Add CSP-compliant variant of undocumented validateButton script feature

[daniel-beck]

Fixes #18688

With CSP protection enabled (https://plugins.jenkins.io/csp/ or #11269), so that script-src does not allow 'unsafe-inline', it is not possible to regenerate a legacy API token. In new instances, this token has been disabled by default for many years.

The reason is a seemingly undocumented feature of f:validateButton: If you provide an inline script in the script HTTP response header to the validation method, that script is executed:

jenkins/war/src/main/webapp/scripts/hudson-behavior.js

Lines 2639 to 2644 in 110014c

	var s = rsp.headers.get("script");
	try {
	geval(s);
	} catch (e) {
	window.alert("failed to evaluate " + s + "\n" + e.message);
	}

There seem to be just two plugins (gitlab-plugin and gitee) and this core functionality using this feature.

This PR adds a new, equally undocumented feature: Set the X-Jenkins-ValidateButton-Callback HTTP response header to a JSON object with the String callback and the list arguments to have a global function of that name be invoked with those arguments.

Testing done

Manually generated a token while CSP plugin was installed and "Report Only" was unchecked, it worked as if there was no CSP enforcement.

Proposed changelog entries

• Bugfix: Make "Change API Token" for legacy API tokens work when Content Security Policy is enforced and prohibits inline JavaScript.
• Developer: Add X-Jenkins-ValidateButton-Callback (a JSON object with callback and arguments keys) as a replacement for the CSP-incompatible script HTTP response header for f:validateButton.

Proposed changelog category

/label

Proposed upgrade guidelines

N/A

Submitter checklist

• The Jira issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests.
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
• UI changes do not introduce regressions when enforcing the current default rules of Content Security Policy Plugin. In particular, new or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
• For dependency updates, there are links to external changelogs and, if possible, full differentials.
• For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
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[timja]

would be better for this to be removed, but if it works good for now

[timja]

/label ready-for-merge

---

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!

[timja]

(linked #18688 to close on merge)

[daniel-beck]

would be better for this to be removed, but if it works good for now

FWIW I've changed both plugins' PRs to not use either iteration of this feature. This way no new Jenkins core dependency is required for them either. So the only remaining known use of this feature would be in core.