While working on ReportOnlyTest recently, a hitherto unnoticed stack trace was noticed in a log file from running jtreg.

Directory ".../open/build/test/Volumes/J2GWork/jtreg/jtreg.dev/open/build/test/ReportOnlyTest" not found: creating
Test results: passed: 93; failed: 44; error: 88
Error: Unexpected exception occurred! java.lang.StringIndexOutOfBoundsException: String index out of range: -1
java.lang.StringIndexOutOfBoundsException: String index out of range: -1
        at java.base/java.lang.StringLatin1.charAt(StringLatin1.java:47)
        at java.base/java.lang.String.charAt(String.java:693)
        at com.sun.javatest.InitialUrlFilter.isInitialUrlMatch(InitialUrlFilter.java:133)
        at com.sun.javatest.InitialUrlFilter.accepts(InitialUrlFilter.java:159)
        at com.sun.javatest.ParameterFilter.accepts(ParameterFilter.java:149)
        at com.sun.javatest.ParameterFilter.accepts(ParameterFilter.java:96)
        at com.sun.javatest.TRT_Iterator.wouldAccept(TRT_Iterator.java:914)
        at com.sun.javatest.TRT_Iterator.findNext(TRT_Iterator.java:752)
        at com.sun.javatest.TRT_Iterator.init(TRT_Iterator.java:697)
        at com.sun.javatest.TRT_Iterator.<init>(TRT_Iterator.java:157)
        at com.sun.javatest.TestResultTable.getIterator(TestResultTable.java:1206)
        at com.sun.javatest.TestResultTable.getIterator(TestResultTable.java:1116)
        at com.sun.javatest.report.ReportSettings.setupSortedResults(ReportSettings.java:523)
        at com.sun.javatest.report.Report.writeReports(Report.java:177)
        at com.sun.javatest.regtest.report.RegressionReporter.report(RegressionReporter.java:106)
        at com.sun.javatest.regtest.tool.Tool.batchHarness(Tool.java:1992)
        at com.sun.javatest.regtest.tool.Tool.run(Tool.java:1338)
        at com.sun.javatest.regtest.tool.Tool.run(Tool.java:1082)
        at com.sun.javatest.regtest.tool.Tool.main(Tool.java:155)
        at com.sun.javatest.regtest.Main.main(Main.java:46)


The immediate cause was an empty string for a test file being passed down into JT Harness from jtreg,

The root cause is a bad check in `TestManager`, comparing the wrong form of a path (relative, not canonical) against the canonical file for the test suite root. This indirectly leads to computing an empty relative path between the two, causing the issue.