Reduce CPU usage of gradle run (#49055)

buildSrc/src/main/java/org/elasticsearch/gradle/testclusters/RunTask.java

@@ -7,11 +7,12 @@
 import org.gradle.api.tasks.options.Option;
 
 import java.io.BufferedReader;
+import java.io.Closeable;
 import java.io.IOException;
 import java.nio.file.Files;
-import java.util.HashSet;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Map;
-import java.util.Set;
 import java.util.stream.Collectors;
 
 public class RunTask extends DefaultTestClustersTask {
@@ -66,17 +67,54 @@ public void beforeStart() {
 
     @TaskAction
     public void runAndWait() throws IOException {
-        Set<BufferedReader> toRead = new HashSet<>();
-        for (ElasticsearchCluster cluster : getClusters()) {
-            for (ElasticsearchNode node : cluster.getNodes()) {
-                toRead.add(Files.newBufferedReader(node.getEsStdoutFile()));
+        List<BufferedReader> toRead = new ArrayList<>();
+        try {
+            for (ElasticsearchCluster cluster : getClusters()) {
+                for (ElasticsearchNode node : cluster.getNodes()) {
+                    BufferedReader reader = Files.newBufferedReader(node.getEsStdoutFile());
+                    toRead.add(reader);
+                }
             }
-        }
-        while (Thread.currentThread().isInterrupted() == false) {
-            for (BufferedReader bufferedReader : toRead) {
-                if (bufferedReader.ready()) {
-                    logger.lifecycle(bufferedReader.readLine());
+
+            while (Thread.currentThread().isInterrupted() == false) {
+                boolean readData = false;
+                for (BufferedReader bufferedReader : toRead) {
+                    if (bufferedReader.ready()) {
+                        readData = true;
+                        logger.lifecycle(bufferedReader.readLine());
+                    }
                 }
+
+                if (readData == false) {
+                    // no data was ready to be consumed and rather than continuously spinning, pause
+                    // for some time to avoid excessive CPU usage. Ideally we would use the JDK
+                    // WatchService to receive change notifications but the WatchService does not have
+                    // a native MacOS implementation and instead relies upon polling with possible
+                    // delays up to 10s before a notification is received. See JDK-7133447.
+                    try {
+                        Thread.sleep(100L);
+                    } catch (InterruptedException e) {
+                        Thread.currentThread().interrupt();
+                        return;
+                    }
+                }
+            }
+        } finally {
+            Exception thrown = null;
+            for (Closeable closeable : toRead) {
+                try {
+                    closeable.close();
+                } catch (Exception e) {
+                    if (thrown == null) {
+                        thrown = e;
+                    } else {
+                        thrown.addSuppressed(e);
+                    }
+                }
+            }
+
+            if (thrown != null) {
+                logger.debug("exception occurred during close of stdout file readers", thrown);
             }
         }
     }