os/signal: NotifyContext should communicate the signal via context.WithCancelCause

[fishy]

What version of Go are you using (go version)?

$ go version
go version go1.20.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/fishy/.cache/go-build"
GOENV="/home/fishy/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/fishy/.gopath/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/fishy/.gopath"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/nix/store/8v5zwymidmry0wd3lhj6zggskzsvqrfk-go-1.20.4/share/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/nix/store/8v5zwymidmry0wd3lhj6zggskzsvqrfk-go-1.20.4/share/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20.4"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build1590600196=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ cat main.go 
package main

import (
        "context"
        "fmt"
        "os"
        "os/signal"
)

func main() {
        ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
        defer cancel()
        select {
        case <-ctx.Done():
                fmt.Println(context.Cause(ctx))
        }
}
$ go run main.go 
^Ccontext canceled

What did you expect to see?

context.Cause(ctx) should return an error containing the info of which signal caused the cancellation.

What did you see instead?

[ianlancetaylor]

CC @henvic @Sajmani

[jdemeyer]

Should be easy to implement in a backwards-compatible way: https://go.dev/play/p/y5FpFPupOAP

[fishy]

@jdemeyer I think we would want the signal to be inspectable programmally, so we would need to define an error type instead of using fmt.Errorf, something like this:

type CanceledBySignalError os.Signal

func (cbse CanceledBySignalError) Error() string {
  return fmt.Sprintf("canceled by signal %v", os.Signal(cbse))
}

then users can inspect it via:

select {
case <-ctx.Done():
  cause := context.Cause(ctx)
  var cbse signal.CanceledBySignalError
  if errors.As(cause, &cbse) {
    sig := os.Signal(cbse)
    // Do something with sig
  }
}

but yes once we can agree on how the error type should look like the actual fix is trivial :)

[gopherbot]

Change https://go.dev/cl/502675 mentions this issue: os/signal: communicate the signal in NotifyContext

[mknyszek]

Am I understanding correctly that this issue has been basically superceded by #60756? Closing as duplicate for now to keep discussion in one place, but feel free to reply and I can reopen. (For some context on process, proposals that get accepted end up tracking work for the proposal in the same issue, so there's no reason to keep this open for that purpose, but I may be missing something.)