os: DirFS has interaction with Chdir

[kortschak]

This is something that may not be significant enough to address, but I thought it was worth raising.

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

$ go version
go version go1.17rc1 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="on"
GOARCH="amd64"
GOBIN="/home/user/bin"
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/user/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/user/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/user/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17rc1"
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-build511087247=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Given a main.go from https://play.golang.org/p/Tccj4KWcX7X

$ cd /home/user/dirfs
$ mkdir dirA
$ go run main.go

What did you see?

I saw exactly what I expected, but it could be something that may surprise some users.

2021/07/15 15:02:04 from "/home/user/dirfs/dirA": open dirA/file: no such file or directory
exit status 1

Given that the DirFS was handed a relative path, it's entirely reasonable for reads subsequent to a chdir to be now relative to the new working directory, but it may be worth clarifying that the DirFS does not hold an absolute path as its root unless it was handed one or some other way to indicate that chdir will alter the effective filesystem that's seen.

[seankhliao]

cc @rsc

[gopherbot]

Change https://go.dev/cl/410334 mentions this issue: os: document that Chdir affects fs.FS returned by DirFS with a relative path