Race condition in JavacPlugin

JavacPlugin.loadJavacParserClass() checks if the classloader contains ManJavacParser, and if not, adds it.

When building with SBT, this method is invoked multiple times, sometimes concurrently, which can result in an “attempted duplicate class definition” error:

java.lang.LinkageError: loader 'app' (instance of jdk.internal.loader.ClassLoaders$AppClassLoader) attempted duplicate class definition for com.sun.tools.javac.parser.ManJavacParser.
    at java.base/java.lang.ClassLoader.defineClass1(Native Method)
    ...

It looks like loadJavacParserClass() should be acquiring a lock first. Adding a synchronized(classLoader) {...} around the body of this method appears to prevent the error.

[rsmckinney]

@iadcode I don’t have any experience with SBT, however using the class lock is probably okay for this use-case. FWIW, SBT is the first reported case of this kind for Manifold. It could be that this issue exists in Maven and/or Gradle too, but that no one uses Manifold in parallel builds. I’ll experiment.

Btw, assuming your project uses Scala, the next Manifold release will support Scala and Kotlin as well as Android. It’s almost there.

[rsmckinney]

Fixed in release 2020.1.7