Emit Jelly file path inside <script> tags to get better CSP samples

[daniel-beck]

This makes it easier to identify the source of a CSP violation from <script> tags in Jelly/Groovy views: Instead of the first 30 characters, we get a substring of the file path.

Additional benefit: If this isn't present, the source is something else (like an adjunct, or browser extension).

This is not a prerequisite for jenkinsci/jenkins#11269 nor vice versa, but this would complement the core change well.

Testing done

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[Wadeck]

FTR from Daniel in Gitter:

It's a bit (very) limited given we're restricted to just 40 chars in Firefox at least, but perhaps this might end up useful?

[Wadeck]

nit suggestion: use //xxx instead of /*...*/ to save 2 characters as we seem to be lacking space

Daniel found the spec: https://www.w3.org/TR/CSP3/#violation-sample

A violation’s sample will be populated with the first 40 characters of an inline script, event handler, or style that caused an violation. Violations which stem from an external file will not include a sample in the violation report.

[daniel-beck]

save 2 characters as we seem to be lacking space

Seems less safe to do to me.

Also FTR I will need to confirm it's fine to not have //<![CDATA[ at the start of a block like this.

[daniel-beck]

Also FTR I will need to confirm it's fine to not have //<![CDATA[ at the start of a block like this.

Examples in https://developer.mozilla.org/en-US/docs/Web/API/CDATASection indicate <!CDATA[ can be put anywhere (and has no meaning in HTML), so inserting something before them should be fine.

[Kevin-CB]

I tested the PR, it works fine, except from my feedback on the line number, we're good to go there.

[Kevin-CB]

Suggested change

	"/*" + getFileName().substring(getFileName().length() - 33) + ":" + getLineNumber()
	"/*" + getFileName().substring(getFileName().length() - 31) + ":" + getLineNumber()

If we’re limited to 40 characters, I think we should reserve 4 for the line number to avoid mistakenly indicating the wrong line. Or at least 3, if you don’t think Jelly files with thousands of lines are that common. In any case, it’s better to be sure we indicate the correct line than to have one extra char at the start of the file name.