add OverrideHeadersIn and OverrideHeadersOut support

This is an attempt to address #108

Instead of dedicated support of header`s removal it allows to return a flag indicating plugin's full control over headers. With this flag set, the conductor won't mix response headers with originals but rather will count on a plugin to provide all the headers.

README.md

@@ -285,6 +285,11 @@ The core functionality of reproxy can be extended with external plugins. Each pl
 - `HeadersIn` - incoming headers. Those will be sent to the proxied url
 - `HeadersOut` - outgoing headers. Will be sent back to the client 
 
+By default headers set by a plugin will be mixed with the original headers. In case if plugin need to control all the headers, for example drop some of them, `OverrideHeaders*` field can be set by a plugin indicating to the core reporxy process the need to overwrite all the headers instead of mixing them in.
+
+- `OverrideHeadersIn` - indicates plugin responsible for all incoming headers.
+- `OverrideHeadersOut` - indicates plugin responsible for all outgoing headers 
+
 To simplify the development process all the building blocks provided. It includes `lib.Plugin` handling registration, listening and dispatching calls as well as `lib.Request` and `lib.Response` defining input and output. Plugin's authors should implement concrete handlers satisfying `func(req lib.Request, res *lib.HandlerResponse) (err error)` signature. Each plugin may contain multiple handlers like this.
 
 

app/plugin/conductor.go

@@ -87,6 +87,20 @@ func (c *Conductor) Run(ctx context.Context) error {
 // Failed plugin calls ignored. Status code from any plugin may stop the chain of calls if not 200. This is needed
 // to allow plugins like auth which has to terminate request in some cases.
 func (c *Conductor) Middleware(next http.Handler) http.Handler {
+
+	setHeaders := func(src, alt http.Header, overrideHeaders bool) {
+		if overrideHeaders {
+			for k := range src {
+				src.Del(k)
+			}
+		}
+		for k, vv := range alt {
+			for _, v := range vv {
+				src.Add(k, v)
+			}
+		}
+	}
+
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 
 		c.lock.RLock()
@@ -101,16 +115,10 @@ func (c *Conductor) Middleware(next http.Handler) http.Handler {
 				http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
 				return
 			}
-			for k, vv := range reply.HeadersIn {
-				for _, v := range vv {
-					r.Header.Add(k, v)
-				}
-			}
-			for k, vv := range reply.HeadersOut {
-				for _, v := range vv {
-					w.Header().Add(k, v)
-				}
-			}
+
+			setHeaders(r.Header, reply.HeadersIn, reply.OverrideHeadersIn)
+			setHeaders(w.Header(), reply.HeadersOut, reply.OverrideHeadersOut)
+
 			if reply.StatusCode >= 400 {
 				c.lock.RUnlock()
 				http.Error(w, http.StatusText(reply.StatusCode), reply.StatusCode)

app/plugin/conductor_test.go

@@ -223,6 +223,7 @@ func TestConductor_Middleware(t *testing.T) {
 				reply.(*lib.Response).StatusCode = 200
 				reply.(*lib.Response).HeadersOut = map[string][]string{}
 				reply.(*lib.Response).HeadersOut.Set("k11", "v11")
+				reply.(*lib.Response).OverrideHeadersOut = true
 			}
 			if serviceMethod == "Test1.Mw3" {
 				t.Fatal("shouldn't be called")
@@ -285,9 +286,10 @@ func TestConductor_Middleware(t *testing.T) {
 	}))
 	h.ServeHTTP(w, rr)
 	assert.Equal(t, 200, w.Result().StatusCode)
-	assert.Equal(t, "v1", w.Result().Header.Get("k1"))
+	assert.Equal(t, "", w.Result().Header.Get("k1"))
 	assert.Equal(t, "v2", w.Result().Header.Get("k2"))
 	assert.Equal(t, "v21", rr.Header.Get("k21"))
+	assert.Equal(t, "v11", w.Result().Header.Get("k11"))
 	t.Logf("req: %+v", rr)
 	t.Logf("resp: %+v", w.Result())
 }

lib/rpc.go

@@ -25,7 +25,9 @@ type Request struct {
 
 // Response from plugin's handler call
 type Response struct {
-	StatusCode int
-	HeadersIn  http.Header
-	HeadersOut http.Header
+	StatusCode         int
+	HeadersIn          http.Header
+	HeadersOut         http.Header
+	OverrideHeadersIn  bool // indicates plugin removing all the original incoming headers
+	OverrideHeadersOut bool // indicates plugin removing all the original outgoing headers
 }