os/exec: Command.CombinedOutput does not return stderr when there is an error

[wingerse]

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

$ go version
go version go1.16.2 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/wingerse/.cache/go-build"
GOENV="/home/wingerse/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/wingerse/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/wingerse/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go-1.16"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go-1.16/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16.2"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build1365221385=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run CombinedOutput on a command that will error.
https://play.golang.org/p/TLNqCABOTCu

What did you expect to see?

I expect to see the content of stderr.
The documentation implies it should be copied to the buffer:

Otherwise, during the execution of the command a separate goroutine
reads from the process over a pipe and delivers that data to the
corresponding Writer. In this case, Wait does not complete until the
goroutine reaches EOF or encounters an error.

The process exists after writing to its stderr, so I should be able to read it to my buffer.

What did you see instead?

I see nothing.
If I set cmd.Stderr = os.Stderr, it is copied properly though: https://play.golang.org/p/bWlSY8F2sLo

[seankhliao]

It's there https://play.golang.org/p/Fs1FxEIEkC-

[wingerse]

My bad. I need more sleep D: