Merge pull request #760 from xeonx/master

Allow CORS on discovery endpoint

cmd/dex/config.go

@@ -98,10 +98,11 @@ type OAuth2 struct {
 
 // Web is the config format for the HTTP server.
 type Web struct {
-	HTTP    string `json:"http"`
-	HTTPS   string `json:"https"`
-	TLSCert string `json:"tlsCert"`
-	TLSKey  string `json:"tlsKey"`
+	HTTP                    string   `json:"http"`
+	HTTPS                   string   `json:"https"`
+	TLSCert                 string   `json:"tlsCert"`
+	TLSKey                  string   `json:"tlsKey"`
+	DiscoveryAllowedOrigins []string `json:"discoveryAllowedOrigins"`
 }
 
 // GRPC is the config for the gRPC API.

cmd/dex/serve.go

@@ -179,20 +179,24 @@ func serve(cmd *cobra.Command, args []string) error {
 	if c.OAuth2.SkipApprovalScreen {
 		logger.Infof("config skipping approval screen")
 	}
+	if len(c.Web.DiscoveryAllowedOrigins) > 0 {
+		logger.Infof("config discovery allowed origins: %s", c.Web.DiscoveryAllowedOrigins)
+	}
 
 	// explicitly convert to UTC.
 	now := func() time.Time { return time.Now().UTC() }
 
 	serverConfig := server.Config{
-		SupportedResponseTypes: c.OAuth2.ResponseTypes,
-		SkipApprovalScreen:     c.OAuth2.SkipApprovalScreen,
-		Issuer:                 c.Issuer,
-		Connectors:             connectors,
-		Storage:                s,
-		Web:                    c.Frontend,
-		EnablePasswordDB:       c.EnablePasswordDB,
-		Logger:                 logger,
-		Now:                    now,
+		SupportedResponseTypes:  c.OAuth2.ResponseTypes,
+		SkipApprovalScreen:      c.OAuth2.SkipApprovalScreen,
+		DiscoveryAllowedOrigins: c.Web.DiscoveryAllowedOrigins,
+		Issuer:                  c.Issuer,
+		Connectors:              connectors,
+		Storage:                 s,
+		Web:                     c.Frontend,
+		EnablePasswordDB:        c.EnablePasswordDB,
+		Logger:                  logger,
+		Now:                     now,
 	}
 	if c.Expiry.SigningKeys != "" {
 		signingKeys, err := time.ParseDuration(c.Expiry.SigningKeys)

glide.lock

-hash: 22c01c4265c210fbf3cd2d55e9451b924a3301e35053c82ebe35171ab7286c83
-updated: 2017-01-06T15:38:29.812891187-08:00
+hash: 4d7d84f09a330d27458fb821ae7ada243cfa825808dc7ab116db28a08f9166a2
+updated: 2017-01-08T19:23:40.352046548+01:00
 imports:
 - name: github.com/cockroachdb/cockroach-go
   version: 31611c0501c812f437d4861d87d117053967c955
@@ -18,6 +18,8 @@ imports:
   - protoc-gen-go
 - name: github.com/gorilla/context
   version: aed02d124ae4a0e94fea4541c8effd05bf0c8296
+- name: github.com/gorilla/handlers
+  version: 3a5767ca75ece5f7f1440b1d16975247f8d8b221
 - name: github.com/gorilla/mux
   version: 9fa818a44c2bf1396a17f9d5a3c0f6dd39d2ff8e
 - name: github.com/gtank/cryptopasta

glide.yaml

@@ -53,6 +53,8 @@ import:
   version: 9fa818a44c2bf1396a17f9d5a3c0f6dd39d2ff8e
 - package: github.com/gorilla/context
   version: aed02d124ae4a0e94fea4541c8effd05bf0c8296
+- package: github.com/gorilla/handlers
+  version: 3a5767ca75ece5f7f1440b1d16975247f8d8b221
 
 # Package with a bunch of sane crypto defaults. Consider just
 # copy the code (as recommended by the repo itself) instead

server/handlers.go

@@ -11,6 +11,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gorilla/handlers"
 	"github.com/gorilla/mux"
 	jose "gopkg.in/square/go-jose.v2"
 
@@ -101,7 +102,7 @@ type discovery struct {
 	Claims        []string `json:"claims_supported"`
 }
 
-func (s *Server) discoveryHandler() (http.HandlerFunc, error) {
+func (s *Server) discoveryHandler() (http.Handler, error) {
 	d := discovery{
 		Issuer:      s.issuerURL.String(),
 		Auth:        s.absURL("/auth"),
@@ -127,11 +128,18 @@ func (s *Server) discoveryHandler() (http.HandlerFunc, error) {
 		return nil, fmt.Errorf("failed to marshal discovery data: %v", err)
 	}
 
-	return func(w http.ResponseWriter, r *http.Request) {
+	var discoveryHandler http.Handler
+	discoveryHandler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "application/json")
 		w.Header().Set("Content-Length", strconv.Itoa(len(data)))
 		w.Write(data)
-	}, nil
+	})
+	if len(s.discoveryAllowedOrigins) > 0 {
+		corsOption := handlers.AllowedOrigins(s.discoveryAllowedOrigins)
+		discoveryHandler = handlers.CORS(corsOption)(discoveryHandler)
+	}
+
+	return discoveryHandler, nil
 }
 
 // handleAuthorization handles the OAuth2 auth endpoint.

server/handlers_test.go

@@ -22,3 +22,61 @@ func TestHandleHealth(t *testing.T) {
 	}
 
 }
+
+var discoveryHandlerCORSTests = []struct {
+	DiscoveryAllowedOrigins []string
+	Origin                  string
+	ResponseAllowOrigin     string //The expected response: same as Origin in case of valid CORS flow
+}{
+	{nil, "http://foo.example", ""}, //Default behavior: cross origin requests not allowed
+	{[]string{}, "http://foo.example", ""},
+	{[]string{"http://foo.example"}, "http://foo.example", "http://foo.example"},
+	{[]string{"http://bar.example", "http://foo.example"}, "http://foo.example", "http://foo.example"},
+	{[]string{"*"}, "http://foo.example", "http://foo.example"},
+	{[]string{"http://bar.example"}, "http://foo.example", ""},
+}
+
+func TestDiscoveryHandlerCORS(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	for _, testcase := range discoveryHandlerCORSTests {
+
+		httpServer, server := newTestServer(ctx, t, func(c *Config) {
+			c.DiscoveryAllowedOrigins = testcase.DiscoveryAllowedOrigins
+		})
+		defer httpServer.Close()
+
+		discoveryHandler, err := server.discoveryHandler()
+		if err != nil {
+			t.Fatalf("failed to get discovery handler: %v", err)
+		}
+
+		//Perform preflight request
+		rrPreflight := httptest.NewRecorder()
+		reqPreflight := httptest.NewRequest("OPTIONS", "/.well-kown/openid-configuration", nil)
+		reqPreflight.Header.Set("Origin", testcase.Origin)
+		reqPreflight.Header.Set("Access-Control-Request-Method", "GET")
+		discoveryHandler.ServeHTTP(rrPreflight, reqPreflight)
+		if rrPreflight.Code != http.StatusOK {
+			t.Errorf("expected 200 got %d", rrPreflight.Code)
+		}
+		headerAccessControlPreflight := rrPreflight.HeaderMap.Get("Access-Control-Allow-Origin")
+		if headerAccessControlPreflight != testcase.ResponseAllowOrigin {
+			t.Errorf("expected '%s' got '%s'", testcase.ResponseAllowOrigin, headerAccessControlPreflight)
+		}
+
+		//Perform request
+		rr := httptest.NewRecorder()
+		req := httptest.NewRequest("GET", "/.well-kown/openid-configuration", nil)
+		req.Header.Set("Origin", testcase.Origin)
+		discoveryHandler.ServeHTTP(rr, req)
+		if rr.Code != http.StatusOK {
+			t.Errorf("expected 200 got %d", rr.Code)
+		}
+		headerAccessControl := rr.HeaderMap.Get("Access-Control-Allow-Origin")
+		if headerAccessControl != testcase.ResponseAllowOrigin {
+			t.Errorf("expected '%s' got '%s'", testcase.ResponseAllowOrigin, headerAccessControl)
+		}
+	}
+}

server/server.go

@@ -42,6 +42,11 @@ type Config struct {
 	// flow. If no response types are supplied this value defaults to "code".
 	SupportedResponseTypes []string
 
+	// List of allowed origins for CORS requests on discovery endpoint.
+	// If none are indicated, CORS requests are disabled. Passing in "*" will allow any
+	// domain.
+	DiscoveryAllowedOrigins []string
+
 	// If enabled, the server won't prompt the user to approve authorization requests.
 	// Logging in implies approval.
 	SkipApprovalScreen bool
@@ -111,6 +116,8 @@ type Server struct {
 
 	supportedResponseTypes map[string]bool
 
+	discoveryAllowedOrigins []string
+
 	now func() time.Time
 
 	idTokensValidFor time.Duration
@@ -178,15 +185,16 @@ func newServer(ctx context.Context, c Config, rotationStrategy rotationStrategy)
 	}
 
 	s := &Server{
-		issuerURL:              *issuerURL,
-		connectors:             make(map[string]Connector),
-		storage:                newKeyCacher(c.Storage, now),
-		supportedResponseTypes: supported,
-		idTokensValidFor:       value(c.IDTokensValidFor, 24*time.Hour),
-		skipApproval:           c.SkipApprovalScreen,
-		now:                    now,
-		templates:              tmpls,
-		logger:                 c.Logger,
+		issuerURL:               *issuerURL,
+		connectors:              make(map[string]Connector),
+		storage:                 newKeyCacher(c.Storage, now),
+		supportedResponseTypes:  supported,
+		discoveryAllowedOrigins: c.DiscoveryAllowedOrigins,
+		idTokensValidFor:        value(c.IDTokensValidFor, 24*time.Hour),
+		skipApproval:            c.SkipApprovalScreen,
+		now:                     now,
+		templates:               tmpls,
+		logger:                  c.Logger,
 	}
 
 	for _, conn := range c.Connectors {
@@ -197,6 +205,9 @@ func newServer(ctx context.Context, c Config, rotationStrategy rotationStrategy)
 	handleFunc := func(p string, h http.HandlerFunc) {
 		r.HandleFunc(path.Join(issuerURL.Path, p), h)
 	}
+	handle := func(p string, h http.Handler) {
+		r.Handle(path.Join(issuerURL.Path, p), h)
+	}
 	handlePrefix := func(p string, h http.Handler) {
 		prefix := path.Join(issuerURL.Path, p)
 		r.PathPrefix(prefix).Handler(http.StripPrefix(prefix, h))
@@ -207,7 +218,7 @@ func newServer(ctx context.Context, c Config, rotationStrategy rotationStrategy)
 	if err != nil {
 		return nil, err
 	}
-	handleFunc("/.well-known/openid-configuration", discoveryHandler)
+	handle("/.well-known/openid-configuration", discoveryHandler)
 
 	// TODO(ericchiang): rate limit certain paths based on IP.
 	handleFunc("/token", s.handleToken)

vendor/github.com/gorilla/handlers/LICENSE

+Copyright (c) 2013 The Gorilla Handlers Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+  Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+  Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

vendor/github.com/gorilla/handlers/canonical.go

+package handlers
+
+import (
+	"net/http"
+	"net/url"
+	"strings"
+)
+
+type canonical struct {
+	h      http.Handler
+	domain string
+	code   int
+}
+
+// CanonicalHost is HTTP middleware that re-directs requests to the canonical
+// domain. It accepts a domain and a status code (e.g. 301 or 302) and
+// re-directs clients to this domain. The existing request path is maintained.
+//
+// Note: If the provided domain is considered invalid by url.Parse or otherwise
+// returns an empty scheme or host, clients are not re-directed.
+//
+// Example:
+//
+//  r := mux.NewRouter()
+//  canonical := handlers.CanonicalHost("http://www.gorillatoolkit.org", 302)
+//  r.HandleFunc("/route", YourHandler)
+//
+//  log.Fatal(http.ListenAndServe(":7000", canonical(r)))
+//
+func CanonicalHost(domain string, code int) func(h http.Handler) http.Handler {
+	fn := func(h http.Handler) http.Handler {
+		return canonical{h, domain, code}
+	}
+
+	return fn
+}
+
+func (c canonical) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	dest, err := url.Parse(c.domain)
+	if err != nil {
+		// Call the next handler if the provided domain fails to parse.
+		c.h.ServeHTTP(w, r)
+		return
+	}
+
+	if dest.Scheme == "" || dest.Host == "" {
+		// Call the next handler if the scheme or host are empty.
+		// Note that url.Parse won't fail on in this case.
+		c.h.ServeHTTP(w, r)
+		return
+	}
+
+	if !strings.EqualFold(cleanHost(r.Host), dest.Host) {
+		// Re-build the destination URL
+		dest := dest.Scheme + "://" + dest.Host + r.URL.Path
+		if r.URL.RawQuery != "" {
+			dest += "?" + r.URL.RawQuery
+		}
+		http.Redirect(w, r, dest, c.code)
+		return
+	}
+
+	c.h.ServeHTTP(w, r)
+}
+
+// cleanHost cleans invalid Host headers by stripping anything after '/' or ' '.
+// This is backported from Go 1.5 (in response to issue #11206) and attempts to
+// mitigate malformed Host headers that do not match the format in RFC7230.
+func cleanHost(in string) string {
+	if i := strings.IndexAny(in, " /"); i != -1 {
+		return in[:i]
+	}
+	return in
+}

vendor/github.com/gorilla/handlers/compress.go

+// Copyright 2013 The Gorilla Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package handlers
+
+import (
+	"compress/flate"
+	"compress/gzip"
+	"io"
+	"net/http"
+	"strings"
+)
+
+type compressResponseWriter struct {
+	io.Writer
+	http.ResponseWriter
+	http.Hijacker
+	http.Flusher
+	http.CloseNotifier
+}
+
+func (w *compressResponseWriter) WriteHeader(c int) {
+	w.ResponseWriter.Header().Del("Content-Length")
+	w.ResponseWriter.WriteHeader(c)
+}
+
+func (w *compressResponseWriter) Header() http.Header {
+	return w.ResponseWriter.Header()
+}
+
+func (w *compressResponseWriter) Write(b []byte) (int, error) {
+	h := w.ResponseWriter.Header()
+	if h.Get("Content-Type") == "" {
+		h.Set("Content-Type", http.DetectContentType(b))
+	}
+	h.Del("Content-Length")
+
+	return w.Writer.Write(b)
+}
+
+type flusher interface {
+	Flush() error
+}
+
+func (w *compressResponseWriter) Flush() {
+	// Flush compressed data if compressor supports it.
+	if f, ok := w.Writer.(flusher); ok {
+		f.Flush()
+	}
+	// Flush HTTP response.
+	if w.Flusher != nil {
+		w.Flusher.Flush()
+	}
+}
+
+// CompressHandler gzip compresses HTTP responses for clients that support it
+// via the 'Accept-Encoding' header.
+//
+// Compressing TLS traffic may leak the page contents to an attacker if the
+// page contains user input: http://security.stackexchange.com/a/102015/12208
+func CompressHandler(h http.Handler) http.Handler {
+	return CompressHandlerLevel(h, gzip.DefaultCompression)
+}
+
+// CompressHandlerLevel gzip compresses HTTP responses with specified compression level
+// for clients that support it via the 'Accept-Encoding' header.
+//
+// The compression level should be gzip.DefaultCompression, gzip.NoCompression,
+// or any integer value between gzip.BestSpeed and gzip.BestCompression inclusive.
+// gzip.DefaultCompression is used in case of invalid compression level.
+func CompressHandlerLevel(h http.Handler, level int) http.Handler {
+	if level < gzip.DefaultCompression || level > gzip.BestCompression {
+		level = gzip.DefaultCompression
+	}
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	L:
+		for _, enc := range strings.Split(r.Header.Get("Accept-Encoding"), ",") {
+			switch strings.TrimSpace(enc) {
+			case "gzip":
+				w.Header().Set("Content-Encoding", "gzip")
+				w.Header().Add("Vary", "Accept-Encoding")
+
+				gw, _ := gzip.NewWriterLevel(w, level)
+				defer gw.Close()
+
+				h, hok := w.(http.Hijacker)
+				if !hok { /* w is not Hijacker... oh well... */
+					h = nil
+				}
+
+				f, fok := w.(http.Flusher)
+				if !fok {
+					f = nil
+				}
+
+				cn, cnok := w.(http.CloseNotifier)
+				if !cnok {
+					cn = nil
+				}
+
+				w = &compressResponseWriter{
+					Writer:         gw,
+					ResponseWriter: w,
+					Hijacker:       h,
+					Flusher:        f,
+					CloseNotifier:  cn,
+				}
+
+				break L
+			case "deflate":
+				w.Header().Set("Content-Encoding", "deflate")
+				w.Header().Add("Vary", "Accept-Encoding")
+
+				fw, _ := flate.NewWriter(w, level)
+				defer fw.Close()
+
+				h, hok := w.(http.Hijacker)
+				if !hok { /* w is not Hijacker... oh well... */
+					h = nil
+				}
+
+				f, fok := w.(http.Flusher)
+				if !fok {
+					f = nil
+				}
+
+				cn, cnok := w.(http.CloseNotifier)
+				if !cnok {
+					cn = nil
+				}
+
+				w = &compressResponseWriter{
+					Writer:         fw,
+					ResponseWriter: w,
+					Hijacker:       h,
+					Flusher:        f,
+					CloseNotifier:  cn,
+				}
+
+				break L
+			}
+		}
+
+		h.ServeHTTP(w, r)
+	})
+}

vendor/github.com/gorilla/handlers/cors.go

+package handlers
+
+import (
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+// CORSOption represents a functional option for configuring the CORS middleware.
+type CORSOption func(*cors) error
+
+type cors struct {
+	h                      http.Handler
+	allowedHeaders         []string
+	allowedMethods         []string
+	allowedOrigins         []string
+	allowedOriginValidator OriginValidator
+	exposedHeaders         []string
+	maxAge                 int
+	ignoreOptions          bool
+	allowCredentials       bool
+}
+
+// OriginValidator takes an origin string and returns whether or not that origin is allowed.
+type OriginValidator func(string) bool
+
+var (
+	defaultCorsMethods = []string{"GET", "HEAD", "POST"}
+	defaultCorsHeaders = []string{"Accept", "Accept-Language", "Content-Language", "Origin"}
+	// (WebKit/Safari v9 sends the Origin header by default in AJAX requests)
+)
+
+const (
+	corsOptionMethod           string = "OPTIONS"
+	corsAllowOriginHeader      string = "Access-Control-Allow-Origin"
+	corsExposeHeadersHeader    string = "Access-Control-Expose-Headers"
+	corsMaxAgeHeader           string = "Access-Control-Max-Age"
+	corsAllowMethodsHeader     string = "Access-Control-Allow-Methods"
+	corsAllowHeadersHeader     string = "Access-Control-Allow-Headers"
+	corsAllowCredentialsHeader string = "Access-Control-Allow-Credentials"
+	corsRequestMethodHeader    string = "Access-Control-Request-Method"
+	corsRequestHeadersHeader   string = "Access-Control-Request-Headers"
+	corsOriginHeader           string = "Origin"
+	corsVaryHeader             string = "Vary"
+	corsOriginMatchAll         string = "*"
+)
+
+func (ch *cors) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	origin := r.Header.Get(corsOriginHeader)
+	if !ch.isOriginAllowed(origin) {
+		ch.h.ServeHTTP(w, r)
+		return
+	}
+
+	if r.Method == corsOptionMethod {
+		if ch.ignoreOptions {
+			ch.h.ServeHTTP(w, r)
+			return
+		}
+
+		if _, ok := r.Header[corsRequestMethodHeader]; !ok {
+			w.WriteHeader(http.StatusBadRequest)
+			return
+		}
+
+		method := r.Header.Get(corsRequestMethodHeader)
+		if !ch.isMatch(method, ch.allowedMethods) {
+			w.WriteHeader(http.StatusMethodNotAllowed)
+			return
+		}
+
+		requestHeaders := strings.Split(r.Header.Get(corsRequestHeadersHeader), ",")
+		allowedHeaders := []string{}
+		for _, v := range requestHeaders {
+			canonicalHeader := http.CanonicalHeaderKey(strings.TrimSpace(v))
+			if canonicalHeader == "" || ch.isMatch(canonicalHeader, defaultCorsHeaders) {
+				continue
+			}
+
+			if !ch.isMatch(canonicalHeader, ch.allowedHeaders) {
+				w.WriteHeader(http.StatusForbidden)
+				return
+			}
+
+			allowedHeaders = append(allowedHeaders, canonicalHeader)
+		}
+
+		if len(allowedHeaders) > 0 {
+			w.Header().Set(corsAllowHeadersHeader, strings.Join(allowedHeaders, ","))
+		}
+
+		if ch.maxAge > 0 {
+			w.Header().Set(corsMaxAgeHeader, strconv.Itoa(ch.maxAge))
+		}
+
+		if !ch.isMatch(method, defaultCorsMethods) {
+			w.Header().Set(corsAllowMethodsHeader, method)
+		}
+	} else {
+		if len(ch.exposedHeaders) > 0 {
+			w.Header().Set(corsExposeHeadersHeader, strings.Join(ch.exposedHeaders, ","))
+		}
+	}
+
+	if ch.allowCredentials {
+		w.Header().Set(corsAllowCredentialsHeader, "true")
+	}
+
+	if len(ch.allowedOrigins) > 1 {
+		w.Header().Set(corsVaryHeader, corsOriginHeader)
+	}
+
+	w.Header().Set(corsAllowOriginHeader, origin)
+
+	if r.Method == corsOptionMethod {
+		return
+	}
+	ch.h.ServeHTTP(w, r)
+}
+
+// CORS provides Cross-Origin Resource Sharing middleware.
+// Example:
+//
+//  import (
+//      "net/http"
+//
+//      "github.com/gorilla/handlers"
+//      "github.com/gorilla/mux"
+//  )
+//
+//  func main() {
+//      r := mux.NewRouter()
+//      r.HandleFunc("/users", UserEndpoint)
+//      r.HandleFunc("/projects", ProjectEndpoint)
+//
+//      // Apply the CORS middleware to our top-level router, with the defaults.
+//      http.ListenAndServe(":8000", handlers.CORS()(r))
+//  }
+//
+func CORS(opts ...CORSOption) func(http.Handler) http.Handler {
+	return func(h http.Handler) http.Handler {
+		ch := parseCORSOptions(opts...)
+		ch.h = h
+		return ch
+	}
+}
+
+func parseCORSOptions(opts ...CORSOption) *cors {
+	ch := &cors{
+		allowedMethods: defaultCorsMethods,
+		allowedHeaders: defaultCorsHeaders,
+		allowedOrigins: []string{corsOriginMatchAll},
+	}
+
+	for _, option := range opts {
+		option(ch)
+	}
+
+	return ch
+}
+
+//
+// Functional options for configuring CORS.
+//
+
+// AllowedHeaders adds the provided headers to the list of allowed headers in a
+// CORS request.
+// This is an append operation so the headers Accept, Accept-Language,
+// and Content-Language are always allowed.
+// Content-Type must be explicitly declared if accepting Content-Types other than
+// application/x-www-form-urlencoded, multipart/form-data, or text/plain.
+func AllowedHeaders(headers []string) CORSOption {
+	return func(ch *cors) error {
+		for _, v := range headers {
+			normalizedHeader := http.CanonicalHeaderKey(strings.TrimSpace(v))
+			if normalizedHeader == "" {
+				continue
+			}
+
+			if !ch.isMatch(normalizedHeader, ch.allowedHeaders) {
+				ch.allowedHeaders = append(ch.allowedHeaders, normalizedHeader)
+			}
+		}
+
+		return nil
+	}
+}
+
+// AllowedMethods can be used to explicitly allow methods in the
+// Access-Control-Allow-Methods header.
+// This is a replacement operation so you must also
+// pass GET, HEAD, and POST if you wish to support those methods.
+func AllowedMethods(methods []string) CORSOption {
+	return func(ch *cors) error {
+		ch.allowedMethods = []string{}
+		for _, v := range methods {
+			normalizedMethod := strings.ToUpper(strings.TrimSpace(v))
+			if normalizedMethod == "" {
+				continue
+			}
+
+			if !ch.isMatch(normalizedMethod, ch.allowedMethods) {
+				ch.allowedMethods = append(ch.allowedMethods, normalizedMethod)
+			}
+		}
+
+		return nil
+	}
+}
+
+// AllowedOrigins sets the allowed origins for CORS requests, as used in the
+// 'Allow-Access-Control-Origin' HTTP header.
+// Note: Passing in a []string{"*"} will allow any domain.
+func AllowedOrigins(origins []string) CORSOption {
+	return func(ch *cors) error {
+		for _, v := range origins {
+			if v == corsOriginMatchAll {
+				ch.allowedOrigins = []string{corsOriginMatchAll}
+				return nil
+			}
+		}
+
+		ch.allowedOrigins = origins
+		return nil
+	}
+}
+
+// AllowedOriginValidator sets a function for evaluating allowed origins in CORS requests, represented by the
+// 'Allow-Access-Control-Origin' HTTP header.
+func AllowedOriginValidator(fn OriginValidator) CORSOption {
+	return func(ch *cors) error {
+		ch.allowedOriginValidator = fn
+		return nil
+	}
+}
+
+// ExposeHeaders can be used to specify headers that are available
+// and will not be stripped out by the user-agent.
+func ExposedHeaders(headers []string) CORSOption {
+	return func(ch *cors) error {
+		ch.exposedHeaders = []string{}
+		for _, v := range headers {
+			normalizedHeader := http.CanonicalHeaderKey(strings.TrimSpace(v))
+			if normalizedHeader == "" {
+				continue
+			}
+
+			if !ch.isMatch(normalizedHeader, ch.exposedHeaders) {
+				ch.exposedHeaders = append(ch.exposedHeaders, normalizedHeader)
+			}
+		}
+
+		return nil
+	}
+}
+
+// MaxAge determines the maximum age (in seconds) between preflight requests. A
+// maximum of 10 minutes is allowed. An age above this value will default to 10
+// minutes.
+func MaxAge(age int) CORSOption {
+	return func(ch *cors) error {
+		// Maximum of 10 minutes.
+		if age > 600 {
+			age = 600
+		}
+
+		ch.maxAge = age
+		return nil
+	}
+}
+
+// IgnoreOptions causes the CORS middleware to ignore OPTIONS requests, instead
+// passing them through to the next handler. This is useful when your application
+// or framework has a pre-existing mechanism for responding to OPTIONS requests.
+func IgnoreOptions() CORSOption {
+	return func(ch *cors) error {
+		ch.ignoreOptions = true
+		return nil
+	}
+}
+
+// AllowCredentials can be used to specify that the user agent may pass
+// authentication details along with the request.
+func AllowCredentials() CORSOption {
+	return func(ch *cors) error {
+		ch.allowCredentials = true
+		return nil
+	}
+}
+
+func (ch *cors) isOriginAllowed(origin string) bool {
+	if origin == "" {
+		return false
+	}
+
+	if ch.allowedOriginValidator != nil {
+		return ch.allowedOriginValidator(origin)
+	}
+
+	for _, allowedOrigin := range ch.allowedOrigins {
+		if allowedOrigin == origin || allowedOrigin == corsOriginMatchAll {
+			return true
+		}
+	}
+
+	return false
+}
+
+func (ch *cors) isMatch(needle string, haystack []string) bool {
+	for _, v := range haystack {
+		if v == needle {
+			return true
+		}
+	}
+
+	return false
+}

vendor/github.com/gorilla/handlers/doc.go

+/*
+Package handlers is a collection of handlers (aka "HTTP middleware") for use
+with Go's net/http package (or any framework supporting http.Handler).
+
+The package includes handlers for logging in standardised formats, compressing
+HTTP responses, validating content types and other useful tools for manipulating
+requests and responses.
+*/
+package handlers

vendor/github.com/gorilla/handlers/handlers.go

+// Copyright 2013 The Gorilla Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package handlers
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"net/url"
+	"sort"
+	"strconv"
+	"strings"
+	"time"
+	"unicode/utf8"
+)
+
+// MethodHandler is an http.Handler that dispatches to a handler whose key in the
+// MethodHandler's map matches the name of the HTTP request's method, eg: GET
+//
+// If the request's method is OPTIONS and OPTIONS is not a key in the map then
+// the handler responds with a status of 200 and sets the Allow header to a
+// comma-separated list of available methods.
+//
+// If the request's method doesn't match any of its keys the handler responds
+// with a status of HTTP 405 "Method Not Allowed" and sets the Allow header to a
+// comma-separated list of available methods.
+type MethodHandler map[string]http.Handler
+
+func (h MethodHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	if handler, ok := h[req.Method]; ok {
+		handler.ServeHTTP(w, req)
+	} else {
+		allow := []string{}
+		for k := range h {
+			allow = append(allow, k)
+		}
+		sort.Strings(allow)
+		w.Header().Set("Allow", strings.Join(allow, ", "))
+		if req.Method == "OPTIONS" {
+			w.WriteHeader(http.StatusOK)
+		} else {
+			http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		}
+	}
+}
+
+// loggingHandler is the http.Handler implementation for LoggingHandlerTo and its
+// friends
+type loggingHandler struct {
+	writer  io.Writer
+	handler http.Handler
+}
+
+// combinedLoggingHandler is the http.Handler implementation for LoggingHandlerTo
+// and its friends
+type combinedLoggingHandler struct {
+	writer  io.Writer
+	handler http.Handler
+}
+
+func (h loggingHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	t := time.Now()
+	logger := makeLogger(w)
+	url := *req.URL
+	h.handler.ServeHTTP(logger, req)
+	writeLog(h.writer, req, url, t, logger.Status(), logger.Size())
+}
+
+func (h combinedLoggingHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	t := time.Now()
+	logger := makeLogger(w)
+	url := *req.URL
+	h.handler.ServeHTTP(logger, req)
+	writeCombinedLog(h.writer, req, url, t, logger.Status(), logger.Size())
+}
+
+func makeLogger(w http.ResponseWriter) loggingResponseWriter {
+	var logger loggingResponseWriter = &responseLogger{w: w}
+	if _, ok := w.(http.Hijacker); ok {
+		logger = &hijackLogger{responseLogger{w: w}}
+	}
+	h, ok1 := logger.(http.Hijacker)
+	c, ok2 := w.(http.CloseNotifier)
+	if ok1 && ok2 {
+		return hijackCloseNotifier{logger, h, c}
+	}
+	if ok2 {
+		return &closeNotifyWriter{logger, c}
+	}
+	return logger
+}
+
+type loggingResponseWriter interface {
+	http.ResponseWriter
+	http.Flusher
+	Status() int
+	Size() int
+}
+
+// responseLogger is wrapper of http.ResponseWriter that keeps track of its HTTP
+// status code and body size
+type responseLogger struct {
+	w      http.ResponseWriter
+	status int
+	size   int
+}
+
+func (l *responseLogger) Header() http.Header {
+	return l.w.Header()
+}
+
+func (l *responseLogger) Write(b []byte) (int, error) {
+	if l.status == 0 {
+		// The status will be StatusOK if WriteHeader has not been called yet
+		l.status = http.StatusOK
+	}
+	size, err := l.w.Write(b)
+	l.size += size
+	return size, err
+}
+
+func (l *responseLogger) WriteHeader(s int) {
+	l.w.WriteHeader(s)
+	l.status = s
+}
+
+func (l *responseLogger) Status() int {
+	return l.status
+}
+
+func (l *responseLogger) Size() int {
+	return l.size
+}
+
+func (l *responseLogger) Flush() {
+	f, ok := l.w.(http.Flusher)
+	if ok {
+		f.Flush()
+	}
+}
+
+type hijackLogger struct {
+	responseLogger
+}
+
+func (l *hijackLogger) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	h := l.responseLogger.w.(http.Hijacker)
+	conn, rw, err := h.Hijack()
+	if err == nil && l.responseLogger.status == 0 {
+		// The status will be StatusSwitchingProtocols if there was no error and
+		// WriteHeader has not been called yet
+		l.responseLogger.status = http.StatusSwitchingProtocols
+	}
+	return conn, rw, err
+}
+
+type closeNotifyWriter struct {
+	loggingResponseWriter
+	http.CloseNotifier
+}
+
+type hijackCloseNotifier struct {
+	loggingResponseWriter
+	http.Hijacker
+	http.CloseNotifier
+}
+
+const lowerhex = "0123456789abcdef"
+
+func appendQuoted(buf []byte, s string) []byte {
+	var runeTmp [utf8.UTFMax]byte
+	for width := 0; len(s) > 0; s = s[width:] {
+		r := rune(s[0])
+		width = 1
+		if r >= utf8.RuneSelf {
+			r, width = utf8.DecodeRuneInString(s)
+		}
+		if width == 1 && r == utf8.RuneError {
+			buf = append(buf, `\x`...)
+			buf = append(buf, lowerhex[s[0]>>4])
+			buf = append(buf, lowerhex[s[0]&0xF])
+			continue
+		}
+		if r == rune('"') || r == '\\' { // always backslashed
+			buf = append(buf, '\\')
+			buf = append(buf, byte(r))
+			continue
+		}
+		if strconv.IsPrint(r) {
+			n := utf8.EncodeRune(runeTmp[:], r)
+			buf = append(buf, runeTmp[:n]...)
+			continue
+		}
+		switch r {
+		case '\a':
+			buf = append(buf, `\a`...)
+		case '\b':
+			buf = append(buf, `\b`...)
+		case '\f':
+			buf = append(buf, `\f`...)
+		case '\n':
+			buf = append(buf, `\n`...)
+		case '\r':
+			buf = append(buf, `\r`...)
+		case '\t':
+			buf = append(buf, `\t`...)
+		case '\v':
+			buf = append(buf, `\v`...)
+		default:
+			switch {
+			case r < ' ':
+				buf = append(buf, `\x`...)
+				buf = append(buf, lowerhex[s[0]>>4])
+				buf = append(buf, lowerhex[s[0]&0xF])
+			case r > utf8.MaxRune:
+				r = 0xFFFD
+				fallthrough
+			case r < 0x10000:
+				buf = append(buf, `\u`...)
+				for s := 12; s >= 0; s -= 4 {
+					buf = append(buf, lowerhex[r>>uint(s)&0xF])
+				}
+			default:
+				buf = append(buf, `\U`...)
+				for s := 28; s >= 0; s -= 4 {
+					buf = append(buf, lowerhex[r>>uint(s)&0xF])
+				}
+			}
+		}
+	}
+	return buf
+
+}
+
+// buildCommonLogLine builds a log entry for req in Apache Common Log Format.
+// ts is the timestamp with which the entry should be logged.
+// status and size are used to provide the response HTTP status and size.
+func buildCommonLogLine(req *http.Request, url url.URL, ts time.Time, status int, size int) []byte {
+	username := "-"
+	if url.User != nil {
+		if name := url.User.Username(); name != "" {
+			username = name
+		}
+	}
+
+	host, _, err := net.SplitHostPort(req.RemoteAddr)
+
+	if err != nil {
+		host = req.RemoteAddr
+	}
+
+	uri := req.RequestURI
+
+	// Requests using the CONNECT method over HTTP/2.0 must use
+	// the authority field (aka r.Host) to identify the target.
+	// Refer: https://httpwg.github.io/specs/rfc7540.html#CONNECT
+	if req.ProtoMajor == 2 && req.Method == "CONNECT" {
+		uri = req.Host
+	}
+	if uri == "" {
+		uri = url.RequestURI()
+	}
+
+	buf := make([]byte, 0, 3*(len(host)+len(username)+len(req.Method)+len(uri)+len(req.Proto)+50)/2)
+	buf = append(buf, host...)
+	buf = append(buf, " - "...)
+	buf = append(buf, username...)
+	buf = append(buf, " ["...)
+	buf = append(buf, ts.Format("02/Jan/2006:15:04:05 -0700")...)
+	buf = append(buf, `] "`...)
+	buf = append(buf, req.Method...)
+	buf = append(buf, " "...)
+	buf = appendQuoted(buf, uri)
+	buf = append(buf, " "...)
+	buf = append(buf, req.Proto...)
+	buf = append(buf, `" `...)
+	buf = append(buf, strconv.Itoa(status)...)
+	buf = append(buf, " "...)
+	buf = append(buf, strconv.Itoa(size)...)
+	return buf
+}
+
+// writeLog writes a log entry for req to w in Apache Common Log Format.
+// ts is the timestamp with which the entry should be logged.
+// status and size are used to provide the response HTTP status and size.
+func writeLog(w io.Writer, req *http.Request, url url.URL, ts time.Time, status, size int) {
+	buf := buildCommonLogLine(req, url, ts, status, size)
+	buf = append(buf, '\n')
+	w.Write(buf)
+}
+
+// writeCombinedLog writes a log entry for req to w in Apache Combined Log Format.
+// ts is the timestamp with which the entry should be logged.
+// status and size are used to provide the response HTTP status and size.
+func writeCombinedLog(w io.Writer, req *http.Request, url url.URL, ts time.Time, status, size int) {
+	buf := buildCommonLogLine(req, url, ts, status, size)
+	buf = append(buf, ` "`...)
+	buf = appendQuoted(buf, req.Referer())
+	buf = append(buf, `" "`...)
+	buf = appendQuoted(buf, req.UserAgent())
+	buf = append(buf, '"', '\n')
+	w.Write(buf)
+}
+
+// CombinedLoggingHandler return a http.Handler that wraps h and logs requests to out in
+// Apache Combined Log Format.
+//
+// See http://httpd.apache.org/docs/2.2/logs.html#combined for a description of this format.
+//
+// LoggingHandler always sets the ident field of the log to -
+func CombinedLoggingHandler(out io.Writer, h http.Handler) http.Handler {
+	return combinedLoggingHandler{out, h}
+}
+
+// LoggingHandler return a http.Handler that wraps h and logs requests to out in
+// Apache Common Log Format (CLF).
+//
+// See http://httpd.apache.org/docs/2.2/logs.html#common for a description of this format.
+//
+// LoggingHandler always sets the ident field of the log to -
+//
+// Example:
+//
+//  r := mux.NewRouter()
+//  r.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+//  	w.Write([]byte("This is a catch-all route"))
+//  })
+//  loggedRouter := handlers.LoggingHandler(os.Stdout, r)
+//  http.ListenAndServe(":1123", loggedRouter)
+//
+func LoggingHandler(out io.Writer, h http.Handler) http.Handler {
+	return loggingHandler{out, h}
+}
+
+// isContentType validates the Content-Type header matches the supplied
+// contentType. That is, its type and subtype match.
+func isContentType(h http.Header, contentType string) bool {
+	ct := h.Get("Content-Type")
+	if i := strings.IndexRune(ct, ';'); i != -1 {
+		ct = ct[0:i]
+	}
+	return ct == contentType
+}
+
+// ContentTypeHandler wraps and returns a http.Handler, validating the request
+// content type is compatible with the contentTypes list. It writes a HTTP 415
+// error if that fails.
+//
+// Only PUT, POST, and PATCH requests are considered.
+func ContentTypeHandler(h http.Handler, contentTypes ...string) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if !(r.Method == "PUT" || r.Method == "POST" || r.Method == "PATCH") {
+			h.ServeHTTP(w, r)
+			return
+		}
+
+		for _, ct := range contentTypes {
+			if isContentType(r.Header, ct) {
+				h.ServeHTTP(w, r)
+				return
+			}
+		}
+		http.Error(w, fmt.Sprintf("Unsupported content type %q; expected one of %q", r.Header.Get("Content-Type"), contentTypes), http.StatusUnsupportedMediaType)
+	})
+}
+
+const (
+	// HTTPMethodOverrideHeader is a commonly used
+	// http header to override a request method.
+	HTTPMethodOverrideHeader = "X-HTTP-Method-Override"
+	// HTTPMethodOverrideFormKey is a commonly used
+	// HTML form key to override a request method.
+	HTTPMethodOverrideFormKey = "_method"
+)
+
+// HTTPMethodOverrideHandler wraps and returns a http.Handler which checks for
+// the X-HTTP-Method-Override header or the _method form key, and overrides (if
+// valid) request.Method with its value.
+//
+// This is especially useful for HTTP clients that don't support many http verbs.
+// It isn't secure to override e.g a GET to a POST, so only POST requests are
+// considered.  Likewise, the override method can only be a "write" method: PUT,
+// PATCH or DELETE.
+//
+// Form method takes precedence over header method.
+func HTTPMethodOverrideHandler(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method == "POST" {
+			om := r.FormValue(HTTPMethodOverrideFormKey)
+			if om == "" {
+				om = r.Header.Get(HTTPMethodOverrideHeader)
+			}
+			if om == "PUT" || om == "PATCH" || om == "DELETE" {
+				r.Method = om
+			}
+		}
+		h.ServeHTTP(w, r)
+	})
+}

vendor/github.com/gorilla/handlers/proxy_headers.go

+package handlers
+
+import (
+	"net/http"
+	"regexp"
+	"strings"
+)
+
+var (
+	// De-facto standard header keys.
+	xForwardedFor    = http.CanonicalHeaderKey("X-Forwarded-For")
+	xForwardedHost   = http.CanonicalHeaderKey("X-Forwarded-Host")
+	xForwardedProto  = http.CanonicalHeaderKey("X-Forwarded-Proto")
+	xForwardedScheme = http.CanonicalHeaderKey("X-Forwarded-Scheme")
+	xRealIP          = http.CanonicalHeaderKey("X-Real-IP")
+)
+
+var (
+	// RFC7239 defines a new "Forwarded: " header designed to replace the
+	// existing use of X-Forwarded-* headers.
+	// e.g. Forwarded: for=192.0.2.60;proto=https;by=203.0.113.43
+	forwarded = http.CanonicalHeaderKey("Forwarded")
+	// Allows for a sub-match of the first value after 'for=' to the next
+	// comma, semi-colon or space. The match is case-insensitive.
+	forRegex = regexp.MustCompile(`(?i)(?:for=)([^(;|,| )]+)`)
+	// Allows for a sub-match for the first instance of scheme (http|https)
+	// prefixed by 'proto='. The match is case-insensitive.
+	protoRegex = regexp.MustCompile(`(?i)(?:proto=)(https|http)`)
+)
+
+// ProxyHeaders inspects common reverse proxy headers and sets the corresponding
+// fields in the HTTP request struct. These are X-Forwarded-For and X-Real-IP
+// for the remote (client) IP address, X-Forwarded-Proto or X-Forwarded-Scheme
+// for the scheme (http|https) and the RFC7239 Forwarded header, which may
+// include both client IPs and schemes.
+//
+// NOTE: This middleware should only be used when behind a reverse
+// proxy like nginx, HAProxy or Apache. Reverse proxies that don't (or are
+// configured not to) strip these headers from client requests, or where these
+// headers are accepted "as is" from a remote client (e.g. when Go is not behind
+// a proxy), can manifest as a vulnerability if your application uses these
+// headers for validating the 'trustworthiness' of a request.
+func ProxyHeaders(h http.Handler) http.Handler {
+	fn := func(w http.ResponseWriter, r *http.Request) {
+		// Set the remote IP with the value passed from the proxy.
+		if fwd := getIP(r); fwd != "" {
+			r.RemoteAddr = fwd
+		}
+
+		// Set the scheme (proto) with the value passed from the proxy.
+		if scheme := getScheme(r); scheme != "" {
+			r.URL.Scheme = scheme
+		}
+		// Set the host with the value passed by the proxy
+		if r.Header.Get(xForwardedHost) != "" {
+			r.Host = r.Header.Get(xForwardedHost)
+		}
+		// Call the next handler in the chain.
+		h.ServeHTTP(w, r)
+	}
+
+	return http.HandlerFunc(fn)
+}
+
+// getIP retrieves the IP from the X-Forwarded-For, X-Real-IP and RFC7239
+// Forwarded headers (in that order).
+func getIP(r *http.Request) string {
+	var addr string
+
+	if fwd := r.Header.Get(xForwardedFor); fwd != "" {
+		// Only grab the first (client) address. Note that '192.168.0.1,
+		// 10.1.1.1' is a valid key for X-Forwarded-For where addresses after
+		// the first may represent forwarding proxies earlier in the chain.
+		s := strings.Index(fwd, ", ")
+		if s == -1 {
+			s = len(fwd)
+		}
+		addr = fwd[:s]
+	} else if fwd := r.Header.Get(xRealIP); fwd != "" {
+		// X-Real-IP should only contain one IP address (the client making the
+		// request).
+		addr = fwd
+	} else if fwd := r.Header.Get(forwarded); fwd != "" {
+		// match should contain at least two elements if the protocol was
+		// specified in the Forwarded header. The first element will always be
+		// the 'for=' capture, which we ignore. In the case of multiple IP
+		// addresses (for=8.8.8.8, 8.8.4.4,172.16.1.20 is valid) we only
+		// extract the first, which should be the client IP.
+		if match := forRegex.FindStringSubmatch(fwd); len(match) > 1 {
+			// IPv6 addresses in Forwarded headers are quoted-strings. We strip
+			// these quotes.
+			addr = strings.Trim(match[1], `"`)
+		}
+	}
+
+	return addr
+}
+
+// getScheme retrieves the scheme from the X-Forwarded-Proto and RFC7239
+// Forwarded headers (in that order).
+func getScheme(r *http.Request) string {
+	var scheme string
+
+	// Retrieve the scheme from X-Forwarded-Proto.
+	if proto := r.Header.Get(xForwardedProto); proto != "" {
+		scheme = strings.ToLower(proto)
+	} else if proto = r.Header.Get(xForwardedScheme); proto != "" {
+		scheme = strings.ToLower(proto)
+	} else if proto = r.Header.Get(forwarded); proto != "" {
+		// match should contain at least two elements if the protocol was
+		// specified in the Forwarded header. The first element will always be
+		// the 'proto=' capture, which we ignore. In the case of multiple proto
+		// parameters (invalid) we only extract the first.
+		if match := protoRegex.FindStringSubmatch(proto); len(match) > 1 {
+			scheme = strings.ToLower(match[1])
+		}
+	}
+
+	return scheme
+}

vendor/github.com/gorilla/handlers/recovery.go

+package handlers
+
+import (
+	"log"
+	"net/http"
+	"runtime/debug"
+)
+
+// RecoveryHandlerLogger is an interface used by the recovering handler to print logs.
+type RecoveryHandlerLogger interface {
+	Println(...interface{})
+}
+
+type recoveryHandler struct {
+	handler    http.Handler
+	logger     RecoveryHandlerLogger
+	printStack bool
+}
+
+// RecoveryOption provides a functional approach to define
+// configuration for a handler; such as setting the logging
+// whether or not to print strack traces on panic.
+type RecoveryOption func(http.Handler)
+
+func parseRecoveryOptions(h http.Handler, opts ...RecoveryOption) http.Handler {
+	for _, option := range opts {
+		option(h)
+	}
+
+	return h
+}
+
+// RecoveryHandler is HTTP middleware that recovers from a panic,
+// logs the panic, writes http.StatusInternalServerError, and
+// continues to the next handler.
+//
+// Example:
+//
+//  r := mux.NewRouter()
+//  r.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+//  	panic("Unexpected error!")
+//  })
+//
+//  http.ListenAndServe(":1123", handlers.RecoveryHandler()(r))
+func RecoveryHandler(opts ...RecoveryOption) func(h http.Handler) http.Handler {
+	return func(h http.Handler) http.Handler {
+		r := &recoveryHandler{handler: h}
+		return parseRecoveryOptions(r, opts...)
+	}
+}
+
+// RecoveryLogger is a functional option to override
+// the default logger
+func RecoveryLogger(logger RecoveryHandlerLogger) RecoveryOption {
+	return func(h http.Handler) {
+		r := h.(*recoveryHandler)
+		r.logger = logger
+	}
+}
+
+// PrintRecoveryStack is a functional option to enable
+// or disable printing stack traces on panic.
+func PrintRecoveryStack(print bool) RecoveryOption {
+	return func(h http.Handler) {
+		r := h.(*recoveryHandler)
+		r.printStack = print
+	}
+}
+
+func (h recoveryHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	defer func() {
+		if err := recover(); err != nil {
+			w.WriteHeader(http.StatusInternalServerError)
+			h.log(err)
+		}
+	}()
+
+	h.handler.ServeHTTP(w, req)
+}
+
+func (h recoveryHandler) log(v ...interface{}) {
+	if h.logger != nil {
+		h.logger.Println(v...)
+	} else {
+		log.Println(v...)
+	}
+
+	if h.printStack {
+		debug.PrintStack()
+	}
+}