Allow users to configure arbitrary headers in config

CHANGELOG.md

@@ -18,6 +18,7 @@ IMPROVEMENTS:
   * drivers: Interpret Nomad variables in environment variables/args [GH-653]
   * core: Populate job status [GH-663]
   * core/cli: Print short identifiers and UX cleanup [GH-675, GH-693, GH-692]
+  * api: Allow users to set arbitrary headers via agent config [GH-699]
 
 BUG FIXES:
   * cli: Handle parsing of un-named ports [GH-604]

command/agent/config.go

@@ -101,6 +101,10 @@ type Config struct {
 
 	// List of config files that have been loaded (in order)
 	Files []string
+
+	// HTTPAPIResponseHeaders allows users to configure the Nomad http agent to
+	// set arbritrary headers on API responses
+	HTTPAPIResponseHeaders map[string]string `hcl:"http_api_response_headers"`
 }
 
 // AtlasConfig is used to enable an parameterize the Atlas integration

command/agent/config_test.go

@@ -453,6 +453,9 @@ func TestConfig_LoadConfigString(t *testing.T) {
 			Join:           true,
 			Endpoint:       "127.0.0.1:1234",
 		},
+		HTTPAPIResponseHeaders: map[string]string{
+			"Access-Control-Allow-Origin": "*",
+		},
 	}
 
 	// Check parsing
@@ -531,4 +534,7 @@ atlas {
 	join = true
 	endpoint = "127.0.0.1:1234"
 }
+http_api_response_headers {
+	Access-Control-Allow-Origin = "*"
+}
 `

command/agent/http.go

@@ -152,6 +152,7 @@ func (e *codedError) Code() int {
 // wrap is used to wrap functions to make them more convenient
 func (s *HTTPServer) wrap(handler func(resp http.ResponseWriter, req *http.Request) (interface{}, error)) func(resp http.ResponseWriter, req *http.Request) {
 	f := func(resp http.ResponseWriter, req *http.Request) {
+		setHeaders(resp, s.agent.config.HTTPAPIResponseHeaders)
 		// Invoke the handler
 		reqURL := req.URL.String()
 		start := time.Now()
@@ -229,6 +230,13 @@ func setMeta(resp http.ResponseWriter, m *structs.QueryMeta) {
 	setKnownLeader(resp, m.KnownLeader)
 }
 
+// setHeaders is used to set canonical response header fields
+func setHeaders(resp http.ResponseWriter, headers map[string]string) {
+	for field, value := range headers {
+		resp.Header().Set(http.CanonicalHeaderKey(field), value)
+	}
+}
+
 // parseWait is used to parse the ?wait and ?index query params
 // Returns true on error
 func parseWait(resp http.ResponseWriter, req *http.Request, b *structs.QueryOptions) bool {

command/agent/http_test.go

@@ -104,6 +104,26 @@ func TestSetMeta(t *testing.T) {
 	}
 }
 
+func TestSetHeaders(t *testing.T) {
+	s := makeHTTPServer(t, nil)
+	s.Agent.config.HTTPAPIResponseHeaders = map[string]string{"foo": "bar"}
+	defer s.Cleanup()
+
+	resp := httptest.NewRecorder()
+	handler := func(resp http.ResponseWriter, req *http.Request) (interface{}, error) {
+		return &structs.Job{Name: "foo"}, nil
+	}
+
+	req, _ := http.NewRequest("GET", "/v1/kv/key", nil)
+	s.Server.wrap(handler)(resp, req)
+	header := resp.Header().Get("foo")
+
+	if header != "bar" {
+		t.Fatalf("expected header: %v, actual: %v", "bar", header)
+	}
+
+}
+
 func TestContentTypeIsJSON(t *testing.T) {
 	s := makeHTTPServer(t, nil)
 	defer s.Cleanup()

website/source/docs/agent/config.html.md

@@ -190,6 +190,15 @@ nodes, unless otherwise specified:
 * `disable_anonymous_signature`: Disables providing an anonymous signature
   for de-duplication with the update check. See `disable_update_check`.
 
+* `http_api_response_headers`: This object allows adding headers to the 
+  HTTP API responses. For example, the following config can be used to enable
+  CORS on the HTTP API endpoints:
+  ```
+  http_api_response_headers {
+      Access-Control-Allow-Origin = "*"
+  }
+  ```
+
 ## Server-specific Options
 
 The following options are applicable to server agents only and need not be