Support a `local` channel for local development

In local development, the Nimbus sidecar isn't available, so we
just explicitly load the default values for the `local` channel.

config/nimbus.yaml

@@ -1,6 +1,7 @@
 about:
   description: Nimbus Feature Manifest for Monitor Web testing
 channels:
+  - local
   - staging
   - production
 features:
@@ -12,6 +13,8 @@ features:
         type: Boolean
         default: false
     defaults:
+      - channel: local
+        value: { "enabled": true }
       - channel: staging
         value: { "enabled": true }
       - channel: production

src/app/functions/server/getExperiments.ts

@@ -7,6 +7,7 @@ import { logger } from "./logging";
 import {
   ExperimentData,
   defaultExperimentData,
+  localExperimentData,
 } from "../../../telemetry/generated/nimbus/experiments";
 import { ExperimentationId } from "./getExperimentationId";
 
@@ -25,34 +26,35 @@ export async function getExperiments(params: {
   locale: string;
   countryCode: string;
 }): Promise<ExperimentData> {
-  if (["stage", "production"].includes(process.env.APP_ENV ?? "local")) {
-    const serverUrl = process.env.NIMBUS_SIDECAR_URL;
-    if (!serverUrl) {
-      throw new Error("env var NIMBUS_SIDECAR_URL not set");
-    }
-
-    try {
-      const response = await fetch(`${serverUrl}/v1/features`, {
-        headers: {
-          "Content-Type": "application/json",
-        },
-        method: "POST",
-        body: JSON.stringify({
-          client_id: params.experimentationId,
-          context: {
-            // Nimbus takes a language, rather than a locale, hence the .split:
-            locale: params.locale.split("-")[0],
-            countryCode: params.countryCode,
-          },
-        }),
-      });
-
-      const experimentData = (await response.json()) as ExperimentData;
-      return experimentData ?? defaultExperimentData;
-    } catch (ex) {
-      logger.error(`Could not connect to Cirrus on ${serverUrl}`, ex);
-      captureException(ex);
-    }
+  if (["local"].includes(process.env.APP_ENV ?? "local")) {
+    return localExperimentData;
+  }
+  const serverUrl = process.env.NIMBUS_SIDECAR_URL;
+  if (!serverUrl) {
+    throw new Error("env var NIMBUS_SIDECAR_URL not set");
+  }
+
+  try {
+    const response = await fetch(`${serverUrl}/v1/features`, {
+      headers: {
+        "Content-Type": "application/json",
+      },
+      method: "POST",
+      body: JSON.stringify({
+        client_id: params.experimentationId,
+        context: {
+          // Nimbus takes a language, rather than a locale, hence the .split:
+          locale: params.locale.split("-")[0],
+          countryCode: params.countryCode,
+        },
+      }),
+    });
+
+    const experimentData = (await response.json()) as ExperimentData;
+    return experimentData ?? defaultExperimentData;
+  } catch (ex) {
+    logger.error(`Could not connect to Cirrus on ${serverUrl}`, ex);
+    captureException(ex);
+    return defaultExperimentData;
   }
-  return defaultExperimentData;
 }

src/scripts/build/nimbusTypes.js

@@ -12,7 +12,7 @@ run();
  * @typedef {"String" | "Boolean" | "Int" | "Text" | "Image" | `Option<${Type}>` | `List<${Type}>` | `Map<${Type}, ${Type}>`} Type
  */
 /**
- * @typedef {"staging" | "production"} Channel
+ * @typedef {"local" | "staging" | "production"} Channel
  */
 /**
  * @typedef {Record<
@@ -32,7 +32,7 @@ run();
  *   features: Record<string, {
  *     description: string;
  *     variables: Variables;
- *     defaults: Array<{
+ *     defaults?: Array<{
  *       channel: Channel;
  *       values: Record<keyof Variables, unknown>;
  *     }>;
@@ -64,7 +64,9 @@ async function run() {
     "// AUTOGENERATED `npm run build-nimbus`. DO NOT EDIT. DO NOT COMMIT.\n\n" +
     getFeaturesTypeDef(nimbusConfig) +
     "\n" +
-    getFallbackObject(nimbusConfig);
+    getFallbackObject(nimbusConfig) +
+    "\n" +
+    getLocalOverrides(nimbusConfig);
   await mkdir("src/telemetry/generated/nimbus/", { recursive: true });
   await writeFile("src/telemetry/generated/nimbus/experiments.ts", typedef);
 }
@@ -90,6 +92,27 @@ function getFallbackObject(nimbusConfig) {
   return `export const defaultExperimentData: ExperimentData = {\n${featureFallbackDefs.join("\n")}};\n`;
 }
 
+/**
+ * @param {NimbusConfig} nimbusConfig
+ * @returns string
+ */
+function getLocalOverrides(nimbusConfig) {
+  const featureLocalOverridesDefs = Object.keys(nimbusConfig.features).map(
+    (featureId) => {
+      const localOverrides = nimbusConfig.features[featureId].defaults?.find(
+        (defaultData) => defaultData.channel === "local",
+      );
+      const overriddenValuesDef =
+        typeof localOverrides === "undefined"
+          ? ""
+          : `    ...${JSON.stringify(localOverrides.value, null, 2).replaceAll("\n", "\n    ")}\n`;
+      return `  "${featureId}": {\n    ...defaultExperimentData["${featureId}"],\n${overriddenValuesDef}  },\n`;
+    },
+  );
+
+  return `export const localExperimentData: ExperimentData = {\n${featureLocalOverridesDefs.join("\n")}};\n`;
+}
+
 /**
  * @param {NimbusConfig} nimbusConfig
  * @returns string