[Yaml ] Use double-quoted for DUMP_MULTI_LINE_LITERAL_BLOCK to allow more characters

[geek-merlin]

Background

Yaml is nice for dumping config in a human-readable and -editable format. E.g. Drupal uses it extensively.

In that context, dumping e.g. CSS as multiline is essential for human-readability.

As discovered in #25842, and confirmed on SO and yaml-multiline.info,

• Yaml multiline is not capable of handling arbitrary characters: escaped unicode characters like \u1234, also "\r" is excluded in current code
• Double-quoted iscapable of handling arbitrary escaped unicode values, and can span multiple lines

Researched thoroughly, so which are valid multiline literal characters:

• l-nb-literal-text = ...nb-char+
• nb-char = c-printable - b-char - c-byte-order-mark = #x9 | [#x20-#x7E] | #x85 | [#xA0-#xD7FF] | [#xE000-#xFFFD] - #xFEFF | [#x10000-#x10FFFF]

As of current code,
a) "\r" is excluded in current code, which means that any multiline string containing it will not be dumped as multiline, but as quoted scalar.
b) All other (non-printable) characters are invalid as multiline literal, so current code will produce invalid yaml.

Which means

• either we extend the "\r" blacklist with above, so to rather
• or switch to the following more robust format

Description

• Do multiline dumping like shown below (or see this fiddle). So we get multiline readability capable to encode every unicode character.

Example

dquote 0: "one\n\
  two (after every litaral newline, we add an escaped yaml newline)"
dquote 1: "one\n\
  two\n\
  "
dquote 2: "one\n\
  two\n\
  \n\
  "
dquote with initial space: "  one  \n\
    two  \n\
  \  as we see, trailing spaces are no problem, but we need to escape initial spaces on non-first line.  "

[geek-merlin]

OK some meta-comment on the lots of IS and title changes: While drilling deeper into the subject, my POV changed from OMG we must do this to This has some benefits WRT non-printable characters.

[carsonbot]

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[stof]

@xabbuh what do you think about this ?

[xabbuh]

This looks reasonable to me. @geek-merlin Would you like to give this a try?

[carsonbot]

Thank you for this suggestion.
There has not been a lot of activity here for a while. Would you still like to see this feature?

[geek-merlin]

No time to prioritize this currently, but yes still wanted.

[carsonbot]

Thank you for this suggestion.
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[andypost]

++ to fix it

[carsonbot]

Thank you for this suggestion.
There has not been a lot of activity here for a while. Would you still like to see this feature?

[geek-merlin]

Yes, a lot.

[carsonbot]

Thank you for this suggestion.
There has not been a lot of activity here for a while. Would you still like to see this feature?

[geek-merlin]

Alot...

[carsonbot]

Thank you for this suggestion.
There has not been a lot of activity here for a while. Would you still like to see this feature?

[geek-merlin]

Yupp