Add support for environment variable injection in kibana.yml (#16988)

src/cli/serve/__fixtures__/en_var_ref_config.yml

@@ -0,0 +1,5 @@
+foo: 1
+bar: "pre-${KBN_ENV_VAR1}-mid-${KBN_ENV_VAR2}-post"
+
+elasticsearch:
+  requestHeadersWhitelist: ["${KBN_ENV_VAR1}", "${KBN_ENV_VAR2}"]

src/cli/serve/__fixtures__/invalid_en_var_ref_config.yml

@@ -0,0 +1 @@
+foo: "${KBN_NON_EXISTENT_ENV_VAR}"

src/cli/serve/read_yaml_config.js

@@ -1,7 +1,16 @@
-import { isArray, isPlainObject, forOwn, set, transform } from 'lodash';
+import { isArray, isPlainObject, forOwn, set, transform, isString } from 'lodash';
 import { readFileSync as read } from 'fs';
 import { safeLoad } from 'js-yaml';
 
+function replaceEnvVarRefs(val) {
+  return val.replace(/\$\{(\w+)\}/g, (match, envVarName) => {
+    if (process.env[envVarName] !== undefined) {
+      return process.env[envVarName];
+    } else {
+      throw new Error(`Unknown environment variable referenced in config : ${envVarName}`);
+    }
+  });
+}
 
 export function merge(sources) {
   return transform(sources, (merged, source) => {
@@ -19,6 +28,10 @@ export function merge(sources) {
         return;
       }
 
+      if (isString(val)) {
+        val = replaceEnvVarRefs(val);
+      }
+
       set(merged, key, val);
     });
   }, {});

src/cli/serve/read_yaml_config.test.js

@@ -28,6 +28,26 @@ describe('cli/serve/read_yaml_config', function () {
     });
   });
 
+  it('should inject an environment variable value when setting a value with ${ENV_VAR}', function () {
+    process.env.KBN_ENV_VAR1 = 'val1';
+    process.env.KBN_ENV_VAR2 = 'val2';
+    const config = readYamlConfig([ fixture('en_var_ref_config.yml') ]);
+
+    expect(config).toEqual({
+      foo: 1,
+      bar: 'pre-val1-mid-val2-post',
+      elasticsearch: {
+        requestHeadersWhitelist: ['val1', 'val2']
+      }
+    });
+  });
+
+  it('should thow an exception when referenced environment variable in a config value does not exist', function () {
+    expect(function () {
+      readYamlConfig([ fixture('invalid_en_var_ref_config.yml') ]);
+    }).toThrow();
+  });
+
   describe('different cwd()', function () {
     const oldCwd = process.cwd();
     const newCwd = join(oldCwd, '..');
@@ -55,4 +75,5 @@ describe('cli/serve/read_yaml_config', function () {
       process.chdir(oldCwd);
     });
   });
+
 });