A single string scalar as output is not quoted if necessary (breaking roundtrip safety)

[sebastianriese]

Describe the bug

If the output from yq is a single string, then it will not be quoted properly in all cases.

If the string's content is the valid YAML encoding of something else (e.g. an integer, a map or a sequence), the output YAML will not encode that string, but the data encoded by that string when interpreted as a YAML value.

In other words: yq fails to detect if encoding a single string scalar as output, whether this string needs to be quoted. You could also say it treats a string scalar output as if you have given the -r flag in all cases.

Version of yq: 4.53.2
Operating system: linux
Installed via: docker

Input Yaml
Concise yaml document(s) (as simple as possible to show the bug, please keep it to 10 lines or less)
data1.yml:

"this: should really work"

Command
The command you ran:

yq eval . data1.yaml

Actual behaviour

this: should really work

This should only happen, if we called yq with the -r option to enable raw output of scalars.

Expected behaviour

"this: should really work"

Additional context

The problem does not depend on the input being top-level, but only on the output being a string scalar. It occurs the same for:

data2.yaml

foo: |
  a: a
  b: b

yq .foo data2.yaml

Which yields the output

a: a
b: b

It also happens if the output is a stream documents (if one of the documents is just a string scalar).

I could not reproduce this behaviour for other cases (such as when returning a sequence or map) and I haven't looked at the code yet to see whether this could be triggered in other cases.