Give info/refs services more control over response

Currently, SmartServiceInfoRefs always prints "# service=serviceName"
followed by a flush packet in response to an info/refs request, and then
hands it off to the specific service class. Printing of "#
service=serviceName" is mandated for protocol v0, but not v2.

Therefore, the existing code works for protocol v0, but whenever a
service that supports protocol v2 receives an info/refs request, it must
first determine which protocol version is to be used (depending on, for
example, the request and any relevant configuration variables), and then
decide if "# service=serviceName" needs to be printed.

Create a new method that v2-supporting service classes can override,
covering the printing of both "# service=serviceName" and everything
that the #advertise method prints. This will be used in a subsequent
commit in which UploadPackServlet (and the other classes it uses) is
updated to support protocol v2.

Change-Id: Ia026b06e96a6b15937514096babd024ef77df1ea
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Jonathan Nieder <jrn@google.com>

org.eclipse.jgit.http.server/src/org/eclipse/jgit/http/server/SmartServiceInfoRefs.java

@@ -133,9 +133,7 @@ abstract class SmartServiceInfoRefs implements Filter {
 			res.setContentType(infoRefsResultType(svc));
 
 			final PacketLineOut out = new PacketLineOut(buf);
-			out.writeString("# service=" + svc + "\n");
+			respond(req, out, svc);
-			out.end();
-			advertise(req, new PacketLineOutRefAdvertiser(out));
 			buf.close();
 		} catch (ServiceNotAuthorizedException e) {
 			res.sendError(SC_UNAUTHORIZED, e.getMessage());
@@ -178,6 +176,37 @@ abstract class SmartServiceInfoRefs implements Filter {
 			PacketLineOutRefAdvertiser pck) throws IOException,
 			ServiceNotEnabledException, ServiceNotAuthorizedException;
 
+	/**
+	 * Writes the appropriate response to an info/refs request received by
+	 * a smart service. In protocol v0, this starts with "#
+	 * service=serviceName" followed by a flush packet, but this is not
+	 * necessarily the case in other protocol versions.
+	 * <p>
+	 * The default implementation writes "# service=serviceName" and a
+	 * flush packet, then calls {@link #advertise}. Subclasses should
+	 * override this method if they support protocol versions other than
+	 * protocol v0.
+	 *
+	 * @param req
+	 *            request
+	 * @param pckOut
+	 *            destination of response
+	 * @param serviceName
+	 *            service name to be written out in protocol v0; may or may
+	 *            not be used in other versions
+	 * @throws IOException
+	 * @throws ServiceNotEnabledException
+	 * @throws ServiceNotAuthorizedException
+	 */
+	protected void respond(HttpServletRequest req,
+			PacketLineOut pckOut, String serviceName)
+			throws IOException, ServiceNotEnabledException,
+			ServiceNotAuthorizedException {
+		pckOut.writeString("# service=" + svc + '\n'); //$NON-NLS-1$
+		pckOut.end();
+		advertise(req, new PacketLineOutRefAdvertiser(pckOut));
+	}
+
 	private class Chain implements FilterChain {
 		private int filterIdx;