os: Stdin.Stat().Size() always returns 0 with GOOS=wasip1 GOARCH=wasm

[williamwebb]

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

go1.21.0 darwin/arm64

Does this issue reproduce with the latest release?

Yes, but only tested against GOOS=wasp1 GOARCH=wasm

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

go env Output

GOOS=wasp1 GOARCH=wasm

What did you do?

read Stdin, check stat.size

stdin := os.Stdin
stat, _ := stdin.Stat()
fmt.Println("size:", stat.Size())             

run via wasi runtime, piping input to stdin

echo "input" | wasmer run wasi_binary

What did you expect to see?

size should match contents of stdin

What did you see instead?

size of 0 is always returned

[dmitshur]

CC @golang/wasm.

[achille-roussel]

Hello @williamwebb, thanks for reporting the issue.

I believe that the behavior you expect is not something that Go can satisfy; the webassembly runtime is in charge of implementing host system calls such as the one involved in getting the size of an open file, the Go standard library (and in particular the os package in this case) only expose what the runtime returns.

in your example, it appears that wasmer returns a zero size for the standard input stream; this behavior will be the same regardless of the original programming language from which the webassembly module was compiled.

That being said, I was curious to explore how other platforms may behave, so I tested your code snippet:

• on darwin/arm64

$ echo input | go run main.go
size: 6

• on linux/amd64

$ echo input | go run main.go
size: 0

As you can see, programs that rely on the behavior of asking the size of stdin will not be portable across platforms; combinations of operating systems and webassembly runtimes will produce different results.

@dmitshur I believe we can close this issue since Go behaves as expected in this case.

[dmitshur]

Thanks for investigating this report.