testing/synctest: receive on synctest channel from outside bubble when not using synctest

[SilvioRispoliAtElectroluxDotCom]

Go version

go version go1.24.3 windows/amd64

Output of go env in your module/workspace:

set AR=ar
set CC=gcc
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_ENABLED=1
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set CXX=g++
set GCCGO=gccgo
set GO111MODULE=
set GOAMD64=v1
set GOARCH=amd64
set GOAUTH=netrc
set GOBIN=
set GOCACHE=C:\Users\RispoSil\AppData\Local\go-build
set GOCACHEPROG=
set GODEBUG=
set GOENV=C:\Users\RispoSil\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFIPS140=off
set GOFLAGS=
set GOGCCFLAGS=-m64 -mthreads -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=C:\Users\RispoSil\AppData\Local\Temp\go-build2564291241=/tmp/go-build -gno-record-gcc-switches
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\RispoSil\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\RispoSil\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Users\RispoSil\scoop\apps\go\current
set GOSUMDB=sum.golang.org
set GOTELEMETRY=local
set GOTELEMETRYDIR=C:\Users\RispoSil\AppData\Roaming\go\telemetry
set GOTMPDIR=
set GOTOOLCHAIN=auto
set GOTOOLDIR=C:\Users\RispoSil\scoop\apps\go\current\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.24.3
set GOWORK=
set PKG_CONFIG=pkg-config

REM Some items might have been omitted as per industry secrets

What did you do?

I executed my project built binary. The project makes heavy usage of channels and has "hopper" strategies to collect items from many channels and output them into one (which I think is the reason for the panic).

The "hopper" strategy (that I call DeMultiplexer) looks something like this:

package tools

import (
	"context"
	"fmt"
)

type DeMultiplexer[T any] struct {
	sources []chan T
}

func CreateDeMultiplexer[T any]() DeMultiplexer[T] {
	return DeMultiplexer[T]{
		sources: []chan T{},
	}
}

func (ec *DeMultiplexer[T]) Subscribe(source chan T) {
	ec.sources = append(ec.sources, source)
}

func (ec *DeMultiplexer[T]) Run(ctx context.Context) (chan T, error) {
	collected := make(chan T, 1)

	if err := ctx.Err(); err != nil {
		close(collected)
		return collected, fmt.Errorf("abort DeMultiplexer due to error: %w", context.Cause(ctx))
	}

	go func() {
		for {
			select {
			case <-ctx.Done():
				return

			default:
				for _, ch := range ec.sources {
					select {
					case el := <-ch:
						collected <- el
					default:

					}
				}
			}
		}
	}()

	return collected, nil
}

What did you see happen?

At times the runtime panics in a non repeatable manner. An example stacktrace:

panic: receive on synctest channel from outside bubble

goroutine 24 [running]:
the/project/tools.(*DeMultiplexer[...]).Run.func1()
        C:/path/to/project/tools/demultiplexer.go:39 +0xab
created by the/project/tools.(*DeMultiplexer[...]).Run in goroutine 1
        C:/path/to/project/tools/demultiplexer.go:30 +0x12a

What did you expect to see?

Why this, albeit being quite lousy work, snippet of code is able to generate a panic that should happen only during tests, and only by manually enabling experimental features (which I did not do) ?

[gabyhelp]

Related Issues

• runtime: waitforsingleobject wait_failed #65255 (closed)
• sync: Once panics if testing.T.FailX or testing.T.SkipX are called #73159
• runtime: panic in select when dereferencing channel's element type #23114 (closed)
• panic: runtime error Crash in Orphaned gorutines cant be handled #47261 (closed)
• log/syslog: test panics on freebsd-386 #4928 (closed)
• runtime: send on channel panics with "unexpected signal during runtime execution" #19468 (closed)
• cmd/cgo/internal/testsanitizers: occasional timeout panic #72996
• cmd/go: unexpected panic when running go test on particular package without test files #25789 (closed)
• affected/package: sync #53769 (closed)
• panic: runtime.errorString("integer divide by zero") #65867 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

I think we need a reproducer.