testing/synctest: receive on synctest channel from outside bubble when not using synctest #73648
Closed as not planned
Description
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 secretsWhat 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) ?