fix(fluid-build): Load default config when no config is found (#22825)

fluid-build was failing to load in repos without a config file. This
behavior was inadvertently broken when the `getFluidBuildConfig`
function was changed to throw an error when a config was not found. This
prevented existing default config fallback logic to be used.

This change clarifies what the default config is, returns it
consistently from `getFluidBuildConfig` when no config is found, and
updates the FluidRepo loading logic to account for the change.

build-tools/packages/build-tools/src/fluidBuild/buildContext.ts

@@ -14,7 +14,7 @@ export interface BuildContext {
 	/**
 	 * The fluid-build configuration for the repo.
 	 */
-	readonly fluidBuildConfig: IFluidBuildConfig | undefined;
+	readonly fluidBuildConfig: IFluidBuildConfig;
 
 	/**
 	 * The absolute path to the root of the repo.

build-tools/packages/build-tools/src/fluidBuild/buildGraph.ts

@@ -67,7 +67,7 @@ class BuildGraphContext implements BuildContext {
 	public readonly fileHashCache = new FileHashCache();
 	public readonly taskStats = new TaskStats();
 	public readonly failedTaskLines: string[] = [];
-	public readonly fluidBuildConfig: IFluidBuildConfig | undefined;
+	public readonly fluidBuildConfig: IFluidBuildConfig;
 	public readonly repoRoot: string;
 	public readonly gitRepo: GitRepo;
 	constructor(

build-tools/packages/build-tools/src/fluidBuild/fluidBuild.ts

@@ -25,7 +25,7 @@ async function main() {
 	log(`Build Root: ${resolvedRoot}`);
 
 	// Load the packages
-	const repo = FluidRepoBuild.create({
+	const repo = new FluidRepoBuild({
 		repoRoot: resolvedRoot,
 		gitRepo: new GitRepo(resolvedRoot),
 		fluidBuildConfig: getFluidBuildConfig(resolvedRoot),

build-tools/packages/build-tools/src/fluidBuild/fluidBuildConfig.ts

@@ -15,6 +15,16 @@ import { TaskDefinitionsOnDisk } from "./fluidTaskDefinitions";
  */
 export const FLUIDBUILD_CONFIG_VERSION = 1;
 
+/**
+ * The default fluid-build config if one is not found.
+ */
+export const DEFAULT_FLUIDBUILD_CONFIG: IFluidBuildConfig = {
+	version: FLUIDBUILD_CONFIG_VERSION,
+	repoPackages: {
+		root: "",
+	},
+};
+
 /**
  * Top-most configuration for repo build settings.
  */

build-tools/packages/build-tools/src/fluidBuild/fluidRepoBuild.ts

@@ -14,7 +14,6 @@ import { Package, Packages } from "../common/npmPackage";
 import { ExecAsyncResult, isSameFileOrDir, lookUpDirSync } from "../common/utils";
 import type { BuildContext } from "./buildContext";
 import { BuildGraph } from "./buildGraph";
-import type { IFluidBuildDirs } from "./fluidBuildConfig";
 import { FluidRepo } from "./fluidRepo";
 import { getFluidBuildConfig } from "./fluidUtils";
 import { NpmDepChecker } from "./npmDepChecker";
@@ -33,21 +32,8 @@ export interface IPackageMatchedOptions {
 }
 
 export class FluidRepoBuild extends FluidRepo {
-	public static create(context: BuildContext) {
-		// Default to just resolveRoot if no config is found
-		const packageManifest = context.fluidBuildConfig ?? {
-			repoPackages: {
-				root: "",
-			},
-		};
-		return new FluidRepoBuild(context.repoRoot, context, packageManifest.repoPackages);
-	}
-	private constructor(
-		resolvedRoot: string,
-		protected context: BuildContext,
-		repoPackages?: IFluidBuildDirs,
-	) {
-		super(resolvedRoot, repoPackages);
+	public constructor(protected context: BuildContext) {
+		super(context.repoRoot, context.fluidBuildConfig.repoPackages);
 	}
 
 	public async clean() {

build-tools/packages/build-tools/src/fluidBuild/fluidUtils.ts

@@ -14,7 +14,11 @@ import { readJson } from "fs-extra";
 
 import { defaultLogger } from "../common/logging";
 import { commonOptions } from "./commonOptions";
-import { FLUIDBUILD_CONFIG_VERSION, type IFluidBuildConfig } from "./fluidBuildConfig";
+import {
+	DEFAULT_FLUIDBUILD_CONFIG,
+	FLUIDBUILD_CONFIG_VERSION,
+	type IFluidBuildConfig,
+} from "./fluidBuildConfig";
 
 // switch to regular import once building ESM
 const findUp = import("find-up");
@@ -140,29 +144,37 @@ const configExplorer = cosmiconfigSync(configName, {
 	packageProp: [configName],
 });
 
+/**
+ * Set to true when the default config is returned by getFluidBuildConfig so that repeated calls to the function don't
+ * result in repeated searches for config.
+ */
+let defaultConfigLoaded = false;
+
 /**
  * Get an IFluidBuildConfig from the fluidBuild property in a package.json file, or from fluidBuild.config.[c]js.
  *
- * @param rootDir - The path to the root package.json to load.
- * @param noCache - If true, the config cache will be cleared and the config will be reloaded.
- * @returns The fluidBuild section of the package.json, or undefined if not found
+ * @param searchDir - The path to search for the config. The search will look up the folder hierarchy for a config in
+ * either a standalone file or package.json
+ * @returns The the loaded fluidBuild config, or the default config if one is not found.
  */
 export function getFluidBuildConfig(
-	rootDir: string,
-	noCache = false,
+	searchDir: string,
 	log = defaultLogger,
 ): IFluidBuildConfig {
-	if (noCache === true) {
-		configExplorer.clearCaches();
+	if (defaultConfigLoaded) {
+		return DEFAULT_FLUIDBUILD_CONFIG;
 	}
 
-	const configResult = configExplorer.search(rootDir);
-	const config = configResult?.config as IFluidBuildConfig | undefined;
-
-	if (config === undefined) {
-		throw new Error("No fluidBuild configuration found.");
+	const configResult = configExplorer.search(searchDir);
+	if (configResult?.config === undefined) {
+		log.warning(
+			`No fluidBuild config found when searching ${searchDir}; default configuration loaded. Packages and tasks will be inferred.`,
+		);
+		defaultConfigLoaded = true;
+		return DEFAULT_FLUIDBUILD_CONFIG;
 	}
 
+	const config = configResult.config;
 	if (config.version === undefined) {
 		log.warning(
 			"fluidBuild config has no version field. This field will be required in a future release.",