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Fallback on CompilerClassLoader
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‎sbt-bridge/src/xsbt/CompilerClassLoader.java

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package xsbt;
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import java.lang.reflect.Field;
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import java.net.URL;
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import java.net.URLClassLoader;
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import java.util.WeakHashMap;
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/**
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* A classloader to run the compiler
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* <p>
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* A CompilerClassLoader is constructed from a list of `urls` that need to be on
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* the classpath to run the compiler and the classloader used by sbt.
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* <p>
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* To understand why a custom classloader is needed for the compiler, let us
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* describe some alternatives that wouldn't work.
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* <ul>
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* <li>`new URLClassLoader(urls)`:
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* The compiler contains sbt phases that callback to sbt using the `xsbti.*`
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* interfaces. If `urls` does not contain the sbt interfaces we'll get a
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* `ClassNotFoundException` in the compiler when we try to use them, if
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* `urls` does contain the interfaces we'll get a `ClassCastException` or a
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* `LinkageError` because if the same class is loaded by two different
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* classloaders, they are considered distinct by the JVM.
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* <li>`new URLClassLoader(urls, sbtLoader)`:
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* Because of the JVM delegation model, this means that we will only load
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* a class from `urls` if it's not present in the parent `sbtLoader`, but
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* sbt uses its own version of the scala compiler and scala library which
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* is not the one we need to run the compiler.
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* </ul>
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* <p>
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* Our solution is to implement a subclass of URLClassLoader with no parent, instead
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* we override `loadClass` to load the `xsbti.*` interfaces from `sbtLoader`.
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*/
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public class CompilerClassLoader extends URLClassLoader {
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private final ClassLoader sbtLoader;
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public CompilerClassLoader(URL[] urls, ClassLoader sbtLoader) {
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super(urls, null);
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this.sbtLoader = sbtLoader;
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}
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@Override
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public Class<?> loadClass(String className, boolean resolve) throws ClassNotFoundException {
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if (className.startsWith("xsbti.")) {
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// We can't use the loadClass overload with two arguments because it's
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// protected, but we can do the same by hand (the classloader instance
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// from which we call resolveClass does not matter).
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Class<?> c = sbtLoader.loadClass(className);
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if (resolve)
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resolveClass(c);
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return c;
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} else {
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return super.loadClass(className, resolve);
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}
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}
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/**
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* Cache the result of `fixBridgeLoader`.
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* <p>
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* Reusing ClassLoaders is important for warm performance since otherwise the
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* JIT code cache for the compiler will be discarded between every call to
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* the sbt `compile` task.
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*/
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private static WeakHashMap<ClassLoader, ClassLoader> fixedLoaderCache = new WeakHashMap<>();
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/**
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* Fix the compiler bridge ClassLoader
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* <p>
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* Soundtrack: https://www.youtube.com/watch?v=imamcajBEJs
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* <p>
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* The classloader that we get from sbt looks like:
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* <p>
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* URLClassLoader(bridgeURLs,
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* DualLoader(scalaLoader, notXsbtiFilter, sbtLoader, xsbtiFilter))
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* <p>
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* DualLoader will load the `xsbti.*` interfaces using `sbtLoader` and
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* everything else with `scalaLoader`. Once we have loaded the dotty Main
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* class using `scalaLoader`, subsequent classes in the dotty compiler will
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* also be loaded by `scalaLoader` and _not_ by the DualLoader. But the sbt
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* compiler phases are part of dotty and still need access to the `xsbti.*`
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* interfaces in `sbtLoader`, therefore DualLoader does not work for us
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* (this issue is not present with scalac because the sbt phases are
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* currently defined in the compiler bridge itself, not in scalac).
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* <p>
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* CompilerClassLoader is a replacement for DualLoader. Until we can fix
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* this in sbt proper, we need to use reflection to construct our own
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* fixed classloader:
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* <p>
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* URLClassLoader(bridgeURLs,
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* CompilerClassLoader(scalaLoader.getURLs, sbtLoader))
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*
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* @param bridgeLoader The classloader that sbt uses to load the compiler bridge
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* @return A fixed classloader that works with dotty
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*/
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synchronized public static ClassLoader fixBridgeLoader(ClassLoader bridgeLoader) {
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return fixedLoaderCache.computeIfAbsent(bridgeLoader, k -> computeFixedLoader(k));
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}
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private static ClassLoader computeFixedLoader(ClassLoader bridgeLoader) {
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URLClassLoader urlBridgeLoader = (URLClassLoader) bridgeLoader;
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ClassLoader dualLoader = urlBridgeLoader.getParent();
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Class<?> dualLoaderClass = dualLoader.getClass();
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try {
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// DualLoader.parentA and DualLoader.parentB are private
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Field parentAField = dualLoaderClass.getDeclaredField("parentA");
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parentAField.setAccessible(true);
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Field parentBField = dualLoaderClass.getDeclaredField("parentB");
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parentBField.setAccessible(true);
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URLClassLoader scalaLoader = (URLClassLoader) parentAField.get(dualLoader);
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ClassLoader sbtLoader = (ClassLoader) parentBField.get(dualLoader);
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URL[] bridgeURLs = urlBridgeLoader.getURLs();
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return new URLClassLoader(bridgeURLs,
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new CompilerClassLoader(scalaLoader.getURLs(), sbtLoader));
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} catch (NoSuchFieldException | IllegalAccessException e) {
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throw new RuntimeException(e);
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}
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}
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}

‎sbt-bridge/src/xsbt/CompilerInterface.java

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import xsbti.Reporter;
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import xsbti.compile.*;
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import dotty.tools.dotc.Main;
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import java.lang.reflect.InvocationTargetException;
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import java.io.File;
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/**
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* The new compiler interface is [[dotty.tools.xsbt.CompilerBridge]] that extends the new `xsbti.CompilerInterface2`.
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* This interface is kept for compatibility with Mill and the sbt 1.3.x series.
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*/
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public final class CompilerInterface {
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public CachedCompiler newCompiler(String[] options, Output output, Logger initialLog, Reporter initialDelegate) {
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return new CachedCompilerImpl(options, output);
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public CachedCompiler newCompiler(String[] options, Output output, Logger initialLog, Reporter initialDelegate) throws java.lang.Exception {
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if(isClassLoaderValid()) {
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return new CachedCompilerImpl(options, output);
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} else {
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initialLog.warn(() ->
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"The compiler class loader is badly configured.\n" +
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"Consider using a more recent version of your build tool:\n" +
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" - sbt >= 1.4.0 and sbt-dotty >= 1.5.0\n" +
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" - Mill >= 0.9.3-21-002361\n" +
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" - Bloop >= 1.4.6-23-20a501bc"
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);
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// To workaround the wrong class loader, we construct our own and run
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// the following code with it:
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// new CachedCompilerImpl(options, output)
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try {
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ClassLoader bridgeLoader = this.getClass().getClassLoader();
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ClassLoader fixedLoader = CompilerClassLoader.fixBridgeLoader(bridgeLoader);
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Class<?> cciClass = fixedLoader.loadClass("xsbt.CachedCompilerImpl");
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return (CachedCompiler) cciClass.getConstructors()[0].newInstance(options, output);
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} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | InvocationTargetException e) {
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throw new RuntimeException(e);
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}
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}
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}
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private boolean isClassLoaderValid() {
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// Check that the `xsbti.*` classes are loaded from the same class loader in the compiler and the bridge
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ClassLoader compilerClassLoader = Main.class.getClassLoader();
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Class<Logger> clazz = Logger.class;
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try {
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return compilerClassLoader.loadClass("xsbti.Logger") == clazz;
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} catch (ClassNotFoundException e) {
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throw new RuntimeException(e);
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}
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}
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public void run(File[] sources, DependencyChanges changes, AnalysisCallback callback, Logger log,

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