caddyhttp: Minor refactoring for preparing requests

While building a layer4 app for Caddy, I discovered that we need the
ability to fill a request's context just like the HTTP server does,
hence this exported function PrepareRequest().

modules/caddyhttp/routes.go

@@ -113,7 +113,7 @@ func (r Route) Empty() bool {
 // create a middleware chain.
 type RouteList []Route
 
-// Provision sets up both the matchers and handlers in the route.
+// Provision sets up both the matchers and handlers in the routes.
 func (routes RouteList) Provision(ctx caddy.Context) error {
 	err := routes.ProvisionMatchers(ctx)
 	if err != nil {
@@ -284,7 +284,7 @@ type RawMatcherSets []caddy.ModuleMap
 type MatcherSets []MatcherSet
 
 // AnyMatch returns true if req matches any of the
-// matcher sets in mss or if there are no matchers,
+// matcher sets in ms or if there are no matchers,
 // in which case the request always matches.
 func (ms MatcherSets) AnyMatch(req *http.Request) bool {
 	for _, m := range ms {

modules/caddyhttp/server.go

@@ -145,19 +145,8 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	// set up the context for the request
 	repl := caddy.NewReplacer()
-	ctx := context.WithValue(r.Context(), caddy.ReplacerCtxKey, repl)
+	r = PrepareRequest(r, repl, w, s)
-	ctx = context.WithValue(ctx, ServerCtxKey, s)
-	ctx = context.WithValue(ctx, VarsCtxKey, make(map[string]interface{}))
-	ctx = context.WithValue(ctx, routeGroupCtxKey, make(map[string]struct{}))
-	var url2 url.URL // avoid letting this escape to the heap
-	ctx = context.WithValue(ctx, OriginalRequestCtxKey, originalRequest(r, &url2))
-	r = r.WithContext(ctx)
-
-	// once the pointer to the request won't change
-	// anymore, finish setting up the replacer
-	addHTTPVarsToReplacer(repl, r, w)
 
 	// encode the request for logging purposes before
 	// it enters any handler chain; this is necessary
@@ -470,6 +459,25 @@ func (slc ServerLogConfig) getLoggerName(host string) string {
 	return slc.DefaultLoggerName
 }
 
+// PrepareRequest fills the request r for use in a Caddy HTTP handler chain. w and s can
+// be nil, but the handlers will lose response placeholders and access to the server.
+func PrepareRequest(r *http.Request, repl *caddy.Replacer, w http.ResponseWriter, s *Server) *http.Request {
+	// set up the context for the request
+	ctx := context.WithValue(r.Context(), caddy.ReplacerCtxKey, repl)
+	ctx = context.WithValue(ctx, ServerCtxKey, s)
+	ctx = context.WithValue(ctx, VarsCtxKey, make(map[string]interface{}))
+	ctx = context.WithValue(ctx, routeGroupCtxKey, make(map[string]struct{}))
+	var url2 url.URL // avoid letting this escape to the heap
+	ctx = context.WithValue(ctx, OriginalRequestCtxKey, originalRequest(r, &url2))
+	r = r.WithContext(ctx)
+
+	// once the pointer to the request won't change
+	// anymore, finish setting up the replacer
+	addHTTPVarsToReplacer(repl, r, w)
+
+	return r
+}
+
 // errLogValues inspects err and returns the status code
 // to use, the error log message, and any extra fields.
 // If err is a HandlerError, the returned values will