more sophisticated output forwarding from exec tasks

Fixes elastic#8583

Dockerfile

@@ -4,6 +4,7 @@ RUN ln -s /tmp/vendor /opt/logstash/vendor
 
 ADD gradlew /opt/logstash/gradlew
 ADD gradle/wrapper /opt/logstash/gradle/wrapper
+ADD buildSrc /opt/logstash/buildSrc
 RUN /opt/logstash/gradlew wrapper
 
 ADD versions.yml /opt/logstash/versions.yml

build.gradle

@@ -15,6 +15,7 @@ apply plugin: 'de.undercouch.download'
 
 import de.undercouch.gradle.tasks.download.Download
 import de.undercouch.gradle.tasks.download.Verify
+import org.logstash.gradle.ExecLogOutputStream
 import org.yaml.snakeyaml.Yaml
 
 allprojects {
@@ -144,11 +145,9 @@ task installTestGems(dependsOn: downloadAndInstallJRuby, type: Exec) {
   outputs.files file("${projectDir}/Gemfile.lock")
   outputs.files fileTree("${projectDir}/vendor/bundle/gems")
   outputs.files fileTree("${projectDir}/vendor/jruby")
+  standardOutput = new ExecLogOutputStream(System.out)
+  errorOutput =  new ExecLogOutputStream(System.err)
   commandLine './vendor/jruby/bin/jruby', "${projectDir}/vendor/jruby/bin/rake".toString(), "test:install-core"
-  standardOutput = new ByteArrayOutputStream()
-  ext.output = {
-    standardOutput.toString()
-  }
 }
 
 // If you are running a JRuby snapshot we will skip the integrity check.

buildSrc/build.gradle

@@ -0,0 +1,3 @@
+group = 'org.logstash'
+
+apply plugin: 'java'

buildSrc/src/main/java/org/logstash/gradle/ExecLogOutputStream.java

@@ -0,0 +1,29 @@
+package org.logstash.gradle;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+
+/**
+ * Stream that can be used to forward Gradle Exec task output to an arbitrary {@link PrintStream}.
+ */
+public final class ExecLogOutputStream extends ByteArrayOutputStream {
+
+    /**
+     * Underlying {@link PrintStream} to flush output to.
+     */
+    private final PrintStream stream;
+
+    /**
+     * Ctor.
+     * @param stream PrintStream to flush to
+     */
+    public ExecLogOutputStream(final PrintStream stream) {
+        this.stream = stream;
+    }
+
+    @Override
+    public synchronized void flush() {
+        stream.print(toString());
+        reset();
+    }
+}