[FIXED JENKINS-20769]

When ProxyWriter is finalized, it should attempt to release a remote reference count,
but it shouldn't try to close the stream. For one, the other side that had exported
the writer might be still using it (imagine exporting System.out), and secondly, the
said object could have already been unexported, for example if the closure that brought
ProxyWriter to this side has already returned, since RemoteWriter is auto-unexporting.

Compare ProxyOutputStream impl vs ProxyWriter impl and the difference becomes clearer.

src/main/java/hudson/remoting/ProxyOutputStream.java

@@ -196,6 +196,7 @@ private void doClose(Throwable error) throws IOException {
     protected void finalize() throws Throwable {
         super.finalize();
         // if we haven't done so, release the exported object on the remote side.
+        // if the object is auto-unexported, the export entry could have already been removed.
         if(channel!=null) {
             channel.send(new Unexport(channel.newIoId(),oid));
             channel = null;

src/main/java/hudson/remoting/ProxyWriter.java

@@ -129,7 +129,13 @@ public synchronized void close() throws IOException {
 
     protected void finalize() throws Throwable {
         super.finalize();
-        close();
+        // if we haven't done so, release the exported object on the remote side.
+        // if the object is auto-unexported, the export entry could have already been removed.
+        if(channel!=null) {
+            channel.send(new Unexport(channel.newIoId(),oid));
+            channel = null;
+            oid = -1;
+        }
     }
 
     /**
@@ -186,4 +192,34 @@ public String toString() {
 
         private static final long serialVersionUID = 1L;
     }
+
+    /**
+     * {@link Command} for releasing an export table.
+     *
+     * <p>
+     * Unlike {@link EOF}, this just unexports but not closes the stream.
+     */
+    private static class Unexport extends Command {
+        private final int oid;
+        private final int ioId;
+
+        public Unexport(int ioId, int oid) {
+            this.ioId = ioId;
+            this.oid = oid;
+        }
+
+        protected void execute(final Channel channel) {
+            channel.pipeWriter.submit(ioId,new Runnable() {
+                public void run() {
+                    channel.unexport(oid);
+                }
+            });
+        }
+
+        public String toString() {
+            return "Pipe.Unexport("+oid+")";
+        }
+
+        private static final long serialVersionUID = 1L;
+    }
 }