fix broken Android build due to 617bd85

617bd85 broke the Android build by creating an unresolvable
order-of-operations bug in classpath-android.cpp's
MyClasspath::preBoot method.

The problem is that, while JNIEnv::FindClass is supposed to initialize
the class that it finds, this causes JniConstants::init to indirectly
invoke native methods which are not registered until JNI_OnLoad is
called (which happens after JniConstants::init is called).  However,
if we call JNI_OnLoad first, that causes methods to be invoked which
rely on JniConstants::init having already been run.

I haven't checked to see how Dalvik handles this, but I don't see any
way around the problem besides disabling initialization by
JNIEnv::FindClass until the preBoot phase is complete.  Moreover, it's
dangerous to allow Java code to be invoked so early anyway, since the
VM is not yet fully initialized.

src/avian/machine.h

@@ -1571,6 +1571,8 @@ class Classpath {
   virtual void
   preBoot(Thread* t) = 0;
 
+  virtual bool mayInitClasses() = 0;
+
   virtual void
   boot(Thread* t) = 0;
 

src/classpath-android.cpp

@@ -228,9 +228,8 @@ translateStackTrace(Thread* t, object raw)
 
 class MyClasspath : public Classpath {
  public:
-  MyClasspath(Allocator* allocator):
-    allocator(allocator),
-    tzdata(0)
+  MyClasspath(Allocator* allocator)
+      : allocator(allocator), tzdata(0), mayInitClasses_(false)
   { }
 
   virtual object
@@ -493,6 +492,13 @@ class MyClasspath : public Classpath {
     JniConstants::init(reinterpret_cast<_JNIEnv*>(t));
 
     JNI_OnLoad(reinterpret_cast< ::JavaVM*>(t->m), 0);
+
+    mayInitClasses_ = true;
+  }
+
+  virtual bool mayInitClasses()
+  {
+    return mayInitClasses_;
   }
 
   virtual void
@@ -579,6 +585,7 @@ class MyClasspath : public Classpath {
 
   Allocator* allocator;
   System::Region* tzdata;
+  bool mayInitClasses_;
 };
 
 int64_t JNICALL

src/classpath-avian.cpp

@@ -121,6 +121,11 @@ class MyClasspath : public Classpath {
     // ignore
   }
 
+  virtual bool mayInitClasses()
+  {
+    return true;
+  }
+
   virtual void
   boot(Thread*)
   {

src/classpath-openjdk.cpp

@@ -681,6 +681,11 @@ class MyClasspath : public Classpath {
     // ignore
   }
 
+  virtual bool mayInitClasses()
+  {
+    return true;
+  }
+
   virtual void
   boot(Thread* t)
   {

src/jnienv.cpp

@@ -354,9 +354,11 @@ findClass(Thread* t, uintptr_t* arguments)
                                  : root(t, Machine::AppLoader),
                           n);
 
-  PROTECT(t, c);
+  if (t->m->classpath->mayInitClasses()) {
+    PROTECT(t, c);
 
-  initClass(t, c);
+    initClass(t, c);
+  }
 
   return reinterpret_cast<uint64_t>(makeLocalReference(t, getJClass(t, c)));
 }