[BUG] util:get-fragment-between returns unparsable XML when attribute contains square brackets

[joewiz]

Describe the bug

When calling util:get-fragment-between() on a tree that contains an attribute with square brackets, the function returns a string that cannot be parsed.

Expected behavior

I expected the function to return parseable XML, as the source is valid XML.

For example, given a document, test.xml:

<root foo="[bar]">
    <x/>
    <y/>
</root>

... then the following query:

let $doc := doc("/db/test.xml")/root
let $beginning-node := $doc/x
let $ending-node := $doc/y
let $make-fragment := true()
let $display-root-namespace := true()
return
    util:get-fragment-between($beginning-node, $ending-node, $make-fragment, $display-root-namespace)

... will return a string that cannot be parsed as XML:

<root foo=" bar">
<x></x>
    </root"]>

Notice that:

1. The attribute doesn't match the original (" bar" instead of "[bar]")
2. The close tag is mangled (</root"]> instead of </root>)

To Reproduce

The following test extends tests already in https://github.com/eXist-db/exist/blob/develop/exist-core/src/test/xquery/util/get-fragment-between.xqm (added in #3328).

xquery version "3.1";

module namespace gfb = "http://exist-db.org/test/util/get-fragment-between";

declare namespace test = "http://exist-db.org/xquery/xqsuite";

declare variable $gfb:DOC3 :=
    <root foo="[bar]" xmlns="http://exist-db.org/xquery/xqsuite">
        <x/>
        <y/>
        <z/>
    </root>;

declare
    %test:setUp
function gfb:setup() {
    xmldb:create-collection("/db", "test-gfb"),
    xmldb:store("/db/test-gfb", "doc3.xml", $gfb:DOC3)
};

declare
    %test:tearDown
function gfb:teardown() {
    xmldb:remove("/db/test-gfb")
};

declare
    %test:assertTrue
function gfb:wrapped-fragment-with-bracked-in-attribute-is-parseable() {
    let $doc := doc("/db/test-gfb/doc3.xml")
    let $elems := $doc/test:root/*
    let $fragment := util:get-fragment-between($elems[1], $elems[2], true(), false())
    let $parsed := try { parse-xml($fragment) } catch * { $err:code }
    return
        $parsed instance of document-node(element(root))
};

The test suite returns:

<testcase name="wrapped-fragment-with-bracked-in-attribute-is-parseable"
    class="gfb:wrapped-fragment-with-bracked-in-attribute-is-parseable">
    <failure message="assertTrue failed." type="failure-error-code-1"/>
    <output>false</output>
</testcase>

... meaning that the result of util:get-fragment-between() can't be parsed.

Context (please always complete the following information):

• OS: macOS 11.1
• eXist-db version: eXist 4.7.1 and 5.3.0-SNAPSHOT 29a5330 20201227092937
• Java Version: 1.8.0_275

Additional context

• How is eXist-db installed? built from source
• Any custom changes in e.g. conf.xml? none