Change XPath available function check from denylist to allowlist

[tompng]

Define an allowlist of available xpath functions Instead of listing up all internal methods which isn't xpath function. REXML::FunctionsClass now strictly checks function names: local-name is allowed but local_name is rejected.
The compatibility layer that accepts local_name is in the call site.

Why allowlist?

Currently, REXML dropped/changed the way to add custom XPath functions in #329 that I opened.
I don't know if REXML officially supported this, it may be just a monkey patch, but perhaps it's an accidental breaking change.

# Add custom_func and custom-func

# REXML 3.4.4
def (REXML::Functions).custom_func() = 42

# Master branch (after #329)
class REXML::FunctionsClass; def custom_func() = 42; end

# After this PR
$VERBOSE = nil # or remove_const
REXML::FunctionsClass::AVAILABLE_FUNCTIONS += %w[custom-func custom_func]

If we're going to drop it forever, I think static allowlist in this PR is the simplest way.
If REXML needs to re-support custom XPath function with the old style: def (REXML::Functions).custom_func() = 42 , something needs to be done before releasing the next version. But I don't think it's easy to do it in a thread-safe way especially when the custom function touches @context.

About fallback: nil

def call(name, args:, fallback: nil) can't distinguish nil fallback from unspecified fallback but it's OK.
Fallback should be either bool, number, string, or NodeSet. nil is not allowed as a value of an expression of XPath.

[Copilot]

🟢 Approval recommended

The changes are localized, internally consistent (parser + functions + tests), and the updated tests directly cover the new allowlist and compatibility behavior.

Pull request overview

This PR tightens XPath function dispatch in REXML by switching REXML::FunctionsClass from a denylist-based “anything except internal helpers” check to a strict allowlist of recognized XPath functions, while keeping backward-compatible underscore aliases at the call site.

Changes:

• Introduces REXML::FunctionsClass::AVAILABLE_FUNCTIONS (a Set) and routes function invocation through a new FunctionsClass#call allowlist gate.
• Updates XPathParser to invoke XPath functions via #call (with a compatibility normalization layer and fallback behavior).
• Adjusts/extends tests to validate the allowlist, strictness for underscored names via #call, and compatibility via #send.

File summaries

File	Description
test/functions/test_base.rb	Updates tests to assert the allowlist contents and stricter function-name handling while preserving compatibility paths.
lib/rexml/xpath_parser.rb	Switches function evaluation from send to FunctionsClass#call, providing underscore-to-hyphen compatibility and fallback behavior.
lib/rexml/functions.rb	Adds AVAILABLE_FUNCTIONS allowlist and implements #call as the strict dispatch mechanism, with #send retained as a compatibility wrapper.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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