JSONDecoder fails, same code works on x86_64

[flavio]

Big disclaimer: I'm new to swift, I spent the last days playing around with it because it seems the WASM support you are working on is already pretty good. Kudos to you 👏

The problem

I'm trying to decode an instance of a custom struct starting from a JSON object. I tried to leverage the JSONDecoder class from the foundation module. It looks like the same code works on x86_64, but does not on WASM.

Any help is appreciated ❤️

How to reproduce the issue

I created an empty folder, inside of that I wrote this simple main.swift file:

import Foundation

struct Message: Decodable {
    enum CodingKeys: String, CodingKey {
        case author
        case body
    }

    let author: String
    let body: String
}

let json = Data("""
{
    "author": "flavio",
    "body": "hello world"
}
""".utf8)

do {
    let msg: Message = try JSONDecoder().decode(Message.self, from: json)
    print("automatic decoding worked fine")
    print(msg)
} catch {
    print("automatic decoding DID NOT work as expected")
    print(error)

    let obj = try? JSONSerialization.jsonObject(with: json, options: [])

    if let dictionary = obj as? [String: Any] {
      for key in ["author", "body"] {
        let value = dictionary[key] as! String
        print("\(key)-> \(value)\n")
      }
    }
}

I compiled the code on my Linux system using the latest version of the ghcr.io/swiftwasm/swift container.

I produced a x86_64 binary via swiftc main.swift, this produces a main binary.
I produced a WASM binary via swiftc -target wasm32-unknown-wasi main.swift -o main.wasm, this produces a main.wasm binary.

The regular x86_64 binary can be run without any issue:

$ ./main 
automatic decoding worked fine
Message(author: "flavio", body: "hello world")

The WASM file instead fails:

$ wasmtime run main.wasm
valueNotFound(json.Message, Swift.DecodingError.Context(codingPath: [], debugDescription: "The given data did not contain a top-level value.", underlyingError: nil))
author-> flavio

body-> hello world

As you can see the WASM binary cannot use the JSONDecode class, but it can use JSONSerialization just fine.

Environment details

Swift compiler

SwiftWasm Swift version 5.3 (swiftlang-5.3.0)
Target: x86_64-unknown-linux-gnu

Coming from the container ghcr.io/swiftwasm/swift@sha256:763cb6e1a18ce18ebc71634186c4d18d718a594bab149077478695ab9b2e0fd4

WASM runtime

I'm using wasmtime version 0.21.0

Host

My host is openSUSE Tumbleweed, running on a x86_64 machine.

[MaxDesiatov]

Thanks for the report @flavio, great catch! Interestingly enough, JSONDecoder works fine in browser environments. I've tried Safari, Firefox and Chrome on macOS. The error is only present when running this with wasmer and wasmtime. @kateinoigakukun do you have suggestions where we should start looking? Maybe there's a difference between how the browser WASI polyfill works and how these standalone WASI hosts do it?

[MaxDesiatov]

@kateinoigakukun Let's wait until a corresponding 5.3 snapshot is built, test if it works, and then release as 5.3.1, WDYT?

[kateinoigakukun]

@MaxDesiatov I support that plan 👍

[MaxDesiatov]

I've verified that the issue is fixed in swift-wasm-5.3-SNAPSHOT-2020-11-19-a.

@kateinoigakukun Do we have a process for tagging a new release? Should I just create the 5.3.1 release that points to the same commit as swift-wasm-5.3-SNAPSHOT-2020-11-19-a and reupload the same assets as this snapshot with new names?

[kateinoigakukun]

@MaxDesiatov Yes, I think it's ready for the next release.

[MaxDesiatov]

I know you've released 5.3.0, so just want to check that I don't miss anything. Is there anything that you've done for the 5.3.0 other than creating a GitHub release and reuploading renamed assets from a corresponding snapshot tag?

[kateinoigakukun]

@MaxDesiatov Those jobs can be done by "Manual distribution" with the params

[MaxDesiatov]

@flavio the issue should be fixed in the latest 5.3.1 release. Does it resolve the problem for you?