Follow up fixes to CASSANDRA-16633

* Don't run rat check if git fails or not available
* Check for presence of rat.txt before running eclipse-warnings task
  (to ensure that the report is always generated by CI runs)

Patch by Sam Tunnicliffe; reviewed by Mick Semb Wever for CASSANDRA-16633

.build/build-rat.xml

@@ -33,9 +33,14 @@
     </target>
 
     <target name="_build_ratinclude" depends="_rat_init">
-        <exec executable="git" failifexecutionfails="true" failonerror="true" output="build/.versioned_files">
+        <exec executable="git" failifexecutionfails="false" failonerror="false" resultproperty="git.success" output="build/.versioned_files">
             <arg line="ls-tree -r HEAD --name-only"/>
         </exec>
+        <condition property="rat.skip" value="true">
+            <not>
+                <equals arg1="${git.success}" arg2="0"/>
+            </not>
+        </condition>
         <copy file="build/.versioned_files" tofile="build/.ratinclude">
             <filterchain>
                 <linecontainsregexp>
@@ -101,4 +106,15 @@
     <target name="write-java-license-headers" unless="without.rat" description="Add missing java license headers">
       <antcall target="_write_java_license_headers" />
     </target>
+
+    <target name="_assert_rat_output">
+        <fail message="The rat report at build/rat.txt was not generated. Please ensure that the rat-check task is able to run successfully. For dev builds only, touch build/rat.txt to skip this check">
+            <condition>
+                <not>
+                    <available file="build/rat.txt" />
+                </not>
+            </condition>
+        </fail>
+    </target>
+
 </project>

build.xml

@@ -1920,8 +1920,7 @@
   	<delete dir="build/eclipse-classes" />
   </target>
 
-
-  <target name="eclipse-warnings" depends="build" description="Run eclipse compiler code analysis">        
+  <target name="eclipse-warnings" depends="build, _assert_rat_output" description="Run eclipse compiler code analysis">
         <property name="ecj.log.dir" value="${build.dir}/ecj" />
         <property name="ecj.warnings.file" value="${ecj.log.dir}/eclipse_compiler_checks.txt"/>
         <delete dir="${ecj.log.dir}" />