httpserver: give each req context a Replacer that preserves custom values (#1937)

This allows custom replacements to be defined in a way that propagates
throughout all plugins.

caddyhttp/httpserver/middleware.go

@@ -214,6 +214,9 @@ func SameNext(next1, next2 Handler) bool {
 
 // Context key constants.
 const (
+	// ReplacerCtxKey is the context key for a per-request replacer.
+	ReplacerCtxKey caddy.CtxKey = "replacer"
+
 	// RemoteUserCtxKey is the key for the remote user of the request, if any (basicauth).
 	RemoteUserCtxKey caddy.CtxKey = "remote_user"
 

caddyhttp/httpserver/replacer.go

@@ -102,20 +102,30 @@ func (lw *limitWriter) String() string {
 // emptyValue should be the string that is used in place
 // of empty string (can still be empty string).
 func NewReplacer(r *http.Request, rr *ResponseRecorder, emptyValue string) Replacer {
-	rb := newLimitWriter(MaxLogBodySize)
-	if r.Body != nil {
-		r.Body = struct {
-			io.Reader
-			io.Closer
-		}{io.TeeReader(r.Body, rb), io.Closer(r.Body)}
+	repl := &replacer{
+		request:          r,
+		responseRecorder: rr,
+		emptyValue:       emptyValue,
 	}
-	return &replacer{
-		request:            r,
-		requestBody:        rb,
-		responseRecorder:   rr,
-		customReplacements: make(map[string]string),
-		emptyValue:         emptyValue,
+
+	// extract customReplacements from a request replacer when present.
+	if existing, ok := r.Context().Value(ReplacerCtxKey).(*replacer); ok {
+		repl.requestBody = existing.requestBody
+		repl.customReplacements = existing.customReplacements
+	} else {
+		// if there is no existing replacer, build one from scratch.
+		rb := newLimitWriter(MaxLogBodySize)
+		if r.Body != nil {
+			r.Body = struct {
+				io.Reader
+				io.Closer
+			}{io.TeeReader(r.Body, rb), io.Closer(r.Body)}
+		}
+		repl.requestBody = rb
+		repl.customReplacements = make(map[string]string)
 	}
+
+	return repl
 }
 
 func canLogRequest(r *http.Request) bool {

caddyhttp/httpserver/server.go

@@ -356,6 +356,12 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	c := context.WithValue(r.Context(), OriginalURLCtxKey, urlCopy)
 	r = r.WithContext(c)
 
+	// Setup a replacer for the request that keeps track of placeholder
+	// values across plugins.
+	replacer := NewReplacer(r, nil, "")
+	c = context.WithValue(r.Context(), ReplacerCtxKey, replacer)
+	r = r.WithContext(c)
+
 	w.Header().Set("Server", caddy.AppName)
 
 	status, _ := s.serveHTTP(w, r)