Concurrent Modification to currentLoader: Cannot invoke "java.lang.ClassLoader.getParent()" because "this.currentLoader" is null

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• Bug report 🪲
• Feature request 🏎
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Description

While executing the process-asciidoc goal on a project of mine, I encountered this error:

Caused by: java.lang.NullPointerException: Cannot invoke "java.lang.ClassLoader.getParent()" because "this.currentLoader" is null
    at java.util.ServiceLoader$ModuleServicesLookupIterator.hasNext (ServiceLoader.java:1075)
    at java.util.ServiceLoader$2.hasNext (ServiceLoader.java:1309)
    at java.util.ServiceLoader$3.hasNext (ServiceLoader.java:1393)
    at org.asciidoctor.jruby.log.internal.LogHandlerRegistryExecutor.registerAllLogHandlers (LogHandlerRegistryExecutor.java:19)
    at org.asciidoctor.jruby.internal.JRubyAsciidoctor.registerLogHandlers (JRubyAsciidoctor.java:112)
    at org.asciidoctor.jruby.internal.JRubyAsciidoctor.processRegistrations (JRubyAsciidoctor.java:92)
    at org.asciidoctor.jruby.internal.JRubyAsciidoctor.create (JRubyAsciidoctor.java:69)
    at org.asciidoctor.jruby.internal.JRubyAsciidoctor.create (JRubyAsciidoctor.java:65)
    at org.asciidoctor.jruby.AsciidoctorJRuby$Factory.create (AsciidoctorJRuby.java:29)
    at org.asciidoctor.maven.AsciidoctorMojo.getAsciidoctorInstance (AsciidoctorMojo.java:398)
    at org.asciidoctor.maven.AsciidoctorMojo.processSources (AsciidoctorMojo.java:214)
    at org.asciidoctor.maven.AsciidoctorMojo.processAllSources (AsciidoctorMojo.java:166)
    at org.asciidoctor.maven.AsciidoctorMojo.execute (AsciidoctorMojo.java:156)

After taking a look at the code that caused the error:

@Override
        public boolean hasNext() {
            while (nextProvider == null && nextError == null) {
                // get next provider to load
                while (!iterator.hasNext()) {
                    if (currentLoader == null) {
                        return false;
                    } else {
                        currentLoader = currentLoader.getParent();
                        iterator = iteratorFor(currentLoader);
                    }
                }

                // attempt to load provider
                ServiceProvider provider = iterator.next();
                try {
                    @SuppressWarnings("unchecked")
                    Provider<T> next = (Provider<T>) loadProvider(provider);
                    nextProvider = next;
                } catch (ServiceConfigurationError e) {
                    nextError = e;
                }
            }
            return true;
        }

it seems that there is an if statement that handles the case in which the currentLoader is null. And yet, currentLoader.getParent() failed with a null pointer exception. This makes me believe that the issue is related to multithreading.

Is it possible that this issue was handled in recent releases so we might need to upgrade to the latest release?

Environment information

• asciidoctor-maven-plugin version: 2.2.2
• asciidoctorj version: 2.5.3

• Maven, Java and OS version: 3.8.4

[abelsromero]

Is it possible that this issue was handled in recent releases so we might need to upgrade to the latest release?

Yes, if you can, I'd suggest to always use the latest 2.2.6 or 3.0.0. As well as the latest AsciidoctorJ v2.5.12, in fact the issue comes from AsciidoctorJ code, so try to upgrade it first.
If you decide to try maven-plugin v3, check the docs on how to migrate from v2.x.x https://docs.asciidoctor.org/maven-tools/latest/plugin/v3-migration-guide/.

[kevinnammour]

Hello @abelsromero, this error still occurs on asciidoctorj version > 2.5.11.

We upgraded the asciidoctor-maven-plugin to 2.26 that uses version 2.5.11 of asciidoctorj and the same failure still occurs.

asciidoctor-maven-plugin 3.0.0 also uses asciidoctorj 2.5.11 so it wouldn't fix the issue since the issue comes from asciidoctorj.

Is there any chance this is going to be fixed later down the road?

[abelsromero]

@kevinnammour I closed since there was no confirmation.

Is there any chance this is going to be fixed later down the road?

Sure thing! If it still happens, can I ask if you can provide a small reproducer? That would help a lot to find the issue.

[kevinnammour]

Hello @abelsromero, sorry for the late reply. I am not sure I would be able to send a small reproduction scenario. Our codeline is very large and there are almost ~120 references to this plugin. The failure rate is almost ~8%.

[abelsromero]

@kevinnammour thanks for the info. Do you confirm this is a flake then? Knowing that also helps, because the way to approach is different. Simply, if this is the case, I'd need to re-run scenarios to see if it fails.

[kevinnammour]

True it is flaky indeed. We also tried to lower the number of threads to build with (maybe it would reduce the concurrency errors' rate), and the failure rate is ~similar.