x/crypto/ssh: Unable to start subsystem without an exec or shell message

[magisterquis]

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

$ go version
go version go1.13.3 openbsd/amd64

Does this issue reproduce with the latest release?

It does.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/stuart/.cache/go-build"
GOENV="/home/stuart/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="openbsd"
GONOPROXY=""
GONOSUMDB=""
GOOS="openbsd"
GOPATH="/home/stuart/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/stuart/.go/1.13.3"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/stuart/.go/1.13.3/pkg/tool/openbsd_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="0"
GOMOD=""
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 -fmessage-length=0"

What did you do?

Wrote a little server which implemented an SSH subsystem and tried to write a client to use that subsystem.
A minimal server: http://play.golang.org/p/HtAmPnyMpDl
A minimal client: http://play.golang.org/p/VqSuWARBHJT

What did you expect to see?

Output from the subsystem

What did you see instead?

That the session hadn't been started.

When starting a subsystem, a channel is opened and a subsystem request is sent with the name of the subsystem. According to RFC4254 exactly one of shell, exec, or subsystem requests can succeed per channel, though it doesn't say that only one may be sent. Currently, the subsystem request is sent with session.RequestSubsystem, but there is no way to call the non-exported session.start without calling session.Start (which sends an exec request) or session.Shell (which sends a shell request). This means that to start subsystem, the server will first receive a subsystem request and then an exec or shell request. Changing session.RequestSubsystem to call session.start (i.e. to parallel session.Shell and session.Start) causes the server to start the subsystem and delivers output to the client.

The OpenSSH client only sends a subsystem message. The OpenSSH server replies with false exec request following a subsystem request.

[dmitshur]

/cc @hanwen @FiloSottile per owners.