[BEAM-5351] Fix missing pom.xml file in artifact jar.

I also validate that the files exist and error out the build otherwise.

Author: lukecwik

buildSrc/src/main/groovy/org/apache/beam/gradle/BeamModulePlugin.groovy

@@ -771,10 +771,11 @@ class BeamModulePlugin implements Plugin<Project> {
 
         // Create a task which emulates the maven-archiver plugin in generating a
         // pom.properties file.
+        def pomPropertiesFile = "${project.buildDir}/publications/mavenJava/pom.properties"
         project.task('generatePomPropertiesFileForMavenJavaPublication') {
-          outputs.file "${project.buildDir}/publications/mavenJava/pom.properties"
+          outputs.file "${pomPropertiesFile}"
           doLast {
-            new File("${project.buildDir}/publications/mavenJava/pom.properties").text =
+            new File("${pomPropertiesFile}").text =
                     """version=${project.version}
 groupId=${project.group}
 artifactId=${project.name}
@@ -785,10 +786,23 @@ artifactId=${project.name}
         // Have the shaded include both the generate pom.xml and its properties file
         // emulating the behavior of the maven-archiver plugin.
         project.shadowJar {
+          def pomFile = "${project.buildDir}/publications/mavenJava/pom-default.xml"
+
+          // Validate that the artifacts exist before copying them into the jar.
+          doFirst {
+            if (!project.file("${pomFile}").exists()) {
+              throw new GradleException("Expected ${pomFile} to have been generated by the 'generatePomFileForMavenJavaPublication' task.")
+            }
+            if (!project.file("${pomPropertiesFile}").exists()) {
+              throw new GradleException("Expected ${pomPropertiesFile} to have been generated by the 'generatePomPropertiesFileForMavenJavaPublication' task.")
+            }
+          }
+
           dependsOn 'generatePomFileForMavenJavaPublication'
-          into("META-INF/maven/${project.group}/${project.name}") { from "${project.buildDir}/publications/mavenJava/pom.xml" }
+          into("META-INF/maven/${project.group}/${project.name}") { from "${pomFile}" }
+
           dependsOn project.generatePomPropertiesFileForMavenJavaPublication
-          into("META-INF/maven/${project.group}/${project.name}") { from "${project.buildDir}/publications/mavenJava/pom.properties" }
+          into("META-INF/maven/${project.group}/${project.name}") { from "${pomPropertiesFile}" }
         }
 
         // Only build artifacts for archives if we are publishing