Zinc 1.0.0-RC3 memory and output improvements (#4807)

### Problem

While working through #4477, it became obvious that:
1. the performance achieved when the analysis cache is missed is pretty abysmal
2. a `log4j` JMX error was being logged during startup
3. the `forkJava` and `javaHome` settings are redundant: the only reason to pass `javaHome` would be to trigger forking Java
4. a large amount of unnecessary classloading was happening due to #4744

### Solution

1. Set the default `zinc.analysis.cache.limit` value to `Int.MaxValue`, which will allow users who want to cap the size to set it lower, but otherwise not lead to performance cliffs when a target has more dependencies than the current limit.
2. Implicitly disable log4j JMX usage by setting the `log4j2.disable.jmx` property if it is not already set.
3. Remove the `forkJava` setting, to prepare to use the `javaHome` setting explicitly in #4729
4. Enable usage of `ClassLoaderCache` 

### Result

Fixes #4744, and removes a few more blockers for #4729.

src/scala/org/pantsbuild/zinc/analysis/AnalysisMap.scala

@@ -134,7 +134,14 @@ class AnalysisMap private[AnalysisMap] (
 }
 
 object AnalysisMap {
-  private val analysisCacheLimit = Util.intProperty("zinc.analysis.cache.limit", 100)
+  // Because the analysis cache uses Weak references, bounding its size is generally
+  // counterproductive.
+  private val analysisCacheLimit =
+    Util.intProperty(
+      "zinc.analysis.cache.limit",
+      Int.MaxValue
+    )
+
   /**
    * Static cache for compile analyses. Values must be Options because in get() we don't yet
    * know if, on a cache miss, the underlying file will yield a valid Analysis.

src/scala/org/pantsbuild/zinc/compiler/CompilerCacheKey.scala

@@ -22,7 +22,6 @@ case class CompilerCacheKey(
   compilerBridgeSrc: File,
   compilerInterface: File,
   javaHome: Option[File],
-  forkJava: Boolean,
   cacheDir: File)
 
 object CompilerCacheKey {
@@ -40,7 +39,6 @@ object CompilerCacheKey {
       compilerBridgeSrc,
       compilerInterface,
       settings.javaHome,
-      settings.forkJava,
       settings.zincCacheDir
     )
   }
@@ -55,7 +53,6 @@ object CompilerCacheKey {
     compilerBridgeSrc: File,
     compilerInterface: File,
     javaHomeDir: Option[File],
-    forkJava: Boolean,
     cacheDir: File
   ): CompilerCacheKey = {
     val normalise: File => File = { _.getAbsoluteFile }
@@ -65,6 +62,6 @@ object CompilerCacheKey {
     val compilerBridgeJar    = normalise(compilerBridgeSrc)
     val compilerInterfaceJar = normalise(compilerInterface)
     val javaHome             = javaHomeDir map normalise
-    CompilerCacheKey(compilerJar, libraryJar, extraJars, compilerBridgeJar, compilerInterfaceJar, javaHome, forkJava, cacheDir)
+    CompilerCacheKey(compilerJar, libraryJar, extraJars, compilerBridgeJar, compilerInterfaceJar, javaHome, cacheDir)
   }
 }

src/scala/org/pantsbuild/zinc/compiler/CompilerUtils.scala

@@ -15,6 +15,7 @@ import sbt.internal.inc.{
   javac,
   ZincUtil
 }
+import sbt.internal.inc.classpath.ClassLoaderCache
 import sbt.io.Path
 import sbt.io.syntax._
 import sbt.util.Logger
@@ -59,6 +60,12 @@ object CompilerUtils {
       CompilerCache.createCacheFor(maxCompilers)
   }
 
+  /**
+   * Cache of classloaders: see https://github.com/pantsbuild/pants/issues/4744
+   */
+  private val classLoaderCache: Option[ClassLoaderCache] =
+    Some(new ClassLoaderCache(new URLClassLoader(Array())))
+
   /**
    * Get or create a zinc compiler based on compiler setup.
    */
@@ -86,9 +93,7 @@ object CompilerUtils {
       ZincCompilerUtil.constantBridgeProvider(instance, interfaceJar),
       ClasspathOptionsUtil.auto,
       _ => (),
-      // TODO: Should likely use the classloader cache here:
-      //   see https://github.com/pantsbuild/pants/issues/4744
-      None
+      classLoaderCache
     )
 
   /**

src/scala/org/pantsbuild/zinc/compiler/Main.scala

@@ -52,6 +52,10 @@ object Main {
   def printVersion(): Unit = println("%s (%s) %s" format (Command, Description, versionString))
 
   def mkLogger(settings: Settings) = {
+    // If someone has not explicitly enabled log4j2 JMX, disable it.
+    if (!Util.isSetProperty("log4j2.disable.jmx")) {
+      Util.setProperty("log4j2.disable.jmx", "true")
+    }
     val cl =
       ConsoleLogger(
         out = ConsoleOut.systemOut,

src/scala/org/pantsbuild/zinc/compiler/Settings.scala

@@ -41,7 +41,6 @@ case class Settings(
   scala: ScalaLocation              = ScalaLocation(),
   scalacOptions: Seq[String]        = Seq.empty,
   javaHome: Option[File]            = None,
-  forkJava: Boolean                 = false,
   _zincCacheDir: Option[File]       = None,
   javaOnly: Boolean                 = false,
   javacOptions: Seq[String]         = Seq.empty,
@@ -255,7 +254,6 @@ object Settings extends OptionSet[Settings] {
 
     header("Java options:"),
     file(      "-java-home", "directory",      "Select javac home directory (and fork)",     (s: Settings, f: File) => s.copy(javaHome = Some(f))),
-    boolean(   "-fork-java",                   "Run java compiler in separate process",      (s: Settings) => s.copy(forkJava = true)),
     string(    "-compile-order", "order",      "Compile order for Scala and Java sources",   (s: Settings, o: String) => s.copy(compileOrder = compileOrder(o))),
     boolean(   "-java-only",                   "Don't add scala library to classpath",       (s: Settings) => s.copy(javaOnly = true)),
     prefix(    "-C", "<javac-option>",         "Pass option to javac",                       (s: Settings, o: String) => s.copy(javacOptions = s.javacOptions :+ o)),

src/scala/org/pantsbuild/zinc/util/Util.scala

@@ -118,6 +118,14 @@ object Util {
   // Properties
   //
 
+  def setProperty(name: String, value: String): Unit = {
+    System.setProperty(name, value)
+  }
+
+  def isSetProperty(name: String): Boolean = {
+    System.getProperty(name) ne null
+  }
+
   /**
    * Create int from system property.
    */