io/fs: Sub should return a fs.FS that implements fs.SubFS

[drew-richardson]

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/drew-richardson/.cache/go-build"
GOENV="/home/drew-richardson/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/drew-richardson/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/drew-richardson/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16.2"
GCCGO="gccgo"
AR="ar"
CC="x86_64-pc-linux-gnu-gcc"
CXX="x86_64-pc-linux-gnu-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-build992509347=/tmp/go-build -gno-record-gcc-switche
s"

What did you do?

https://play.golang.org/p/uSfpnL8qxCC

What did you expect to see?

main.noopFS.Open
io/fs.(*subFS).Open
io/fs.(*subFS).Open
io/fs.(*subFS).Open
io/fs.(*subFS).Open
io/fs.(*subFS).Open
main.main
runtime.main
runtime.goexit

What did you see instead?

main.noopFS.Open
io/fs.(*subFS).Open
main.main
runtime.main
runtime.goexit

Comments

fs.subFS could implement fs.SubFS and return a new appropriately scoped fs.subFS and the callstack would collapse. Assuming there are no objections, I'll start working on a change

[gopherbot]

Change https://golang.org/cl/306769 mentions this issue: io/fs: implement subFS.Sub