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go panic when using group() and try/finally #5713
Description
Brief summary
In a script, I started to get runtime panic() when adding a try/finally block. I reduced the issue to its simplest form, it appears that a specific mix of group(), variables allocated globally and try/finally causes sobek to crash.
I attached several slightly altered versions of the crashing one that run fine.
This use case was important to me as finalizer was actually a Counter instance to track successes and failures. However, it's not the root cause.
k6 version
k6 v1.6.1 (commit/2ac2bb560e, go1.25.7, linux/amd64)
OS
Linux
Docker version and image (if applicable)
No response
Steps to reproduce the problem
Run this script: ./k6 run crash.js
import { group } from "k6";
const finalizer = () => {
};
export default function () {
try {
const work = () => {
};
group("Home page", function () {
work();
});
if (2 > Math.random()) {
return;
}
} finally {
finalizer();
}
};
Expected behaviour
No panic
Actual behaviour
panics with k6 1.6.1:
panic: runtime error: index out of range [2] with length 1 [recovered, repanicked]
goroutine 81 [running]:
github.com/grafana/sobek.(*Runtime).runWrapped.func1()
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/runtime.go:2521 +0xe5
panic({0x1dbbf40?, 0xc000046b28?})
runtime/panic.go:783 +0x132
github.com/grafana/sobek.(*vm).handleThrow(0xc000412360, {0x1dbbf40, 0xc000046b28})
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/vm.go:847 +0x425
github.com/grafana/sobek.(*vm).try.func1()
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/vm.go:866 +0x3f
panic({0x1dbbf40?, 0xc000046b28?})
runtime/panic.go:783 +0x132
github.com/grafana/sobek.(*vm).handleThrow(0xc000412360, {0x1dbbf40, 0xc000046b28})
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/vm.go:847 +0x425
github.com/grafana/sobek.(*vm).runTryInner.func1()
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/vm.go:889 +0x3f
panic({0x1dbbf40?, 0xc000046b28?})
runtime/panic.go:783 +0x132
github.com/grafana/sobek.(*stash).getByIdx(...)
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/vm.go:496
github.com/grafana/sobek.loadStashLex.exec(0xf7fe25?, 0x6?)
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/vm.go:3145 +0x1db
github.com/grafana/sobek.(*vm).run(0xc000412360)
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/vm.go:642 +0x5b
github.com/grafana/sobek.(*vm).runTryInner(0xc000412360?)
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/vm.go:893 +0x52
github.com/grafana/sobek.(*baseJsFuncObject).__call(0xc0005fe240, {0xc0000cd700?, 0x1, 0x1dbcf00?}, {0x0, 0x0}, {0x228a848?, 0x33a6a00?})
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/func.go:434 +0x711
github.com/grafana/sobek.(*baseJsFuncObject)._call(...)
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/func.go:450
github.com/grafana/sobek.(*baseJsFuncObject).call(0x1dbcf00?, {{0x228a848, 0x33a6a00}, {0xc0000cd700, 0x1, 0x1}}, {0x0?, 0x0?})
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/func.go:458 +0x6d
github.com/grafana/sobek.(*baseJsFuncObject).Call(...)
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/func.go:390
github.com/grafana/sobek.AssertFunction.func1.1()
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/runtime.go:2481 +0x56
github.com/grafana/sobek.(*vm).try(0xc000412360, 0xc000487908)
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/vm.go:870 +0x234
github.com/grafana/sobek.(*Runtime).runWrapped(0xc00033a408, 0x47e685?)
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/runtime.go:2525 +0x5f
github.com/grafana/sobek.AssertFunction.func1({0x228a848?, 0x33a6a00?}, {0xc0000cd700?, 0x1?, 0x1?})
github.com/grafana/sobek@v0.0.0-20260121195222-d8d9202018c5/runtime.go:2480 +0x8c
go.k6.io/k6/internal/js.(*VU).runFn.func2()
go.k6.io/k6/internal/js/runner.go:942 +0x3e
go.k6.io/k6/internal/js/eventloop.(*EventLoop).Start(0xc0002d23c0, 0xc0004ac540)
go.k6.io/k6/internal/js/eventloop/eventloop.go:179 +0x197
go.k6.io/k6/internal/js.(*VU).runFn(0xc0005903c0, {0x2272178, 0xc000350be0}, 0x1, 0xc0004d7c98, 0xc0000cd6e0, {0xc0000cd700, 0x1, 0x1})
go.k6.io/k6/internal/js/runner.go:941 +0x270
go.k6.io/k6/internal/js.(*ActiveVU).RunOnce(0xc0003d6cc0)
go.k6.io/k6/internal/js/runner.go:874 +0x487
go.k6.io/k6/lib/executor.PerVUIterations.Run.getIterationRunner.func7({0x2272140, 0xc00058f080}, {0x225fd20?, 0xc0003d6cc0?})
go.k6.io/k6/lib/executor/helpers.go:81 +0x4a
go.k6.io/k6/lib/executor.PerVUIterations.Run.func5({0x2269620, 0xc0005903c0})
go.k6.io/k6/lib/executor/per_vu_iterations.go:228 +0x39a
created by go.k6.io/k6/lib/executor.PerVUIterations.Run in goroutine 7
go.k6.io/k6/lib/executor/per_vu_iterations.go:241 +0xb85
Crashing script as well as several very close versions that do work fine:
Activity
mstoykov commentedon Mar 6, 2026
Hi @amontfaucon , thank you for reporting this. I have been able to reproduce it and unfortunately some recent updates upstream are not fixing it.
I have also confirmed it started in k6 v0.40.0 (at least for the main script) and tha v0.39.0 and bunch of previous versions do not have the problem.
I will try to work on this next week so it hopefulyl will land in the next release.
fix: restore stash in leaveTry before jumping to finally block
taras commentedon Mar 9, 2026
@mstoykov I created PR 122, I hope it's useful.