercism. It involves parsing a JSON object containing a Dict, but unfortunately this is blocked by #33 . I thought I could work around this by replacing the Dict Str Dec with a List (Str, Dec). Unfortunately, I'm now running into a segfault.

Here's a code example to reproduce the issue (I'm using roc nightly pre-release, built from commit 54cd967 on Fri Sep 6 09:02:04 UTC 2024):

app [main] {
    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br",
    json: "https://github.com/lukewilliamboswell/roc-json/releases/download/0.10.2/FH4N0Sw-JSFXJfG3j54VEDPtXOoN-6I9v_IA8S18IGk.tar.br",
}

import json.Json

main =
    Task.ok {}

ListOfTuples: List (Str, Dec)

expect
    jsonStr = "[[\"Alice\", 1.1], [\"Bob\", 2.2], [\"Charles\", 3.3]]"
    result : Result ListOfTuples _
    result = jsonStr |> Str.toUtf8 |> Decode.fromBytes Json.utf8
    result == Ok [("Alice", 1.1), ("Bob", 2.2), ("Charles", 3.3)]

[Anton-4]

Thanks for reporting this @ageron!

I've converted this to an app to facilitate investigation:

app [main] {
    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br",
    json: "https://github.com/lukewilliamboswell/roc-json/releases/download/0.10.2/FH4N0Sw-JSFXJfG3j54VEDPtXOoN-6I9v_IA8S18IGk.tar.br"
}

import pf.Stdout
import json.Json

ListOfTuples: List (Str, Dec)

jsonStr = "[[\"Alice\", 1.1], [\"Bob\", 2.2], [\"Charles\", 3.3]]"

result : Result ListOfTuples _
result = jsonStr |> Str.toUtf8 |> Decode.fromBytes Json.utf8

main =
    Stdout.line! (Inspect.toStr result)

Valgrind output with the legacy linker:

❯ valgrind ./examples/helloWorld 
==21969== Memcheck, a memory error detector
==21969== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==21969== Using Valgrind-3.22.0 and LibVEX; rerun with -h for copyright info
==21969== Command: ./examples/helloWorld
==21969== 
==21969== Invalid read of size 8
==21969==    at 0x147E33: ??? (roc_app:0)
==21969==    by 0x148F70: ??? (roc_app:0)
==21969==    by 0x144B8F: List_walkHelp_dbefccae6de790f8e3497ad3c6c1c58a12a744edb0d65ec1ec4ade8b1151a59b (roc_app:0)
==21969==    by 0x13AB00: List_walk_52aff1341cf42f5e6559a2cf028663f7bbbc7576ac1948fc58784a0613b79 (roc_app:0)
==21969==    by 0x12FEAA: Inspect_158_d9e2d7d318b97522751e8d862a897cd552d26040fa25d41678f9ac7b36cd7 (roc_app:0)
==21969==    by 0x145C00: Inspect_156_35d6cb78c74f84df82ab12c37bc71bbe5193f93e9d36506abc7c9c8ca124d1ab (roc_app:0)
==21969==    by 0x136BB3: Inspect_apply_5eb6a2599d3097c754d93922b84522fd22c626afbfca9a48d724fa1945e3ca9 (roc_app:0)
==21969==    by 0x12FDD0: #Derived_custom8_52459ae5e05017996bb4298dd9ac3944ffe997fa2e2ad98ba6fd7348395f63 (roc_app:0)
==21969==    by 0x13CB5E: Inspect_apply_8e7a40fb7cb2175e9c8b7aee60f44cef84959b742d3a14c483b6e3b14f05c2f (roc_app:0)
==21969==    by 0x13458D: Inspect_213_3994ebd10847f51a1ba443e4f3b9fb75da3f81a354da59de9bd34aaa2e927d (roc_app:0)
==21969==    by 0x139E8A: Inspect_210_a51293a4c3ce80beb92fd22c82b6b69bd26ee8bc815b483e3cf291f486236c (roc_app:0)
==21969==    by 0x1399EC: List_walkHelp_1971ed175c5339d8a493ee2a719f3ca8f50fbcc2a26feaf7b54a27898e3f (roc_app:0)
==21969==  Address 0xfffffffffffffff8 is not stack'd, malloc'd or (recently) free'd
==21969== 
==21969== 
==21969== Process terminating with default action of signal 11 (SIGSEGV): dumping core
==21969==  Access not within mapped region at address 0xFFFFFFFFFFFFFFF8
==21969==    at 0x147E33: ??? (roc_app:0)
==21969==    by 0x148F70: ??? (roc_app:0)
==21969==    by 0x144B8F: List_walkHelp_dbefccae6de790f8e3497ad3c6c1c58a12a744edb0d65ec1ec4ade8b1151a59b (roc_app:0)
==21969==    by 0x13AB00: List_walk_52aff1341cf42f5e6559a2cf028663f7bbbc7576ac1948fc58784a0613b79 (roc_app:0)
==21969==    by 0x12FEAA: Inspect_158_d9e2d7d318b97522751e8d862a897cd552d26040fa25d41678f9ac7b36cd7 (roc_app:0)
==21969==    by 0x145C00: Inspect_156_35d6cb78c74f84df82ab12c37bc71bbe5193f93e9d36506abc7c9c8ca124d1ab (roc_app:0)
==21969==    by 0x136BB3: Inspect_apply_5eb6a2599d3097c754d93922b84522fd22c626afbfca9a48d724fa1945e3ca9 (roc_app:0)
==21969==    by 0x12FDD0: #Derived_custom8_52459ae5e05017996bb4298dd9ac3944ffe997fa2e2ad98ba6fd7348395f63 (roc_app:0)
==21969==    by 0x13CB5E: Inspect_apply_8e7a40fb7cb2175e9c8b7aee60f44cef84959b742d3a14c483b6e3b14f05c2f (roc_app:0)
==21969==    by 0x13458D: Inspect_213_3994ebd10847f51a1ba443e4f3b9fb75da3f81a354da59de9bd34aaa2e927d (roc_app:0)
==21969==    by 0x139E8A: Inspect_210_a51293a4c3ce80beb92fd22c82b6b69bd26ee8bc815b483e3cf291f486236c (roc_app:0)
==21969==    by 0x1399EC: List_walkHelp_1971ed175c5339d8a493ee2a719f3ca8f50fbcc2a26feaf7b54a27898e3f (roc_app:0)
==21969==  If you believe this happened as a result of a stack
==21969==  overflow in your program's main thread (unlikely but
==21969==  possible), you can try to increase the size of the
==21969==  main thread stack using the --main-stacksize= flag.
==21969==  The main thread stack size used in this run was 8388608.
==21969== 
==21969== HEAP SUMMARY:
==21969==     in use at exit: 504 bytes in 6 blocks
==21969==   total heap usage: 12 allocs, 6 frees, 1,008 bytes allocated
==21969== 
==21969== LEAK SUMMARY:
==21969==    definitely lost: 0 bytes in 0 blocks
==21969==    indirectly lost: 0 bytes in 0 blocks
==21969==      possibly lost: 480 bytes in 5 blocks
==21969==    still reachable: 24 bytes in 1 blocks
==21969==         suppressed: 0 bytes in 0 blocks
==21969== Rerun with --leak-check=full to see details of leaked memory
==21969== 
==21969== For lists of detected and suppressed errors, rerun with: -s
==21969== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
Segmentation fault (core dumped)

This valgrind output is similar to parts of roc-lang/roc#6813, these issues could have the same cause.