Allow a health/ping request to be identified by User-Agent (#567)

* Add an option to allow health checks based on User-Agent.

* Formatting fix

* Rename field and avoid unnecessary interface.

* Skip the redirect fix so it can be put into a different PR.

* Add CHANGELOG entry

* Adding a couple tests for the PingUserAgent option.

oauthproxy.go

@@ -82,6 +82,8 @@ type OAuthProxy struct {
 
 	RobotsPath        string
 	PingPath          string
+	PingUserAgent     string
+	SilencePings      bool
 	SignInPath        string
 	SignOutPath       string
 	OAuthStartPath    string
@@ -312,6 +314,8 @@ func NewOAuthProxy(opts *options.Options, validator func(string) bool) *OAuthPro
 
 		RobotsPath:        "/robots.txt",
 		PingPath:          opts.PingPath,
+		PingUserAgent:     opts.PingUserAgent,
+		SilencePings:      opts.Logging.SilencePing,
 		SignInPath:        fmt.Sprintf("%s/sign_in", opts.ProxyPrefix),
 		SignOutPath:       fmt.Sprintf("%s/sign_out", opts.ProxyPrefix),
 		OAuthStartPath:    fmt.Sprintf("%s/start", opts.ProxyPrefix),
@@ -466,6 +470,11 @@ func (p *OAuthProxy) RobotsTxt(rw http.ResponseWriter) {
 
 // PingPage responds 200 OK to requests
 func (p *OAuthProxy) PingPage(rw http.ResponseWriter) {
+	if p.SilencePings {
+		if rl, ok := rw.(*responseLogger); ok {
+			rl.silent = true
+		}
+	}
 	rw.WriteHeader(http.StatusOK)
 	fmt.Fprintf(rw, "OK")
 }
@@ -675,6 +684,17 @@ func prepareNoCache(w http.ResponseWriter) {
 	}
 }
 
+// IsPingRequest will check if the request appears to be performing a health check
+// either via the path it's requesting or by a special User-Agent configuration.
+func (p *OAuthProxy) IsPingRequest(req *http.Request) bool {
+
+	if req.URL.EscapedPath() == p.PingPath {
+		return true
+	}
+
+	return p.PingUserAgent != "" && req.Header.Get("User-Agent") == p.PingUserAgent
+}
+
 func (p *OAuthProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	if strings.HasPrefix(req.URL.Path, p.ProxyPrefix) {
 		prepareNoCache(rw)
@@ -683,7 +703,7 @@ func (p *OAuthProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	switch path := req.URL.Path; {
 	case path == p.RobotsPath:
 		p.RobotsTxt(rw)
-	case path == p.PingPath:
+	case p.IsPingRequest(req):
 		p.PingPage(rw)
 	case p.IsWhitelistedRequest(req):
 		p.serveMux.ServeHTTP(rw, req)