[BUG] Non-deterministic test failures in fn-matches.re

[alanpaxton]

Describe the bug

Some number of tests in this test set fail (fn-matches.re) when the suite is run as a whole.

re00063	Failure	Expected: 'AssertTrue', but query returned an error: QueryError(FORX0002,err:FORX0002 Invalid regular expression: Syntax error at char 9 in regular expression: Unknown Unicode block: BasicLatin [at line 1, column 43, source: String/3859996392143586125])junit.framework.AssertionFailedError: Expected: 'AssertTrue', but query returned an error: QueryError(FORX0002,err:FORX0002 Invalid regular expression: Syntax error at char 9 in regular expression: Unknown Unicode block: BasicLatin [at line 1, column 43, source: String/3859996392143586125])at org.exist.xqts.runner.JUnitResultsSerializerActor.$anonfun$formatJunitTestSet$6(JUnitResultsSerializerActor.scala:147)at org.exist.xqts.runner.JUnitResultsSerializerActor.$anonfun$formatJunitTestSet$6$adapted(JUnitResultsSerializerActor.scala:134)at scala.collection.immutable.List.foreach(List.scala:333)at org.exist.xqts.runner.JUnitResultsSerializerActor.$anonfun$formatJunitTestSet$1(JUnitResultsSerializerActor.scala:134)at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)at cats.effect.IOFiber.runLoop(IOFiber.scala:262)at cats.effect.IOFiber.execR(IOFiber.scala:1319)at cats.effect.IOFiber.run(IOFiber.scala:118)at cats.effect.unsafe.WorkerThread.run(WorkerThread.scala:555)	0.007
re00061	Failure	Expected: 'AssertTrue', but query returned an error: QueryError(FORX0002,err:FORX0002 Invalid regular expression: Syntax error at char 9 in regular expression: Unknown Unicode block: BasicLatin [at line 1, column 43, source: String/-8379167466134022914])junit.framework.AssertionFailedError: Expected: 'AssertTrue', but query returned an error: QueryError(FORX0002,err:FORX0002 Invalid regular expression: Syntax error at char 9 in regular expression: Unknown Unicode block: BasicLatin [at line 1, column 43, source: String/-8379167466134022914])at org.exist.xqts.runner.JUnitResultsSerializerActor.$anonfun$formatJunitTestSet$6(JUnitResultsSerializerActor.scala:147)at org.exist.xqts.runner.JUnitResultsSerializerActor.$anonfun$formatJunitTestSet$6$adapted(JUnitResultsSerializerActor.scala:134)at scala.collection.immutable.List.foreach(List.scala:333)at org.exist.xqts.runner.JUnitResultsSerializerActor.$anonfun$formatJunitTestSet$1(JUnitResultsSerializerActor.scala:134)at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)at cats.effect.IOFiber.runLoop(IOFiber.scala:262)at cats.effect.IOFiber.execR(IOFiber.scala:1319)at cats.effect.IOFiber.run(IOFiber.scala:118)at cats.effect.unsafe.WorkerThread.run(WorkerThread.scala:555)

Expected behavior
All tests should pass.

To Reproduce

The entry in my VSCode launch.json
{
"type": "scala",
"request": "launch",
"name": "XQTS fn:matches.re",
"mainClass": "org.exist.xqts.runner.XQTSRunner",
"args": ["--xqts-version", "HEAD", "--test-set", "fn-matches.re"],
"jvmOptions": [],
"env": {}
},

For clarity I have been testing against the code at eXist-db/exist#4900, but the behaviour does not seem
dependent on the change.

All (intermittently failing) tests succeed when run individually:

    {
        "type": "scala",
        "request": "launch",
        "name": "XQTS fn:matches.re-single",
        "mainClass": "org.exist.xqts.runner.XQTSRunner",
        "args": ["--xqts-version", "HEAD", "--test-set", "fn-matches.re", "--test-case", "re00063"],
        "jvmOptions": [],
        "env": {}
    },

(note that re00063 is one that often fails in this context)

Context (please always complete the following information):

• OS: macOS 13.3.1
• eXist-db Version: 7-0-0.SNAPSHOT (see above for details)
• Java Version: java version "17.0.6" 2023-01-17 LTS
• App Version: commit 21abfbb (1.4.0-SNAPSHOT)

Additional context

• How is eXist-db installed? Development build
• Any custom changes - it's complicated
• This ticket is primarily intended as a record of the behaviour for future reference; it's not impossible that the problem is truly a genuine concurrent/non-deterministic issue in Saxon.