Fix class count discrepancy when using DataDog agent. (#890)

Resolves #887.

This fix will go through the DataDog agent file and count the number of `.classdata` hidden classes. It then creates a shadow JAR file with the same number of fake classes in it. The Java buildpack's normal class counting mechanism will now see these classes and pick a correct class count.

Author: Daniel Mikusa

lib/java_buildpack/framework/datadog_javaagent.rb

@@ -27,15 +27,18 @@ class DatadogJavaagent < JavaBuildpack::Component::VersionedDependencyComponent
 
       def initialize(context)
         super(context)
-        @datadog_buildpack = File.exist? File.join(@droplet.root, 'datadog')
+        @datadog_buildpack = File.exist? File.join(@droplet.root, '.datadog')
         @logger = JavaBuildpack::Logging::LoggerFactory.instance.get_logger DatadogJavaagent
       end
 
       # (see JavaBuildpack::Component::BaseComponent#compile)
       def compile
         @logger.error 'Datadog Buildpack is required, but not found' unless @datadog_buildpack
 
-        download_jar if @datadog_buildpack
+        return unless @datadog_buildpack
+
+        download_jar
+        fix_class_count
       end
 
       # (see JavaBuildpack::Component::BaseComponent#release)
@@ -63,6 +66,31 @@ def supports?
         apm_disabled = @application.environment['DD_APM_ENABLED'] == 'false'
         (api_key_defined && !apm_disabled)
       end
+
+      # fixes issue where some classes are not counted by adding shadow class files
+      def fix_class_count
+        cnt = classdata_count(@droplet.sandbox + jar_name)
+        zipdir = "#{@droplet.sandbox}/datadog_fakeclasses"
+        zipfile = "#{@droplet.sandbox}/datadog_fakeclasses.jar"
+
+        File.delete(zipfile) if File.exist? zipfile
+        FileUtils.rm_rf(zipdir)
+        FileUtils.mkdir_p(zipdir)
+
+        1.upto(cnt) do |i|
+          File.open("#{zipdir}/#{i}.class", 'w') do |f|
+            f.write(i.to_s)
+          end
+        end
+
+        `cd #{zipdir} && zip -r #{zipfile} .`
+        FileUtils.rm_rf(zipdir)
+      end
+
+      # count hidden class files in the agent JAR
+      def classdata_count(archive)
+        `unzip -l #{archive} | grep '\\(\\.classdata\\)$' | wc -l`.to_i
+      end
     end
   end
 end

spec/fixtures/stub-datadog-javaagent.jar

@@ -67,18 +67,28 @@
 
     context 'when datadog buildpack is present' do
       before do
-        FileUtils.mkdir_p File.join(context[:droplet].root, 'datadog')
+        FileUtils.mkdir_p File.join(context[:droplet].root, '.datadog')
       end
 
       after do
-        FileUtils.rmdir File.join(context[:droplet].root, 'datadog')
+        FileUtils.rmdir File.join(context[:droplet].root, '.datadog')
       end
 
       it 'compile downloads datadog-javaagent JAR', cache_fixture: 'stub-datadog-javaagent.jar' do
         component.compile
         expect(sandbox + "datadog_javaagent-#{version}.jar").to exist
       end
 
+      it 'makes a jar with fake class files', cache_fixture: 'stub-datadog-javaagent.jar' do
+        component.compile
+        expect(sandbox + "datadog_javaagent-#{version}.jar").to exist
+        expect(sandbox + 'datadog_fakeclasses.jar').to exist
+        expect(sandbox + 'datadog_fakeclasses').not_to exist
+
+        cnt = `unzip -l #{sandbox}/datadog_fakeclasses.jar | grep '\\(\\.class\\)$' | wc -l`.to_i
+        expect(cnt).to equal(34)
+      end
+
       it 'release updates JAVA_OPTS' do
         component.release
 
@@ -114,11 +124,11 @@
     end
 
     before do
-      FileUtils.mkdir_p File.join(context[:droplet].root, 'datadog')
+      FileUtils.mkdir_p File.join(context[:droplet].root, '.datadog')
     end
 
     after do
-      FileUtils.rmdir File.join(context[:droplet].root, 'datadog')
+      FileUtils.rmdir File.join(context[:droplet].root, '.datadog')
     end
 
     it 'release updates JAVA_OPTS with env variable version' do