YamlStream.Load with JSON with emojis (even escaped) fails: "While scanning a quoted scalar, found invalid Unicode character escape code."

[cmeeren]

I am serializing something to JSON with System.Text.Json, and then converting it to YAML using YamlStream. However, if the JSON contains an emoji, even if it's escaped, YamlStream.Load throws:

(Line: 1, Col: 1, Idx: 0) - (Line: 1, Col: 4, Idx: 3): While scanning a quoted scalar, found invalid Unicode character escape code.

Code to reproduce:

open System.IO
open System.Text.Json
open YamlDotNet.RepresentationModel

let input = "👍"

let json = JsonSerializer.Serialize input

assert (json = "\"\\uD83D\\uDC4D\"")

let yamlStream = YamlStream()
yamlStream.Load(new StringReader(json))

[EdwardCooke]

Those utf-8 codes are actually invalid codes for utf-8. According to Wikipedia anyways. I wonder if the dot net core json library sees your character as 2 separate characters because it might be utf-16. I’m not able to actually dig in to this too much right now, but I’ll try and take a closer look tonight. I wonder if you can specify the encoding in the json serializer and set it to utf-16 and see what happens.

[cmeeren]

UTF-16? But JSON is always UTF-8 by definition, isn't it? In any case, I can find nothing in System.Text.Json about UTF-16. Let me know if I'm wrong.

[cmeeren]

Also, AFAIK many emojis are composed of 2 or even more unicode characters. This SO question and answers may be helpful.

Finally, I highly doubt that System.Text.Json (the official .NET JSON serialization API) is doing things wrong. That would require very concrete proof.

[EdwardCooke]

This will also help me in getting a fix in

https://github.com/dotnet/runtime/issues/42847 There’s some other linked issues that should help clarify the nuances in Unicode.

not sure when I’ll get it done though.

[cmeeren]

Is there any workaround I can apply now, for converting JSON to YAML? The emojis etc. doesn't have to be unescaped; I'm OK with anything that preserves the escape codes.

I simply want my JSON content converted to YAML (tweaked using a YAML visitor). Now it seems that the YAML conversion is attempting to decode escaped stuff, and is failing at that.

[cmeeren]

Possibly relevant: The Wikipedia article on JSON, section "Character encoding", says:

JSON exchange in an open ecosystem must be encoded in UTF-8. The encoding supports the full Unicode character set, including those characters outside the Basic Multilingual Plane (U+0000 to U+FFFF). However, if escaped, those characters must be written using UTF-16 surrogate pairs. For example, to include the Emoji character U+1F610 😐 NEUTRAL FACE in JSON:

{ "face": "😐" }
// or
{ "face": "\uD83D\uDE10" }

The latter is exactly what System.Text.Json does. And since YAML is a superset of JSON, I would expect any YAML implementation, such as YamlDotNet, to support such surrogate pairs.

[cmeeren]

Ideally I would like unescaped output (i.e., emojis in the YAML), but at least the following workaround lets me preserve the surrogate pair escape codes:

let unicodeEscapeCodePlaceholder = Guid.NewGuid().ToString()

let escapeUnicodeEscapeCodes (str: string) =
    str.Replace(@"\u", unicodeEscapeCodePlaceholder)

let unEscapeUnicodeEscapeCodes (str: string) =
    str.Replace(unicodeEscapeCodePlaceholder, @"\u")

let formatAsYaml json =
    let json = escapeUnicodeEscapeCodes json
    let yaml = (* load with YamlStream and transform to YAML *)
    unEscapeUnicodeEscapeCodes yaml

[EdwardCooke]

I definitely do agree that utf8 surrogate pairs should be supported. I’m hoping to have some time this weekend to look at it. I did find where it was throwing a fit though.

[cmeeren]

Happy to hear it! 😁

[ecooke-macu]

I've been doing a lot of research on this, through the unicode spec and all that. The surrogate pairs are a way of putting utf-16 and utf-32 characters in a utf-8 file. JSON does this by escaping them, in raw UTF-8 files, its done at the byte level (from my understanding). We just need to support the escaped version, just like the YAML spec shows (if I recall). It has a lot to deal with bit masking and what not, it's pretty complicated so it may take some time, but it may go quicker than I think. I'll let you know when I have a PR ready for it by linking it to this issue.

[cmeeren]

Thanks! I have no idea what the internals of YamlDotNet is doing, but wouldn't it work to just keep the loaded text as-is and not attempting to decode the escape sequences, which is seems like it's doing now?

[EdwardCooke]

This fix is going to be released in the latest nuget package which should be available in about 10-15 minutes.

[cmeeren]

It works. Thanks a lot! 😊