allow configs to be passed directly to loadBackendConfig

Signed-off-by: Oleg S <97077423+RobotSail@users.noreply.github.com>

packages/backend-app-api/src/config/ObservableConfigProxy.ts

@@ -16,7 +16,8 @@
 
 import { ConfigService } from '@backstage/backend-plugin-api';
 import { ConfigReader } from '@backstage/config';
-import { JsonValue } from '@backstage/types';
+import type { JsonObject, JsonValue } from '@backstage/types';
+import { mergeJson } from '@backstage/types';
 
 export class ObservableConfigProxy implements ConfigService {
   private config: ConfigService = new ConfigReader({});
@@ -61,6 +62,12 @@ export class ObservableConfigProxy implements ConfigService {
       },
     };
   }
+  mergeConfig(configData: JsonObject) {
+    if (this.parent) {
+      throw new Error('immutable');
+    }
+    this.setConfig(new ConfigReader(mergeJson(this.config.get(), configData)));
+  }
 
   private select(required: true): ConfigService;
   private select(required: false): ConfigService | undefined;

packages/backend-app-api/src/config/config.ts

@@ -24,10 +24,11 @@ import {
   ConfigTarget,
   LoadConfigOptionsRemote,
 } from '@backstage/config-loader';
-import { Config, ConfigReader } from '@backstage/config';
+import { type Config, ConfigReader } from '@backstage/config';
 import { getPackages } from '@manypkg/get-packages';
 import { ObservableConfigProxy } from './ObservableConfigProxy';
 import { isValidUrl } from '../lib/urls';
+import type { JsonObject } from '@backstage/types';
 
 /** @public */
 export async function createConfigSecretEnumerator(options: {
@@ -70,6 +71,7 @@ export async function createConfigSecretEnumerator(options: {
 export async function loadBackendConfig(options: {
   remote?: LoadConfigOptionsRemote;
   argv: string[];
+  config?: JsonObject;
 }): Promise<{ config: Config }> {
   const args = parseArgs(options.argv);
 
@@ -109,12 +111,12 @@ export async function loadBackendConfig(options: {
       }),
     },
   });
-
   console.info(
     `Loaded config from ${appConfigs.map(c => c.context).join(', ')}`,
   );
 
   config.setConfig(ConfigReader.fromConfigs(appConfigs));
+  config.mergeConfig(options.config ?? {});
 
   return { config };
 }

packages/types/src/index.ts

@@ -21,5 +21,6 @@
  */
 
 export type { JsonArray, JsonObject, JsonPrimitive, JsonValue } from './json';
+export { mergeJson } from './json';
 export type { Observable, Observer, Subscription } from './observable';
 export type { HumanDuration } from './time';

packages/types/src/json.ts

@@ -41,3 +41,61 @@ export interface JsonArray extends Array<JsonValue> {}
  * @public
  */
 export type JsonValue = JsonObject | JsonArray | JsonPrimitive;
+
+/**
+ * Attempts to merge two JsonObjects together. In the case of collisions, this function
+ * prefers values from b, unless the value is an object, in which case it recursively
+ * merges the values.
+ *
+ * @param a The base object
+ * @param b The object to merge into a
+ * @returns The merged object
+ */
+export const mergeJson = (a: JsonObject, b: JsonObject): JsonObject => {
+  const final: JsonObject = {};
+  const bKeys = new Set(Object.keys(b));
+  const aKeys = new Set(Object.keys(a));
+  const intersectingKeys = new Set([...aKeys].filter(x => bKeys.has(x)));
+
+  // add all mutually exclusive keys to the final object
+  for (const key of aKeys.values()) {
+    if (!intersectingKeys.has(key)) {
+      final[key] = a[key];
+      continue;
+    }
+  }
+  for (const key of bKeys.values()) {
+    if (!intersectingKeys.has(key)) {
+      final[key] = b[key];
+      continue;
+    }
+  }
+
+  // values now are all overlapping and are either primitives, arrays, or objects.
+  // for all primitives and arrays, we want to assign the value from b
+  // for all objects, we want to recursively merge the values
+  for (const key of intersectingKeys.values()) {
+    // check if value is an array or primitive
+    const value = b[key];
+    if (Array.isArray(value) || typeof value !== 'object') {
+      final[key] = value;
+      continue;
+    }
+
+    // check if either value is undefined and default to the defined one
+    const aValue = a[key];
+    const bValue = b[key];
+    if (!aValue) {
+      final[key] = bValue;
+      continue;
+    }
+    if (!bValue) {
+      final[key] = aValue;
+      continue;
+    }
+
+    // recursively merge the values
+    final[key] = mergeJson(aValue as JsonObject, bValue as JsonObject);
+  }
+  return final;
+};