Shut down the ExecutorService to properly clean up the thread

Because projects might reuse the Compiler instance for multiple compilations, set a new ExecutorService at the beginning of runInCompilerThread().

Should fix #1607

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=160981960

src/com/google/javascript/jscomp/Compiler.java

@@ -257,7 +257,7 @@ public SourceFile loadSource(String filename) {
       DiagnosticType.error("JSC_OPTIMIZE_LOOP_ERROR",
           "Exceeded max number of code motion iterations: {0}");
 
-  private final CompilerExecutor compilerExecutor = getCompilerExecutor();
+  private final CompilerExecutor compilerExecutor = new CompilerExecutor();
 
   /**
    * Logger for the whole com.google.javascript.jscomp domain -
@@ -3441,10 +3441,6 @@ private void renameModules(List<JSModule> newModules, List<JSModule> deserialize
     return;
   }
 
-  protected CompilerExecutor getCompilerExecutor() {
-    return new CompilerExecutor();
-  }
-
   /**
    * Serializable state of the compiler.
    */

src/com/google/javascript/jscomp/CompilerExecutor.java

@@ -30,13 +30,11 @@
 import java.util.concurrent.TimeoutException;
 
 /** Run the compiler in a separate thread with a larger stack */
-final class CompilerExecutor {
+class CompilerExecutor {
   // We use many recursive algorithms that use O(d) memory in the depth
   // of the tree.
   private static final long COMPILER_STACK_SIZE = (1 << 24); // About 16MB
 
-  private final ExecutorService compilerExecutor;
-
   /**
    * Use a dedicated compiler thread per Compiler instance.
    */
@@ -47,15 +45,6 @@ final class CompilerExecutor {
 
   private int timeout = 0;
 
-  CompilerExecutor() {
-    this(getDefaultExecutorService());
-  }
-
-  @GwtIncompatible("java.util.concurrent.ExecutorService")
-  CompilerExecutor(ExecutorService compilerExecutorService) {
-    this.compilerExecutor = compilerExecutorService;
-  }
-
   /**
    * Under JRE 1.6, the JS Compiler overflows the stack when running on some
    * large or complex JS code. When threads are available, we run all compile
@@ -65,7 +54,7 @@ final class CompilerExecutor {
    * (which is what -Xss does).
    */
   @GwtIncompatible("java.util.concurrent.ExecutorService")
-  static ExecutorService getDefaultExecutorService() {
+  ExecutorService getExecutorService() {
     return Executors.newSingleThreadExecutor(new ThreadFactory() {
         @Override
         public Thread newThread(Runnable r) {
@@ -86,6 +75,7 @@ void setTimeout(int timeout) {
 
   @SuppressWarnings("unchecked")
   <T> T runInCompilerThread(final Callable<T> callable, final boolean dumpTraceReport) {
+    ExecutorService executor = getExecutorService();
     T result = null;
     final Throwable[] exception = new Throwable[1];
 
@@ -118,14 +108,16 @@ public T call() {
           }
         };
 
-        Future<T> future = compilerExecutor.submit(bootCompilerThread);
+        Future<T> future = executor.submit(bootCompilerThread);
         if (timeout > 0) {
           result = future.get(timeout, TimeUnit.SECONDS);
         } else {
           result = future.get();
         }
       } catch (InterruptedException | TimeoutException | ExecutionException e) {
         throw new RuntimeException(e);
+      } finally {
+        executor.shutdown();
       }
     } else {
       try {

src/com/google/javascript/jscomp/gwt/super/com/google/javascript/jscomp/CompilerExecutor.java

@@ -20,9 +20,6 @@
 
 /** GWT compatible replacement for {@code CompilerExecutor} */
 final class CompilerExecutor {
-
-  CompilerExecutor() {}
-
   <T> T runInCompilerThread(Callable<T> callable, boolean dumpTraceReport) {
     try {
       return callable.call();