Repository: go
Issue: path/filepath: WebAssembly uses *nix conventions on Windows
Labels: OS-Windows,NeedsInvestigation,arch-wasm

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### What version of Go are you using (`go version`)?

<pre>
$ go version
go version go1.15.6 windows/amd64
</pre>

### Does this issue reproduce with the latest release?

Yes

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\User\AppData\Local\go-build
set GOENV=C:\Users\User\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\User\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\User\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=c:\go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=c:\go\pkg\tool\windows_amd64
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\User\AppData\Local\Temp\go-build012101774=/tmp/go-build -gno-record-gcc-switches
</pre></details>

### What did you do?

<!--
If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.
-->

Here's a quick demonstration of the problem. Make a file called `main.go` that looks like this:

```go
package main

import "fmt"
import "path/filepath"

func main() {
  fmt.Printf("IsAbs: %v\n", filepath.IsAbs("c:\\test"))
}
```

Run this program natively on Windows:

```
> go build -o .\main.exe .\main.go
> .\main.exe
IsAbs: true
```

Run this program via WebAssembly on Windows:

```
> set GOOS=js
> set GOARCH=wasm
> go build -o .\main.wasm .\main.go
> copy c:\go\misc\wasm\wasm_exec.js .\wasm_exec.js
> node .\wasm_exec.js .\main.wasm
IsAbs: false
```

### What did you expect to see?

I expected to see `IsAbs: true` in both cases.

### What did you see instead?

I see `IsAbs: true` when running a native executable on Windows but I see `IsAbs: false` when running via WebAssembly on Windows.

I'm the developer behind [esbuild](https://esbuild.github.io/), a JavaScript bundler written in Go. I publish native executables for all major operating system and architecture combinations, and I also separately publish a WebAssembly executable. The WebAssembly executable is intended to enable people to use esbuild in the browser and also in scenarios where they want to use esbuild outside of the browser in an os-independent context, which is the promise of WebAssembly.

Here is the context for this issue: https://github.com/evanw/esbuild/issues/687. The [Yarn](https://yarnpkg.com/) JavaScript package manager would like to use the WebAssembly version of esbuild instead of the native version. They try to avoid install scripts with platform-specific behavior since it messes with their preferred package installation strategy. Thus the same directory tree must be able to work on all platforms.

I have (perhaps incorrectly) assumed that Go's WebAssembly build target produces a cross-platform executable. This doesn't appear to be the case. It also doesn't appear to be something that can be fixed in a backwards-compatible manner, so I can understand if you either won't or can't fix it. It does seem like a bug to me because it violated my expectations of what the behavior should be, so I have reported it as a bug. But it's also possible that the WebAssembly build target only works in Unix-style environments by design and I was incorrect in assuming it would be cross-platform.

Regardless, I could use some advice for what to do here. Am I on my own for how to fix this? I imagine I could essentially fork the Go standard library and try to include both copies in the WebAssembly build. But I would prefer not to do that if possible.