testing/testfs: tests including absolute root-based paths fail

[clohr-vc]

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

$ go version
go version go1.17.5 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/xxx/.cache/go-build"
GOENV="/home/xxx/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/xxx/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/xxx/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib64/golang"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib64/golang/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.5"
GCCGO="gccgo"
AR="ar"
CC="x86_64-solus-linux-gcc"
CXX="x86_64-solus-linux-g++"
CGO_ENABLED="1"
GOMOD="/home/xxx/git/gitlab.com/xxx/go.mod"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build599005499=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://go.dev/play/p/psQhY3pZTkA

package main

import (
	"testing/fstest"
)

func main() {

	// Let's try to access file setup using relative path
	// This will work as expected.
	if err := testFS("mount/drive/file"); err != nil {
		panic(err)
	}

	// Now let's switch to an absolute path
	// This will fail with error 'file does not exist'
	if err := testFS("/mount/drive/file"); err != nil {
		panic(err)
	}
}

func testFS(path string) error {
	m := fstest.MapFS{
		path: {},
	}

	_, err := m.ReadFile(path)
	return err
}

What did you expect to see?

Judging from the documentation on testfs.MapFS, I'd expect any path present in the map to be treated as existing, regardless of containing the root directory or not.

What did you see instead?

When configuring paths and files with absolute paths including the root directory, any access to those will fail with file does not exist error messages.

I'm not 100% sure about the reason behind this, I suspect it derives from unclear definitions of what a root volume is on non unix systems (as discussed here #44279 ). Since the testing package requires explicit setup of file structures and hierarchies and essentially abstracts the real file system for tests, I don't really understand why it wouldn't work here either, though?

[seankhliao]

MapFS implements io/fs.FS which disallows absolute paths.

[clohr-vc]

@seankhliao is it maybe worth adding some hint about this to the docs? I see quite a few issues around this misconception.