Append to X-Forwarded-Prefix in case it already exists

Fixes gh-993

spring-cloud-netflix-core/src/main/java/org/springframework/cloud/netflix/zuul/filters/pre/PreDecorationFilter.java

@@ -114,7 +114,18 @@ public class PreDecorationFilter extends ZuulFilter {
 					ctx.addZuulRequestHeader(ZuulHeaders.X_FORWARDED_PROTO,
 							ctx.getRequest().getScheme());
 					if (StringUtils.hasText(route.getPrefix())) {
-						ctx.addZuulRequestHeader("X-Forwarded-Prefix", route.getPrefix());
+						String existingPrefix = ctx.getRequest().getHeader("X-Forwarded-Prefix");
+						StringBuilder newPrefixBuilder = new StringBuilder();
+						if (StringUtils.hasLength(existingPrefix)) {
+							if (existingPrefix.endsWith("/") && route.getPrefix().startsWith("/")) {
+								newPrefixBuilder.append(existingPrefix, 0, existingPrefix.length() - 1);
+							}
+							else {
+								newPrefixBuilder.append(existingPrefix);
+							}
+						}
+						newPrefixBuilder.append(route.getPrefix());
+						ctx.addZuulRequestHeader("X-Forwarded-Prefix", newPrefixBuilder.toString());
 					}
 					String xforwardedfor = ctx.getRequest().getHeader("X-Forwarded-For");
 					String remoteAddr = ctx.getRequest().getRemoteAddr();

spring-cloud-netflix-core/src/test/java/org/springframework/cloud/netflix/zuul/filters/pre/PreDecorationFilterTests.java

@@ -106,6 +106,28 @@ public class PreDecorationFilterTests {
 				getHeader(ctx.getOriginResponseHeaders(), "x-zuul-serviceid"));
 	}
 
+	@Test
+	public void prefixRouteWithPrefixHeaderConcatsHeader() throws Exception {
+		this.properties.setPrefix("/api");
+		this.properties.setStripPrefix(true);
+		this.request.setRequestURI("/api/foo/1");
+		this.request.setRemoteAddr("5.6.7.8");
+		this.request.addHeader("X-Forwarded-For", "1.2.3.4");
+		this.request.addHeader("X-Forwarded-Prefix", "/prefix");
+		this.routeLocator.addRoute(
+				new ZuulRoute("foo", "/foo/**", "foo", null, false, null, null));
+		this.filter.run();
+		RequestContext ctx = RequestContext.getCurrentContext();
+		assertEquals("/foo/1", ctx.get("requestURI"));
+		assertEquals("localhost", ctx.getZuulRequestHeaders().get("x-forwarded-host"));
+		assertEquals("80", ctx.getZuulRequestHeaders().get("x-forwarded-port"));
+		assertEquals("http", ctx.getZuulRequestHeaders().get("x-forwarded-proto"));
+		assertEquals("/prefix/api", ctx.getZuulRequestHeaders().get("x-forwarded-prefix"));
+		assertEquals("1.2.3.4, 5.6.7.8", ctx.getZuulRequestHeaders().get("x-forwarded-for"));
+		assertEquals("foo",
+				getHeader(ctx.getOriginResponseHeaders(), "x-zuul-serviceid"));
+	}
+
 	@Test
 	public void forwardRouteAddsLocation() throws Exception {
 		this.properties.setPrefix("/api");