Unconditionally add Access-Control-Expose-Headers HTTP header

.changelog/20220.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+cloud: unconditionally add Access-Control-Expose-Headers HTTP header
+```

agent/hcp/bootstrap/bootstrap.go

@@ -106,6 +106,30 @@ func LoadConfig(ctx context.Context, client hcpclient.Client, dataDir string, lo
 	return newLoader, nil
 }
 
+func AddAclPolicyAccessControlHeader(baseLoader ConfigLoader) ConfigLoader {
+	return func(source config.Source) (config.LoadResult, error) {
+		res, err := baseLoader(source)
+		if err != nil {
+			return res, err
+		}
+
+		rc := res.RuntimeConfig
+
+		// HTTP response headers are modified for the HCP UI to work.
+		if rc.HTTPResponseHeaders == nil {
+			rc.HTTPResponseHeaders = make(map[string]string)
+		}
+		prevValue, ok := rc.HTTPResponseHeaders[accessControlHeaderName]
+		if !ok {
+			rc.HTTPResponseHeaders[accessControlHeaderName] = accessControlHeaderValue
+		} else {
+			rc.HTTPResponseHeaders[accessControlHeaderName] = prevValue + "," + accessControlHeaderValue
+		}
+
+		return res, err
+	}
+}
+
 // bootstrapConfigLoader is a ConfigLoader for passing bootstrap JSON config received from HCP
 // to the config.builder. ConfigLoaders are functions used to build an agent's RuntimeConfig
 // from various sources like files and flags. This config is contained in the config.LoadResult.
@@ -166,17 +190,6 @@ const (
 // handled by the config.builder.
 func finalizeRuntimeConfig(rc *config.RuntimeConfig, cfg *RawBootstrapConfig) {
 	rc.Cloud.ManagementToken = cfg.ManagementToken
-
-	// HTTP response headers are modified for the HCP UI to work.
-	if rc.HTTPResponseHeaders == nil {
-		rc.HTTPResponseHeaders = make(map[string]string)
-	}
-	prevValue, ok := rc.HTTPResponseHeaders[accessControlHeaderName]
-	if !ok {
-		rc.HTTPResponseHeaders[accessControlHeaderName] = accessControlHeaderValue
-	} else {
-		rc.HTTPResponseHeaders[accessControlHeaderName] = prevValue + "," + accessControlHeaderValue
-	}
 }
 
 // fetchBootstrapConfig will fetch boostrap configuration from remote servers and persist it to disk.

agent/hcp/bootstrap/bootstrap_test.go

@@ -48,9 +48,6 @@ func TestBootstrapConfigLoader(t *testing.T) {
 	// bootstrap_expect and management token are injected from bootstrap config received from HCP.
 	require.Equal(t, 8, result.RuntimeConfig.BootstrapExpect)
 	require.Equal(t, "test-token", result.RuntimeConfig.Cloud.ManagementToken)
-
-	// Response header is always injected from a constant.
-	require.Equal(t, "x-consul-default-acl-policy", result.RuntimeConfig.HTTPResponseHeaders[accessControlHeaderName])
 }
 
 func Test_finalizeRuntimeConfig(t *testing.T) {
@@ -65,28 +62,68 @@ func Test_finalizeRuntimeConfig(t *testing.T) {
 	}
 
 	tt := map[string]testCase{
-		"set header if not present": {
+		"set management token": {
 			rc: &config.RuntimeConfig{},
 			cfg: &RawBootstrapConfig{
 				ManagementToken: "test-token",
 			},
 			verifyFn: func(t *testing.T, rc *config.RuntimeConfig) {
 				require.Equal(t, "test-token", rc.Cloud.ManagementToken)
-				require.Equal(t, "x-consul-default-acl-policy", rc.HTTPResponseHeaders[accessControlHeaderName])
 			},
 		},
+	}
+
+	for name, tc := range tt {
+		t.Run(name, func(t *testing.T) {
+			run(t, tc)
+		})
+	}
+}
+
+func Test_AddAclPolicyAccessControlHeader(t *testing.T) {
+	type testCase struct {
+		rc         *config.RuntimeConfig
+		cfg        *RawBootstrapConfig
+		baseLoader ConfigLoader
+		verifyFn   func(t *testing.T, rc *config.RuntimeConfig)
+	}
+	run := func(t *testing.T, tc testCase) {
+		loader := AddAclPolicyAccessControlHeader(tc.baseLoader)
+		result, err := loader(nil)
+		require.NoError(t, err)
+		tc.verifyFn(t, result.RuntimeConfig)
+	}
+
+	tt := map[string]testCase{
 		"append to header if present": {
-			rc: &config.RuntimeConfig{
-				HTTPResponseHeaders: map[string]string{
-					accessControlHeaderName: "Content-Encoding",
-				},
+			baseLoader: func(source config.Source) (config.LoadResult, error) {
+				return config.Load(config.LoadOpts{
+					DefaultConfig: config.DefaultSource(),
+					HCL: []string{
+						`server = true`,
+						`bind_addr = "127.0.0.1"`,
+						`data_dir = "/tmp/consul-data"`,
+						fmt.Sprintf(`http_config = { response_headers = { %s = "test" } }`, accessControlHeaderName),
+					},
+				})
 			},
-			cfg: &RawBootstrapConfig{
-				ManagementToken: "test-token",
+			verifyFn: func(t *testing.T, rc *config.RuntimeConfig) {
+				require.Equal(t, "test,x-consul-default-acl-policy", rc.HTTPResponseHeaders[accessControlHeaderName])
+			},
+		},
+		"set header if not present": {
+			baseLoader: func(source config.Source) (config.LoadResult, error) {
+				return config.Load(config.LoadOpts{
+					DefaultConfig: config.DefaultSource(),
+					HCL: []string{
+						`server = true`,
+						`bind_addr = "127.0.0.1"`,
+						`data_dir = "/tmp/consul-data"`,
+					},
+				})
 			},
 			verifyFn: func(t *testing.T, rc *config.RuntimeConfig) {
-				require.Equal(t, "test-token", rc.Cloud.ManagementToken)
-				require.Equal(t, "Content-Encoding,x-consul-default-acl-policy", rc.HTTPResponseHeaders[accessControlHeaderName])
+				require.Equal(t, "x-consul-default-acl-policy", rc.HTTPResponseHeaders[accessControlHeaderName])
 			},
 		},
 	}

command/agent/agent.go

@@ -183,6 +183,11 @@ func (c *cmd) run(args []string) int {
 		}
 	}
 
+	// We unconditionally add an Access Control header to our config in order to allow the HCP UI to work.
+	// We do this unconditionally because the cluster can be linked to HCP at any time (not just at startup) and this
+	// is simpler than selectively reloading parts of config at runtime.
+	loader = hcpbootstrap.AddAclPolicyAccessControlHeader(loader)
+
 	bd, err := agent.NewBaseDeps(loader, logGate, nil)
 	if err != nil {
 		ui.Error(err.Error())