idempotent change propagation

using lastModified instead of a non-idempotent needsUpdate flag
this fixes a bug where dependees would not be rebuilt if cbt exited
or was killed after dependencies were already rebuilt.

7 files changed, 245 insertions, 150 deletions

diff --git a/nailgun_launcher/BuildStage1Result.java b/nailgun_launcher/BuildStage1Result.java
index 312871d..64a660d 100644
--- a/nailgun_launcher/BuildStage1Result.java
+++ b/nailgun_launcher/BuildStage1Result.java
@@ -1,12 +1,14 @@
 package cbt;
 public class BuildStage1Result{
-  public Boolean changed;
+  public long start;
+  public long stage1LastModified;
   public ClassLoader classLoader;
   public String stage1Classpath;
   public String nailgunClasspath;
   public String compatibilityClasspath;
-  public BuildStage1Result( Boolean changed, ClassLoader classLoader, String stage1Classpath, String nailgunClasspath, String compatibilityClasspath ){
-    this.changed = changed;
+  public BuildStage1Result( long start, long stage1LastModified, ClassLoader classLoader, String stage1Classpath, String nailgunClasspath, String compatibilityClasspath ){
+    this.start = start;
+    this.stage1LastModified = stage1LastModified;
     this.classLoader = classLoader;
     this.stage1Classpath = stage1Classpath;
     this.nailgunClasspath = nailgunClasspath;
diff --git a/nailgun_launcher/CbtURLClassLoader.java b/nailgun_launcher/CbtURLClassLoader.java
index 43d07f4..fac7050 100644
--- a/nailgun_launcher/CbtURLClassLoader.java
+++ b/nailgun_launcher/CbtURLClassLoader.java
@@ -15,7 +15,7 @@ public class CbtURLClassLoader extends java.net.URLClassLoader{
       + "\n)"
     );
   }
-  JavaCache<Class> cache = new JavaCache<Class>( new ConcurrentHashMap<Object,Object>() );
+  ConcurrentHashMap<String,Class> cache = new ConcurrentHashMap<String,Class>();
   public Class loadClass(String name) throws ClassNotFoundException{
     Class _class = super.loadClass(name);
     if(_class == null) throw new ClassNotFoundException(name);
@@ -24,11 +24,11 @@ public class CbtURLClassLoader extends java.net.URLClassLoader{
   public Class loadClass(String name, Boolean resolve) throws ClassNotFoundException{
     //System.out.println("loadClass("+name+") on \n"+this);
     synchronized( cache ){
-      if(!cache.contains(name))
+      if(!cache.containsKey(name))
         try{
-          cache.put(super.loadClass(name, resolve), name);
+          cache.put(name, super.loadClass(name, resolve));
         } catch (ClassNotFoundException e){
-          cache.put(Object.class, name);
+          cache.put(name, Object.class);
         }
       Class _class = cache.get(name);
       if(_class == Object.class){
diff --git a/nailgun_launcher/ClassLoaderCache.java b/nailgun_launcher/ClassLoaderCache.java
new file mode 100644
index 0000000..6bffad0
--- /dev/null
+++ b/nailgun_launcher/ClassLoaderCache.java
@@ -0,0 +1,66 @@
+package cbt;
+
+import java.util.*;
+import static java.io.File.pathSeparator;
+import static cbt.Stage0Lib.*;
+
+final public class ClassLoaderCache{
+  public Map<Object,Object> hashMap;
+  final ThreadLocal<HashSet<String>> seen = new ThreadLocal<HashSet<String>>(){
+    @Override protected HashSet<String> initialValue(){
+      return new HashSet<String>();
+    }
+  };
+
+  public ClassLoaderCache(
+    Map<Object,Object> hashMap
+  ){
+    this.hashMap = hashMap;
+  }
+
+  public ClassLoader get( String key, long timestamp ){
+    seen.get().add( key );
+    @SuppressWarnings("unchecked")
+    ClassLoader t = (ClassLoader) hashMap.get(
+      hashMap.get( key )
+    );
+    assert hashMap.get(t).equals(timestamp);
+    return t;
+  }
+
+  public boolean containsKey( String key, long timestamp ){
+    boolean contains = hashMap.containsKey( key );
+    if( contains ){
+      Object keyObject = hashMap.get( key );
+      Object classLoader = hashMap.get( keyObject );
+      long oldTimestamp = (long) hashMap.get( classLoader );
+      boolean res = oldTimestamp == timestamp;
+      return res;
+    } else {
+      return false;
+    }
+  }
+
+  public void put( String key, ClassLoader value, long timestamp ){
+    assert !seen.get().contains( key ): "Thread tries to update cache key after observing it: " + key;
+    LockableJavaKey keyObject = new LockableJavaKey();
+    hashMap.put( key, keyObject );
+    hashMap.put( keyObject, value );
+    hashMap.put( value, timestamp );
+  }
+
+  @Override public String toString(){
+    StringBuilder res = new StringBuilder();
+    res.append("ClassLoaderCache(\n\n");
+    for( Object key: hashMap.keySet() ){
+      if( key instanceof String )
+        res.append(
+          join( "\n", key.toString().split(":") ) + " -> " + hashMap.get( hashMap.get(key) )
+          + "\n\n"
+        );
+    }
+    res.append("\n\n");
+    return res.toString();
+  }
+}
+class LockableJavaKey{}
diff --git a/nailgun_launcher/EarlyDependencies.java b/nailgun_launcher/EarlyDependencies.java
index fdb54b5..8709e69 100644
--- a/nailgun_launcher/EarlyDependencies.java
+++ b/nailgun_launcher/EarlyDependencies.java
@@ -4,6 +4,7 @@ import java.io.*;
 import java.nio.file.*;
 import java.net.*;
 import java.security.*;
+import java.util.*;
 import static cbt.Stage0Lib.*;
 import static cbt.NailgunLauncher.*;
 
@@ -28,7 +29,7 @@ class EarlyDependencies{
   String scalaLibrary_2_10_6_File;
 
   public EarlyDependencies(
-    String mavenCache, String mavenUrl, JavaCache<ClassLoader> classLoaderCache, ClassLoader rootClassLoader
+    String mavenCache, String mavenUrl, ClassLoaderCache classLoaderCache, ClassLoader rootClassLoader
   ) throws Throwable {
     scalaReflect_2_11_8_File = mavenCache + "/org/scala-lang/scala-reflect/2.11.8/scala-reflect-2.11.8.jar";
     scalaCompiler_2_11_8_File = mavenCache + "/org/scala-lang/scala-compiler/2.11.8/scala-compiler-2.11.8.jar";
@@ -46,95 +47,104 @@ class EarlyDependencies{
     download(new URL(mavenUrl + "/org/scala-lang/scala-compiler/2.11.8/scala-compiler-2.11.8.jar"), Paths.get(scalaCompiler_2_11_8_File), "fe1285c9f7b58954c5ef6d80b59063569c065e9a");
 
     // org.scala-lang:scala-library:2.10.6
-    download(new URL(mavenUrl + "/org/scala-lang/scala-library/2.10.6/scala-library-2.10.6.jar"), Paths.get(scalaLibrary_2_10_6_File), "421989aa8f95a05a4f894630aad96b8c7b828732");
-
     String[] scalaLibrary_2_10_6_ClasspathArray = new String[]{scalaLibrary_2_10_6_File};
-    String scalaLibrary_2_10_6_Classpath = classpath( scalaLibrary_2_10_6_ClasspathArray );
-    ClassLoader scalaLibrary_2_10_6_ =
-      classLoaderCache.contains( scalaLibrary_2_10_6_Classpath )
-      ? classLoaderCache.get( scalaLibrary_2_10_6_Classpath )
-      : classLoaderCache.put( classLoader( scalaLibrary_2_10_6_File, rootClassLoader ), scalaLibrary_2_10_6_Classpath );
+    ClassLoader scalaLibrary_2_10_6_ = loadDependency(
+      mavenUrl + "/org/scala-lang/scala-library/2.10.6/scala-library-2.10.6.jar",
+      scalaLibrary_2_10_6_File,
+      "421989aa8f95a05a4f894630aad96b8c7b828732",
+      classLoaderCache,
+      rootClassLoader,
+      scalaLibrary_2_10_6_ClasspathArray
+    );
 
     // org.scala-lang:scala-reflect:2.10.6
-    download(new URL(mavenUrl + "/org/scala-lang/scala-reflect/2.10.6/scala-reflect-2.10.6.jar"), Paths.get(scalaReflect_2_10_6_File), "3259f3df0f166f017ef5b2d385445808398c316c");
-
     String[] scalaReflect_2_10_6_ClasspathArray = new String[]{scalaLibrary_2_10_6_File, scalaReflect_2_10_6_File};
-    String scalaReflect_2_10_6_Classpath = classpath( scalaReflect_2_10_6_ClasspathArray );
-    ClassLoader scalaReflect_2_10_6_ =
-      classLoaderCache.contains( scalaReflect_2_10_6_Classpath )
-      ? classLoaderCache.get( scalaReflect_2_10_6_Classpath )
-      : classLoaderCache.put( classLoader( scalaReflect_2_10_6_File, scalaLibrary_2_10_6_ ), scalaReflect_2_10_6_Classpath );
+    ClassLoader scalaReflect_2_10_6_ = loadDependency(
+      mavenUrl + "/org/scala-lang/scala-reflect/2.10.6/scala-reflect-2.10.6.jar",
+      scalaReflect_2_10_6_File,
+      "3259f3df0f166f017ef5b2d385445808398c316c",
+      classLoaderCache,
+      scalaLibrary_2_10_6_,
+      scalaReflect_2_10_6_ClasspathArray
+    );
 
     // com.typesafe.sbt:sbt-interface:0.13.12
-    download(new URL(mavenUrl + "/com/typesafe/sbt/sbt-interface/0.13.12/sbt-interface-0.13.12.jar"), Paths.get(sbtInterface_0_13_12_File), "fcc7875c02f0d4641fac0518121bd71475d3909b");
-
     String[] sbtInterface_0_13_12_ClasspathArray = new String[]{sbtInterface_0_13_12_File, scalaLibrary_2_10_6_File, scalaReflect_2_10_6_File};
-    String sbtInterface_0_13_12_Classpath = classpath( sbtInterface_0_13_12_ClasspathArray );
-    ClassLoader sbtInterface_0_13_12_ =
-      classLoaderCache.contains( sbtInterface_0_13_12_Classpath )
-      ? classLoaderCache.get( sbtInterface_0_13_12_Classpath )
-      : classLoaderCache.put( classLoader( sbtInterface_0_13_12_File, scalaReflect_2_10_6_ ), sbtInterface_0_13_12_Classpath );
+    ClassLoader sbtInterface_0_13_12_ = loadDependency(
+      mavenUrl + "/com/typesafe/sbt/sbt-interface/0.13.12/sbt-interface-0.13.12.jar",
+      sbtInterface_0_13_12_File,
+      "fcc7875c02f0d4641fac0518121bd71475d3909b",
+      classLoaderCache,
+      scalaReflect_2_10_6_,
+      sbtInterface_0_13_12_ClasspathArray
+    );
 
     // org.scala-lang:scala-compiler:2.10.6
-    download(new URL(mavenUrl + "/org/scala-lang/scala-compiler/2.10.6/scala-compiler-2.10.6.jar"), Paths.get(scalaCompiler_2_10_6_File), "9b15174852f5b6bb1edbf303d5722286a0a54011");
-
     String[] scalaCompiler_2_10_6_ClasspathArray = new String[]{sbtInterface_0_13_12_File, scalaCompiler_2_10_6_File, scalaLibrary_2_10_6_File, scalaReflect_2_10_6_File};
-    String scalaCompiler_2_10_6_Classpath = classpath( scalaCompiler_2_10_6_ClasspathArray );
-    ClassLoader scalaCompiler_2_10_6_ =
-      classLoaderCache.contains( scalaCompiler_2_10_6_Classpath )
-      ? classLoaderCache.get( scalaCompiler_2_10_6_Classpath )
-      : classLoaderCache.put( classLoader( scalaCompiler_2_10_6_File, sbtInterface_0_13_12_ ), scalaCompiler_2_10_6_Classpath );
+    ClassLoader scalaCompiler_2_10_6_ = loadDependency(
+      mavenUrl + "/org/scala-lang/scala-compiler/2.10.6/scala-compiler-2.10.6.jar",
+      scalaCompiler_2_10_6_File,
+      "9b15174852f5b6bb1edbf303d5722286a0a54011",
+      classLoaderCache,
+      sbtInterface_0_13_12_,
+      scalaCompiler_2_10_6_ClasspathArray
+    );
 
     // com.typesafe.sbt:compiler-interface:0.13.12
-    download(new URL(mavenUrl + "/com/typesafe/sbt/compiler-interface/0.13.12/compiler-interface-0.13.12-sources.jar"), Paths.get(compilerInterface_0_13_12_File), "d9c3270576e162bf017b146af262364c2db87a32");
-
     String[] compilerInterface_0_13_12_ClasspathArray = new String[]{compilerInterface_0_13_12_File, sbtInterface_0_13_12_File, scalaCompiler_2_10_6_File, scalaLibrary_2_10_6_File, scalaReflect_2_10_6_File};
-    String compilerInterface_0_13_12_Classpath = classpath( compilerInterface_0_13_12_ClasspathArray );
-    ClassLoader compilerInterface_0_13_12_ =
-      classLoaderCache.contains( compilerInterface_0_13_12_Classpath )
-      ? classLoaderCache.get( compilerInterface_0_13_12_Classpath )
-      : classLoaderCache.put( classLoader( compilerInterface_0_13_12_File, scalaCompiler_2_10_6_ ), compilerInterface_0_13_12_Classpath );
+    ClassLoader compilerInterface_0_13_12_ = loadDependency(
+      mavenUrl + "/com/typesafe/sbt/compiler-interface/0.13.12/compiler-interface-0.13.12-sources.jar",
+      compilerInterface_0_13_12_File,
+      "d9c3270576e162bf017b146af262364c2db87a32",
+      classLoaderCache,
+      scalaCompiler_2_10_6_,
+      compilerInterface_0_13_12_ClasspathArray
+    );
 
     // com.typesafe.sbt:incremental-compiler:0.13.12
-    download(new URL(mavenUrl + "/com/typesafe/sbt/incremental-compiler/0.13.12/incremental-compiler-0.13.12.jar"), Paths.get(incrementalCompiler_0_13_12_File), "259f6d24a5a3791bb233787d6a8e639c4ab86fe5");
-
     String[] incrementalCompiler_0_13_12_ClasspathArray = new String[]{compilerInterface_0_13_12_File, incrementalCompiler_0_13_12_File, sbtInterface_0_13_12_File, scalaCompiler_2_10_6_File, scalaLibrary_2_10_6_File, scalaReflect_2_10_6_File};
-    String incrementalCompiler_0_13_12_Classpath = classpath( incrementalCompiler_0_13_12_ClasspathArray );
-    ClassLoader incrementalCompiler_0_13_12_ =
-      classLoaderCache.contains( incrementalCompiler_0_13_12_Classpath )
-      ? classLoaderCache.get( incrementalCompiler_0_13_12_Classpath )
-      : classLoaderCache.put( classLoader( incrementalCompiler_0_13_12_File, compilerInterface_0_13_12_ ), incrementalCompiler_0_13_12_Classpath );
+    ClassLoader incrementalCompiler_0_13_12_ = loadDependency(
+      mavenUrl + "/com/typesafe/sbt/incremental-compiler/0.13.12/incremental-compiler-0.13.12.jar",
+      incrementalCompiler_0_13_12_File,
+      "259f6d24a5a3791bb233787d6a8e639c4ab86fe5",
+      classLoaderCache,
+      compilerInterface_0_13_12_,
+      incrementalCompiler_0_13_12_ClasspathArray
+    );
 
     // com.typesafe.zinc:zinc:0.3.12
-    download(new URL(mavenUrl + "/com/typesafe/zinc/zinc/0.3.12/zinc-0.3.12.jar"), Paths.get(zinc_0_3_12_File), "c4339e93f5b7273f49ad026248f4fdb1d4d6c7c4");
-
     String[] zinc_0_3_12_ClasspathArray = new String[]{compilerInterface_0_13_12_File, incrementalCompiler_0_13_12_File, sbtInterface_0_13_12_File, zinc_0_3_12_File, scalaCompiler_2_10_6_File, scalaLibrary_2_10_6_File, scalaReflect_2_10_6_File};
-    String zinc_0_3_12_Classpath = classpath( zinc_0_3_12_ClasspathArray );
-    ClassLoader zinc_0_3_12_ =
-      classLoaderCache.contains( zinc_0_3_12_Classpath )
-      ? classLoaderCache.get( zinc_0_3_12_Classpath )
-      : classLoaderCache.put( classLoader( zinc_0_3_12_File, incrementalCompiler_0_13_12_ ), zinc_0_3_12_Classpath );
+    ClassLoader zinc_0_3_12_ = loadDependency(
+      mavenUrl + "/com/typesafe/zinc/zinc/0.3.12/zinc-0.3.12.jar",
+      zinc_0_3_12_File,
+      "c4339e93f5b7273f49ad026248f4fdb1d4d6c7c4",
+      classLoaderCache,
+      incrementalCompiler_0_13_12_,
+      zinc_0_3_12_ClasspathArray
+    );
 
     // org.scala-lang:scala-library:2.11.8
-    download(new URL(mavenUrl + "/org/scala-lang/scala-library/2.11.8/scala-library-2.11.8.jar"), Paths.get(scalaLibrary_2_11_8_File), "ddd5a8bced249bedd86fb4578a39b9fb71480573");
-
     String[] scalaLibrary_2_11_8_ClasspathArray = new String[]{scalaLibrary_2_11_8_File};
-    String scalaLibrary_2_11_8_Classpath = classpath( scalaLibrary_2_11_8_ClasspathArray );
-    ClassLoader scalaLibrary_2_11_8_ =
-      classLoaderCache.contains( scalaLibrary_2_11_8_Classpath )
-      ? classLoaderCache.get( scalaLibrary_2_11_8_Classpath )
-      : classLoaderCache.put( classLoader( scalaLibrary_2_11_8_File, rootClassLoader ), scalaLibrary_2_11_8_Classpath );
+    ClassLoader scalaLibrary_2_11_8_ = loadDependency(
+      mavenUrl + "/org/scala-lang/scala-library/2.11.8/scala-library-2.11.8.jar",
+      scalaLibrary_2_11_8_File,
+      "ddd5a8bced249bedd86fb4578a39b9fb71480573",
+      classLoaderCache,
+      rootClassLoader,
+      scalaLibrary_2_11_8_ClasspathArray
+    );
 
     // org.scala-lang.modules:scala-xml_2.11:1.0.5
-    download(new URL(mavenUrl + "/org/scala-lang/modules/scala-xml_2.11/1.0.5/scala-xml_2.11-1.0.5.jar"), Paths.get(scalaXml_1_0_5_File), "77ac9be4033768cf03cc04fbd1fc5e5711de2459");
-
     String[] scalaXml_1_0_5_ClasspathArray = new String[]{scalaXml_1_0_5_File, scalaLibrary_2_11_8_File};
-    String scalaXml_1_0_5_Classpath = classpath( scalaXml_1_0_5_ClasspathArray );
-    ClassLoader scalaXml_1_0_5_ =
-      classLoaderCache.contains( scalaXml_1_0_5_Classpath )
-      ? classLoaderCache.get( scalaXml_1_0_5_Classpath )
-      : classLoaderCache.put( classLoader( scalaXml_1_0_5_File, scalaLibrary_2_11_8_ ), scalaXml_1_0_5_Classpath );
-  
+    ClassLoader scalaXml_1_0_5_ = loadDependency(
+      mavenUrl + "/org/scala-lang/modules/scala-xml_2.11/1.0.5/scala-xml_2.11-1.0.5.jar",
+      scalaXml_1_0_5_File,
+      "77ac9be4033768cf03cc04fbd1fc5e5711de2459",
+      classLoaderCache,
+      scalaLibrary_2_11_8_,
+      scalaXml_1_0_5_ClasspathArray
+    );
+
     classLoader = scalaXml_1_0_5_;
     classpathArray = scalaXml_1_0_5_ClasspathArray;
 
diff --git a/nailgun_launcher/JavaCache.java b/nailgun_launcher/JavaCache.java
deleted file mode 100644
index 3ba12ab..0000000
--- a/nailgun_launcher/JavaCache.java
+++ /dev/null
@@ -1,40 +0,0 @@
-package cbt;
-
-import java.util.*;
-import static java.io.File.pathSeparator;
-import static cbt.Stage0Lib.*;
-
-final class JavaCache<T>{
-  Map<Object,Object> hashMap;
-
-  public JavaCache(
-    Map<Object,Object> hashMap
-  ){
-    this.hashMap = hashMap;
-  }
-
-  public T get( Object key ){
-    @SuppressWarnings("unchecked")
-    T t = (T) hashMap.get(
-      hashMap.get( key )
-    );
-    return t;
-  }
-
-  public Boolean contains( Object key/*, Long timestamp*/ ){
-    return hashMap.containsKey( key );/* && (
-      (Long) hashMap.get( hashMap.get( hashMap.get(key) ) ) >= timestamp
-    );*/
-  }
-
-  public T put( Object value, Object key/*, Long timestamp*/ ){
-    LockableJavaKey keyObject = new LockableJavaKey();
-    hashMap.put( key, keyObject );
-    hashMap.put( keyObject, value );
-    //hashMap.put( value, timestamp );
-    @SuppressWarnings("unchecked")
-    T t = (T) value;
-    return t;
-  }
-}
-class LockableJavaKey{}
diff --git a/nailgun_launcher/NailgunLauncher.java b/nailgun_launcher/NailgunLauncher.java
index c397810..8a330d8 100644
--- a/nailgun_launcher/NailgunLauncher.java
+++ b/nailgun_launcher/NailgunLauncher.java
@@ -26,12 +26,12 @@ public class NailgunLauncher{
   @SuppressWarnings("unchecked")
   public static Object getBuild( Object context ) throws Throwable{
     BuildStage1Result res = buildStage1(
-      (Boolean) get(context, "cbtHasChangedCompat"),
-      (Long) get(context, "startCompat"),
+      (long) get(context, "cbtLastModified"),
+      (long) get(context, "start"),
       ((File) get(context, "cache")).toString() + "/",
       ((File) get(context, "cbtHome")).toString(),
       ((File) get(context, "compatibilityTarget")).toString() + "/",
-      new JavaCache<ClassLoader>(
+      new ClassLoaderCache(
         (HashMap) get(context, "persistentCache")
       )
     );
@@ -43,6 +43,8 @@ public class NailgunLauncher{
         .invoke(null, context, res);
   }
 
+  public static long nailgunLauncherLastModified = -1; // this initial value should be overwritten, never read
+
   public static void main( String[] args ) throws Throwable {
     Long _start = System.currentTimeMillis();
     if(args[0].equals("check-alive")){
@@ -76,13 +78,18 @@ public class NailgunLauncher{
     String CBT_HOME = System.getenv("CBT_HOME");
     String cache = CBT_HOME + "/cache/";
     String compatibilityTarget = CBT_HOME + "/compatibility/" + TARGET;
-    // copy cache, so that this thread has a consistent view
-    // replace before returning, see below
-    JavaCache<ClassLoader> classLoaderCache = new JavaCache<ClassLoader>(
+    // copy cache, so that this thread has a consistent view despite other threads
+    // changing their copies
+    // replace again before returning, see below
+    ClassLoaderCache classLoaderCache = new ClassLoaderCache(
       new HashMap<Object,Object>(classLoaderCacheHashMap)
     );
+
+    String nailgunTarget = CBT_HOME + "/" + NAILGUN + TARGET;
+    long nailgunLauncherLastModified = new File( nailgunTarget + "../classes.last-success" ).lastModified();
+
     BuildStage1Result res = buildStage1(
-      false, start, cache, CBT_HOME, compatibilityTarget, classLoaderCache
+      nailgunLauncherLastModified, start, cache, CBT_HOME, compatibilityTarget, classLoaderCache
     );
 
     try{
@@ -90,9 +97,9 @@ public class NailgunLauncher{
         .classLoader
         .loadClass("cbt.Stage1")
         .getMethod(
-          "run", String[].class, File.class, File.class, BuildStage1Result.class, Long.class, Map.class
+          "run", String[].class, File.class, File.class, BuildStage1Result.class, Map.class
         ).invoke(
-          null, (Object) args, new File(cache), new File(CBT_HOME), res, start, classLoaderCache.hashMap
+          null, (Object) args, new File(cache), new File(CBT_HOME), res, classLoaderCache.hashMap
         );
 
       System.exit( exitCode );
@@ -115,7 +122,8 @@ public class NailgunLauncher{
   }
 
   public static BuildStage1Result buildStage1(
-    Boolean changed, long start, String cache, String cbtHome, String compatibilityTarget, JavaCache<ClassLoader> classLoaderCache
+    final long lastModified, final long start, final String cache, final String cbtHome,
+    final String compatibilityTarget, final ClassLoaderCache classLoaderCache
   ) throws Throwable {
     _assert(TARGET != null, "environment variable TARGET not defined");
     String nailgunTarget = cbtHome + "/" + NAILGUN + TARGET;
@@ -128,9 +136,11 @@ public class NailgunLauncher{
     ClassLoader rootClassLoader = new CbtURLClassLoader( new URL[]{}, ClassLoader.getSystemClassLoader().getParent() ); // wrap for caching
     EarlyDependencies earlyDeps = new EarlyDependencies(mavenCache, mavenUrl, classLoaderCache, rootClassLoader);
 
-    ClassLoader compatibilityClassLoader;
+    long nailgunLauncherLastModified = new File( nailgunTarget + "../classes.last-success" ).lastModified();
+
+    long compatibilityLastModified;
     if(!compatibilityTarget.startsWith(cbtHome)){
-      compatibilityClassLoader = classLoaderCache.get( compatibilityTarget );
+      compatibilityLastModified = new File( compatibilityTarget + "../classes.last-success" ).lastModified();
     } else {
       List<File> compatibilitySourceFiles = new ArrayList<File>();
       for( File f: compatibilitySources.listFiles() ){
@@ -138,20 +148,20 @@ public class NailgunLauncher{
           compatibilitySourceFiles.add(f);
         }
       }
-      changed = compile(changed, start, "", compatibilityTarget, earlyDeps, compatibilitySourceFiles);
-      
-      if( classLoaderCache.contains( compatibilityTarget ) ){
-        compatibilityClassLoader = classLoaderCache.get( compatibilityTarget );
-      } else {
-        compatibilityClassLoader = classLoaderCache.put( classLoader(compatibilityTarget, rootClassLoader), compatibilityTarget );
+
+      compatibilityLastModified = compile( 0L, "", compatibilityTarget, earlyDeps, compatibilitySourceFiles);
+
+      if( !classLoaderCache.containsKey( compatibilityTarget, compatibilityLastModified ) ){
+        classLoaderCache.put( compatibilityTarget, classLoader(compatibilityTarget, rootClassLoader), compatibilityLastModified );
       }
     }
+    final ClassLoader compatibilityClassLoader = classLoaderCache.get( compatibilityTarget, compatibilityLastModified );
 
     String[] nailgunClasspathArray = append( earlyDeps.classpathArray, nailgunTarget );
     String nailgunClasspath = classpath( nailgunClasspathArray );
-    ClassLoader nailgunClassLoader = new CbtURLClassLoader( new URL[]{}, NailgunLauncher.class.getClassLoader() ); // wrap for caching
-    if( !classLoaderCache.contains( nailgunClasspath ) ){
-      nailgunClassLoader = classLoaderCache.put( nailgunClassLoader, nailgunClasspath );
+    final ClassLoader nailgunClassLoader = new CbtURLClassLoader( new URL[]{}, NailgunLauncher.class.getClassLoader() ); // wrap for caching
+    if( !classLoaderCache.containsKey( nailgunClasspath, nailgunLauncherLastModified ) ){
+      classLoaderCache.put( nailgunClasspath, nailgunClassLoader, nailgunLauncherLastModified );
     }
 
     String[] stage1ClasspathArray =
@@ -164,14 +174,24 @@ public class NailgunLauncher{
         stage1SourceFiles.add(f);
       }
     }
-    changed = compile(changed, start, stage1Classpath, stage1Target, earlyDeps, stage1SourceFiles);
 
-    ClassLoader stage1classLoader;
-    if( !changed && classLoaderCache.contains( stage1Classpath ) ){
-      stage1classLoader = classLoaderCache.get( stage1Classpath );
-    } else {
-      stage1classLoader =
+    final long stage1BeforeCompiled = System.currentTimeMillis();
+    final long stage0LastModified = Math.max(
+      lastModified,
+      Math.max( lastModified, compatibilityLastModified )
+    );
+    final long stage1LastModified = compile(
+      stage0LastModified, stage1Classpath, stage1Target, earlyDeps, stage1SourceFiles
+    );
+
+    if( stage1LastModified < compatibilityLastModified )
+      throw new AssertionError(
+        "Cache invalidation bug: cbt compatibility layer recompiled, but cbt stage1 did not."
+      );
+
+    if( !classLoaderCache.containsKey( stage1Classpath, stage1LastModified ) ){
       classLoaderCache.put(
+        stage1Classpath,
         classLoader(
           stage1Target,
           new MultiClassLoader2(
@@ -180,12 +200,14 @@ public class NailgunLauncher{
             earlyDeps.classLoader
           )
         ),
-        stage1Classpath
+        stage1LastModified
       );
     }
+    final ClassLoader stage1classLoader = classLoaderCache.get( stage1Classpath, stage1LastModified );
 
     return new BuildStage1Result(
-      changed,
+      start,
+      stage1LastModified,
       stage1classLoader,
       stage1Classpath,
       nailgunClasspath,
diff --git a/nailgun_launcher/Stage0Lib.java b/nailgun_launcher/Stage0Lib.java
index 865b1cb..425ced3 100644
--- a/nailgun_launcher/Stage0Lib.java
+++ b/nailgun_launcher/Stage0Lib.java
@@ -11,6 +11,7 @@ import static java.io.File.pathSeparator;
 import static cbt.NailgunLauncher.*;
 import java.nio.file.*;
 import java.nio.file.attribute.FileTime;
+import static java.lang.Math.min;
 
 public class Stage0Lib{
   public static void _assert(Boolean condition, Object msg){
@@ -47,25 +48,57 @@ public class Stage0Lib{
     return join( pathSeparator, files );
   }
 
+  public static long lastModified( String... files ){
+    List<Long> lastModified = new ArrayList<Long>();
+    for( String file: files ){
+      lastModified.add( new File(file).lastModified() );
+    }
+    return Collections.max( lastModified );
+  }
+
+  public static ClassLoader loadDependency(
+    String url,
+    String file,
+    String hash,
+    ClassLoaderCache classLoaderCache,
+    ClassLoader parent,
+    String... classpathArray
+  ) throws Throwable {
+    download(new URL(url), Paths.get(file), hash);
+
+    final long lastModified = lastModified( classpathArray );
+    final String classpath = classpath( classpathArray );
+
+    if( !classLoaderCache.containsKey( classpath, lastModified ) )
+      classLoaderCache.put( classpath, classLoader( file, parent ), lastModified );
+
+    return classLoaderCache.get( classpath, lastModified );
+  }
+
   public static File write(File file, String content, OpenOption... options) throws Throwable{
     file.getParentFile().mkdirs();
     Files.write(file.toPath(), content.getBytes(), options);
     return file;
   }
 
-  public static Boolean compile(
-    Boolean changed, Long start, String classpath, String target,
+  public static long compile(
+    long lastModified, String classpath, String target,
     EarlyDependencies earlyDeps, List<File> sourceFiles
   ) throws Throwable{
     File statusFile = new File( new File(target) + ".last-success" );
-    Long lastSuccessfullCompile = statusFile.lastModified();
+    long lastCompiled = statusFile.lastModified();
+
+    long maxLastModified = lastModified;
+    final long start = System.currentTimeMillis(); // <- before recursing, so we catch them all
+
     for( File file: sourceFiles ){
-      if( file.lastModified() > lastSuccessfullCompile ){
-        changed = true;
-        break;
-      }
+      long l = file.lastModified();
+      if( l > maxLastModified ) maxLastModified = l;
+      // performance optimization because we'll recompile and don't need to check other files
+      if( l > lastCompiled ) break;
     }
-    if(changed){
+
+    if( maxLastModified > lastCompiled ){
       List<String> zincArgs = new ArrayList<String>(
         Arrays.asList(
           new String[]{
@@ -100,8 +133,10 @@ public class Stage0Lib{
       } finally {
         System.setOut(oldOut);
       }
+      return statusFile.lastModified(); // can't just use `start` here as system time precision is less than milliseconds on OSX
+    } else {
+      return lastCompiled;
     }
-    return changed;
   }
 
   public static ClassLoader classLoader( String file ) throws Throwable{