Use NullOutputStream not DisabledOutputStream in UploadPack

The stream should not throw IllegalStateException if it is off.
Flush the stream after the hook runs, in case any messages need
to be sent ahead of the pack.

Change-Id: I21c7a0258ab1308406d226293fa0e7da69b4f57b

org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java

@@ -82,6 +82,7 @@ import org.eclipse.jgit.transport.BasePackFetchConnection.MultiAck;
 import org.eclipse.jgit.transport.RefAdvertiser.PacketLineOutRefAdvertiser;
 import org.eclipse.jgit.util.io.DisabledOutputStream;
 import org.eclipse.jgit.util.io.InterruptTimer;
+import org.eclipse.jgit.util.io.NullOutputStream;
 import org.eclipse.jgit.util.io.TimeoutInputStream;
 import org.eclipse.jgit.util.io.TimeoutOutputStream;
 
@@ -193,7 +194,7 @@ public class UploadPack {
 
 	private PacketLineOut pckOut;
 
-	private OutputStream msgOut = DisabledOutputStream.INSTANCE;
+	private OutputStream msgOut = NullOutputStream.INSTANCE;
 
 	/** The refs we advertised as existing at the start of the connection. */
 	private Map<String, Ref> refs;
@@ -525,7 +526,7 @@ public class UploadPack {
 			pckOut = new PacketLineOut(rawOut);
 			service();
 		} finally {
-			msgOut = DisabledOutputStream.INSTANCE;
+			msgOut = NullOutputStream.INSTANCE;
 			walk.release();
 			if (timer != null) {
 				try {
@@ -1126,6 +1127,7 @@ public class UploadPack {
 			} else {
 				preUploadHook.onSendPack(this, wantAll, commonBase);
 			}
+			msgOut.flush();
 		} catch (ServiceMayNotContinueException noPack) {
 			if (sideband && noPack.getMessage() != null) {
 				noPack.setOutput();
@@ -1209,7 +1211,7 @@ public class UploadPack {
 			pw.writePack(pm, NullProgressMonitor.INSTANCE, packOut);
 			statistics = pw.getStatistics();
 
-			if (msgOut != DisabledOutputStream.INSTANCE) {
+			if (msgOut != NullOutputStream.INSTANCE) {
 				String msg = pw.getStatistics().getMessage() + '\n';
 				msgOut.write(Constants.encode(msg));
 				msgOut.flush();