net/http: panic on duplicate registrations

Otherwise, the registration semantics are
init-order-dependent, which I was trying very hard
to avoid in the API.  This may break broken programs.

Fixes #2900.

R=golang-dev, r, bradfitz, dsymonds, balasanjay, kevlar
CC=golang-dev
https://golang.org/cl/5644051

doc/go1.html

@@ -1146,10 +1146,16 @@ The affected items are:
 </ul>
 
 <p>
-Also, the <code>Request.RawURL</code> field has been removed; it was a
+The <code>Request.RawURL</code> field has been removed; it was a
 historical artifact.
 </p>
 
+<p>
+The <code>Handle</code> and <code>HandleFunc</code>
+functions, and the similarly-named methods of <code>ServeMux</code>,
+now panic if an attempt is made to register the same pattern twice.
+</p>
+
 <p>
 <em>Updating</em>:
 Running <code>go fix</code> will update the few programs that are affected except for

doc/go1.tmpl

@@ -1049,10 +1049,16 @@ The affected items are:
 </ul>
 
 <p>
-Also, the <code>Request.RawURL</code> field has been removed; it was a
+The <code>Request.RawURL</code> field has been removed; it was a
 historical artifact.
 </p>
 
+<p>
+The <code>Handle</code> and <code>HandleFunc</code>
+functions, and the similarly-named methods of <code>ServeMux</code>,
+now panic if an attempt is made to register the same pattern twice.
+</p>
+
 <p>
 <em>Updating</em>:
 Running <code>go fix</code> will update the few programs that are affected except for

src/pkg/net/http/server.go

@@ -833,11 +833,17 @@ func RedirectHandler(url string, code int) Handler {
 // redirecting any request containing . or .. elements to an
 // equivalent .- and ..-free URL.
 type ServeMux struct {
-	m map[string]Handler
+	mu sync.RWMutex
+	m  map[string]muxEntry
+}
+
+type muxEntry struct {
+	explicit bool
+	h        Handler
 }
 
 // NewServeMux allocates and returns a new ServeMux.
-func NewServeMux() *ServeMux { return &ServeMux{make(map[string]Handler)} }
+func NewServeMux() *ServeMux { return &ServeMux{m: make(map[string]muxEntry)} }
 
 // DefaultServeMux is the default ServeMux used by Serve.
 var DefaultServeMux = NewServeMux()
@@ -883,12 +889,28 @@ func (mux *ServeMux) match(path string) Handler {
 		}
 		if h == nil || len(k) > n {
 			n = len(k)
-			h = v
+			h = v.h
 		}
 	}
 	return h
 }
 
+// handler returns the handler to use for the request r.
+func (mux *ServeMux) handler(r *Request) Handler {
+	mux.mu.RLock()
+	defer mux.mu.RUnlock()
+
+	// Host-specific pattern takes precedence over generic ones
+	h := mux.match(r.Host + r.URL.Path)
+	if h == nil {
+		h = mux.match(r.URL.Path)
+	}
+	if h == nil {
+		h = NotFoundHandler()
+	}
+	return h
+}
+
 // ServeHTTP dispatches the request to the handler whose
 // pattern most closely matches the request URL.
 func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
@@ -898,30 +920,33 @@ func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
 		w.WriteHeader(StatusMovedPermanently)
 		return
 	}
-	// Host-specific pattern takes precedence over generic ones
-	h := mux.match(r.Host + r.URL.Path)
-	if h == nil {
-		h = mux.match(r.URL.Path)
-	}
-	if h == nil {
-		h = NotFoundHandler()
-	}
-	h.ServeHTTP(w, r)
+	mux.handler(r).ServeHTTP(w, r)
 }
 
 // Handle registers the handler for the given pattern.
+// If a handler already exists for pattern, Handle panics.
 func (mux *ServeMux) Handle(pattern string, handler Handler) {
+	mux.mu.Lock()
+	defer mux.mu.Unlock()
+
 	if pattern == "" {
 		panic("http: invalid pattern " + pattern)
 	}
+	if handler == nil {
+		panic("http: nil handler")
+	}
+	if mux.m[pattern].explicit {
+		panic("http: multiple registrations for " + pattern)
+	}
 
-	mux.m[pattern] = handler
+	mux.m[pattern] = muxEntry{explicit: true, h: handler}
 
 	// Helpful behavior:
-	// If pattern is /tree/, insert permanent redirect for /tree.
+	// If pattern is /tree/, insert an implicit permanent redirect for /tree.
+	// It can be overridden by an explicit registration.
 	n := len(pattern)
-	if n > 0 && pattern[n-1] == '/' {
-		mux.m[pattern[0:n-1]] = RedirectHandler(pattern, StatusMovedPermanently)
+	if n > 0 && pattern[n-1] == '/' && !mux.m[pattern[0:n-1]].explicit {
+		mux.m[pattern[0:n-1]] = muxEntry{h: RedirectHandler(pattern, StatusMovedPermanently)}
 	}
 }