WASIP2: Output truncated/repeats after 4096 bytes

[ErikFrankling]

This example code:

package main

import "fmt"

func main() {
	start := "start"

	var a [4099]byte
	for j := 0; j < len(a); j++ {
		if j%3 == 0 {
			a[j] = 'A'
		} else if j%3 == 1 {
			a[j] = 'B'
		} else if j%3 == 2 {
			a[j] = 'C'
		}
	}
	copy(a[:], start)
	fmt.Printf("%s", a)
	fmt.Println()
}

Run with this:

tinygo build -target=wasip2 -o test.wasm main.go
wasmtime run test.wasm

Gives an output that starts repeating after 4096 bytes

But running normally:

tinygo run .
# or
go run .

Gives the expected output.

I think this is the offending code (have not had much time to look into it yet):
https://github.com/tinygo-org/tinygo/blob/3869f76887feef6c444308e7e1531b7cac1bbd10/src/runtime/runtime_tinygowasmp2.go#L24C1-L32C2

So the issue would be using this function incorrectly:
https://github.com/WebAssembly/wasi-io/blob/73222408bd3de90c8a9144c9ece02bb606c834f6/wit/streams.wit#L154C1-L167C38

I am new to WASM so any pointers are welcome :)

[dgryski]

I'll take a look at this.

[aykevl]

The putchar buffer is only up to 120 bytes, so that can't be it. But I think os.Stdout doesn't go through there anyway. It might be a bug in os.File.Write.