Explicitly Set MALLOC_ARENA_MAX

Because the JVM isn't very aware of the memory model of the OS that it's
running on, it would often request memory in a way that caused a large number
of malloc arenas, especially under a full heap.  Unfortunately, it would not
use the malloc arena count in it's internal accounting of it's current memory
usage, causing the container to kill the process.  This change fixes the
number of arenas, taking the performance hit for better behavior at the edge
of a container.

[resolves #550]

lib/java_buildpack/jre/open_jdk_like_memory_calculator.rb

@@ -59,7 +59,9 @@ def memory_calculation_command
       end
 
       # (see JavaBuildpack::Component::BaseComponent#release)
-      def release; end
+      def release
+        @droplet.environment_variables.add_environment_variable 'MALLOC_ARENA_MAX', 2
+      end
 
       protected
 

spec/java_buildpack/jre/open_jdk_like_memory_calculator_spec.rb

@@ -99,4 +99,10 @@
     component.compile
   end
 
+  it 'adds MALLOC_ARENA_MAX to environment' do
+    component.release
+
+    expect(environment_variables).to include('MALLOC_ARENA_MAX=2')
+  end
+
 end