handle legacy slave.jar gracefully

core/src/main/java/hudson/FilePath.java

@@ -39,6 +39,7 @@ import hudson.remoting.Pipe;
 import hudson.remoting.RemoteOutputStream;
 import hudson.remoting.VirtualChannel;
 import hudson.remoting.RemoteInputStream;
+import hudson.remoting.Which;
 import hudson.security.AccessControlled;
 import hudson.util.DirScanner;
 import hudson.util.IOException2;
@@ -85,6 +86,7 @@ import java.util.List;
 import java.util.StringTokenizer;
 import java.util.Arrays;
 import java.util.Comparator;
+import java.util.logging.Level;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import java.util.concurrent.ExecutionException;
@@ -1419,8 +1421,29 @@ public final class FilePath implements Serializable {
         });
 
         // make sure the write fully happens before we return.
-        if (channel!=null)
-            channel.syncLocalIO();
+        syncIO();
+    }
+
+    private void syncIO() throws InterruptedException {
+        try {
+            if (channel!=null)
+                _syncIO();
+        } catch (AbstractMethodError e) {
+            // legacy slave.jar. Handle this gracefully
+            try {
+                LOGGER.log(Level.WARNING,"Looks like an old slave.jar. Please update "+ Which.jarFile(Channel.class)+" to the new version",e);
+            } catch (IOException _) {
+                // really ignore this time
+            }
+        }
+    }
+
+    /**
+     * A pointless function to work around what appears to be a HotSpot problem. See JENKINS-5756 and bug 6933067
+     * on BugParade for more details.
+     */
+    private void _syncIO() throws InterruptedException {
+        channel.syncLocalIO();
     }
 
     /**