:@ reported as an invalid plain value in some flow contexts

[cemerick]

Describe the bug
The character sequence :@ is a valid flow plain value, but is reported as invalid in some contexts.

Other sequences involving characters that are barred from starting a plain scalar are also reported as invalid, including `, %, and probably others.

To Reproduce

yaml.parseDocument("[[], :@]")

The reported error is Plain value cannot start with reserved character @ at line 1, column 7:\n\n[[], :@]\n ^\n

Expected behaviour

A successful parse, with document contents of a sequence with two members (one empty sequence, and the string scalar ":@").

Versions (please complete the following information):

• Environment: node v10.20.1
• yaml: yaml@2.0.0-8

Additional context
I found mixed performance on this example (libfyaml and libyaml both parse successfully, but pyyaml and others do not), so I asked in the main YAML matrix chat. ingy confirmed that the input text is valid yaml, and suggested filing an issue on the yaml test suite, which I will be doing shortly.

[cemerick]

ingy further suggested that this might be a regression, and so it is: with yaml@1.10.2, the repro in the report here works as expected.

[ingydotnet]

See https://spec.yaml.io/main/playground/javascript?input=W1tdLCA6QF0=

[ingydotnet]

Discussion happened in https://matrix.to/#/#chat:yaml.io so read that too...

[eemeli]

Thank you for spotting this! This was a lexer bug: a flag for tracking if a flow-collection key is a json-node was not being reset after the , character. The first [] set this flag, and then when parsing :@ it was still mistakenly on, leading to the : to be considered as a separator that does not need a subsequent space, and hence ending up with a mis-parsed value of @, i.e. a reserved character. And that's then the error that you actually saw.

[cemerick]

I'm glad the report was helpful.

FYI, I've upgraded my installation to master, and I can confirm the fix is effective. I really appreciate the super-fast turnaround, thank you for the great library. 😃