Finally figured out how to suppress the ReadPendingException from Jetty.

Something¹ seems to interrupt the HTTP duplex service thread,
and Jetty apparently deals poorly with interruptions.
So introduce an extra sleep to soak up the interrupt.

¹Who? http://stackoverflow.com/questions/2126997/who-is-calling-the-java-thread-interrupt-method-if-im-not#comment40865453_2127397

cli/src/main/java/hudson/cli/PlainCLIProtocol.java

@@ -135,12 +135,12 @@ class PlainCLIProtocol {
                     }
                 } catch (ClosedChannelException x) {
                     LOGGER.log(Level.FINE, null, x);
+                    handleClose();
                 } catch (IOException x) {
                     LOGGER.log(Level.WARNING, null, flightRecorder.analyzeCrash(x, "broken stream"));
                 } catch (ReadPendingException x) {
+                    // in case trick in CLIAction does not work
                     LOGGER.log(Level.FINE, null, x);
-                    // TODO what does this signify? Seems to be thrown randomly by org.eclipse.jetty.io.FillInterest.register. No obvious impact.
-                    // Contrary to https://github.com/eclipse/jetty.project/issues/1047 this still happens in 9.4.3.v20170317 as in https://github.com/joakime/jetty-async-bug/issues/1
                     handleClose();
                 }
             }

core/src/main/java/hudson/cli/CLIAction.java

@@ -221,6 +221,11 @@ public class CLIAction implements UnprotectedRootAction, StaplerProxy {
                             int exit = command.main(connection.args.subList(1, connection.args.size()), connection.locale, connection.stdin, stdout, stderr);
                             stdout.flush();
                             connection.sendExit(exit);
+                            try { // seems to avoid ReadPendingException from Jetty
+                                Thread.sleep(1000);
+                            } catch (InterruptedException x) {
+                                // expected; ignore
+                            }
                         } finally {
                             CLICommand.setCurrent(orig);
                             runningThread.set(null);

test/src/test/java/hudson/cli/CLIActionTest.java

@@ -27,6 +27,7 @@ import java.util.UUID;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
+import java.util.logging.Level;
 import jenkins.model.Jenkins;
 import jenkins.security.ApiTokenProperty;
 import jenkins.util.Timer;
@@ -39,6 +40,7 @@ import org.junit.Test;
 import org.junit.rules.TemporaryFolder;
 import org.jvnet.hudson.test.Issue;
 import org.jvnet.hudson.test.JenkinsRule;
+import org.jvnet.hudson.test.LoggerRule;
 import org.jvnet.hudson.test.MockAuthorizationStrategy;
 import org.jvnet.hudson.test.TestExtension;
 import org.jvnet.hudson.test.recipes.PresetData;
@@ -54,6 +56,9 @@ public class CLIActionTest {
     @Rule
     public TemporaryFolder tmp = new TemporaryFolder();
 
+    @Rule
+    public LoggerRule logging = new LoggerRule();
+
     private ExecutorService pool;
 
     /**
@@ -131,6 +136,7 @@ public class CLIActionTest {
     @Issue({"JENKINS-12543", "JENKINS-41745"})
     @Test
     public void authentication() throws Exception {
+        logging.record(PlainCLIProtocol.class, Level.FINE);
         File jar = tmp.newFile("jenkins-cli.jar");
         FileUtils.copyURLToFile(j.jenkins.getJnlpJars("jenkins-cli.jar").getURL(), jar);
         j.jenkins.setSecurityRealm(j.createDummySecurityRealm());