pkg/server/api: Add /healthz for load-balancer health checks

The server currently 404s the root path.  From a master:

  [core@ip-10-0-26-134 ~]$ curl -ik https://wking-api.devcluster.openshift.com:49500/
  HTTP/1.1 404 Not Found
  Content-Type: text/plain; charset=utf-8
  X-Content-Type-Options: nosniff
  Date: Sun, 16 Dec 2018 06:30:14 GMT
  Content-Length: 19

  404 page not found

but we need a reliable response on the range 200-399 to satisfy our
network load balancer health checks, which do not support configurable
response status codes [1,2] (these are Terraform links, but they
discuss an AWS restriction that is not Terraform-specific).  This
commit adds a /healthz endpoint which always 204s (when the server is
alive to handle it).

[1]: hashicorp/terraform-provider-aws#2708 (comment)
[2]: https://github.com/terraform-providers/terraform-provider-aws/pull/2906/files#diff-375aea487c27a6ada86edfd817ba2401R612

pkg/server/api.go

@@ -22,6 +22,7 @@ type getConfig func(request poolRequest) (*ignv2_2types.Config, error)
 
 func newHandler(getConfig getConfig) http.Handler {
 	mux := http.NewServeMux()
+	mux.Handle("/healthz", &healthHandler{})
 	mux.Handle(apiPathConfig, &configHandler{getConfig: getConfig})
 	mux.Handle("/", &defaultHandler{})
 	return mux
@@ -85,6 +86,26 @@ func (h *configHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+type healthHandler struct{}
+
+// ServeHTTP handles /healthz requests.
+func (h *healthHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Length", "0")
+
+	if r.URL.Path == "/healthz" {
+		if r.Method == http.MethodGet || r.Method == http.MethodHead {
+			w.WriteHeader(http.StatusNoContent)
+			return
+		}
+
+		w.WriteHeader(http.StatusMethodNotAllowed)
+		return
+	}
+
+	w.WriteHeader(http.StatusNotFound)
+	return
+}
+
 // defaultHandler is the HTTP Handler for backstopping invalid requests.
 type defaultHandler struct{}
 

pkg/server/api_test.go

@@ -79,6 +79,78 @@ func TestHandler(t *testing.T) {
 				checkBodyLength(t, response, 0)
 			},
 		},
+		{
+			name:    "get /healthz",
+			request: httptest.NewRequest(http.MethodGet, "http://testrequest/healthz", nil),
+			getConfig: func(poolRequest) (*ignv2_2types.Config, error) {
+				return new(ignv2_2types.Config), nil
+			},
+			checkResponse: func(t *testing.T, response *http.Response) {
+				checkStatus(t, response, http.StatusNoContent)
+				checkContentLength(t, response, 0)
+				checkBodyLength(t, response, 0)
+			},
+		},
+		{
+			name:    "head /healthz",
+			request: httptest.NewRequest(http.MethodHead, "http://testrequest/healthz", nil),
+			getConfig: func(poolRequest) (*ignv2_2types.Config, error) {
+				return new(ignv2_2types.Config), nil
+			},
+			checkResponse: func(t *testing.T, response *http.Response) {
+				checkStatus(t, response, http.StatusNoContent)
+				checkContentLength(t, response, 0)
+				checkBodyLength(t, response, 0)
+			},
+		},
+		{
+			name:    "get health path that does not exist",
+			request: httptest.NewRequest(http.MethodGet, "http://testrequest/healthz-does-not-exist", nil),
+			getConfig: func(poolRequest) (*ignv2_2types.Config, error) {
+				return nil, nil
+			},
+			checkResponse: func(t *testing.T, response *http.Response) {
+				checkStatus(t, response, http.StatusNotFound)
+				checkContentLength(t, response, 0)
+				checkBodyLength(t, response, 0)
+			},
+		},
+		{
+			name:    "get health child that does not exist",
+			request: httptest.NewRequest(http.MethodGet, "http://testrequest/healthz/does-not-exist", nil),
+			getConfig: func(poolRequest) (*ignv2_2types.Config, error) {
+				return nil, nil
+			},
+			checkResponse: func(t *testing.T, response *http.Response) {
+				checkStatus(t, response, http.StatusNotFound)
+				checkContentLength(t, response, 0)
+				checkBodyLength(t, response, 0)
+			},
+		},
+		{
+			name:    "get root",
+			request: httptest.NewRequest(http.MethodGet, "http://testrequest/", nil),
+			getConfig: func(poolRequest) (*ignv2_2types.Config, error) {
+				return nil, nil
+			},
+			checkResponse: func(t *testing.T, response *http.Response) {
+				checkStatus(t, response, http.StatusNotFound)
+				checkContentLength(t, response, 0)
+				checkBodyLength(t, response, 0)
+			},
+		},
+		{
+			name:    "post root",
+			request: httptest.NewRequest(http.MethodPost, "http://testrequest/", nil),
+			getConfig: func(poolRequest) (*ignv2_2types.Config, error) {
+				return nil, nil
+			},
+			checkResponse: func(t *testing.T, response *http.Response) {
+				checkStatus(t, response, http.StatusNotFound)
+				checkContentLength(t, response, 0)
+				checkBodyLength(t, response, 0)
+			},
+		},
 		{
 			name:    "post non-config path that does not exist",
 			request: httptest.NewRequest(http.MethodPost, "http://testrequest/post", nil),
@@ -103,6 +175,18 @@ func TestHandler(t *testing.T) {
 				checkBodyLength(t, response, 0)
 			},
 		},
+		{
+			name:    "post /healthz",
+			request: httptest.NewRequest(http.MethodPost, "http://testrequest/healthz", nil),
+			getConfig: func(poolRequest) (*ignv2_2types.Config, error) {
+				return nil, nil
+			},
+			checkResponse: func(t *testing.T, response *http.Response) {
+				checkStatus(t, response, http.StatusMethodNotAllowed)
+				checkContentLength(t, response, 0)
+				checkBodyLength(t, response, 0)
+			},
+		},
 	}
 
 	for _, scenario := range scenarios {