feat: better error messages when resolving undefined variables (#419)

* feat: better error messages when resolving undefined variables

Test before calling resolveVariable(), since by that point the information
has already been lost and it's too late to provide a more detailed message

Fixes #262

* feat: include API namespace in missing variable errors

Helps finding where the error is

* chore: reword error messages, introduce custom Error type

packages/sdk/src/configure/variables.ts

@@ -29,7 +29,7 @@ export type InputVariable<T = string> = T | EnvironmentVariable<T> | PlaceHolder
  */
 export const resolveVariable = (variable: string | EnvironmentVariable): string => {
 	if (variable === undefined) {
-		throw new Error(`could not resolve empty data variable: ${variable}`);
+		throw new Error('could not resolve undefined data variable');
 	}
 	if (variable instanceof EnvironmentVariable) {
 		const environmentVariable = variable as EnvironmentVariable;

packages/sdk/src/db/introspection.ts

@@ -82,6 +82,9 @@ export const introspectPrismaDatabaseWithRetries = async (
 	databaseSchema: DatabaseSchema,
 	maxRetries: number
 ): Promise<PrismaDatabaseIntrospectionResult> => {
+	if (!introspection.databaseURL) {
+		throw new Error('database URL is not defined');
+	}
 	const resolvedURL = resolveVariable(introspection.databaseURL);
 	for (let i = 0; i < maxRetries; i++) {
 		try {

packages/sdk/src/definition/federation-introspection.ts

@@ -93,6 +93,7 @@ export const introspectFederation = async (introspection: GraphQLFederationIntro
 				}
 				const introspectionHeaders = resolveGraphqlIntrospectionHeaders(mapHeaders(introspectionHeadersBuilder));
 
+				// upstream.url is truthy at this point, no need to check
 				schema = await fetchFederationServiceSDL(resolveVariable(upstream.url), introspectionHeaders, {
 					apiNamespace: introspection.apiNamespace,
 					upstreamName: name,

packages/sdk/src/definition/graphql-introspection.ts

@@ -35,6 +35,13 @@ import { Logger } from '../logger';
 import { mergeSchemas } from '@graphql-tools/schema';
 import transformSchema from '../transformations/shema';
 
+class MissingKeyError extends Error {
+	constructor(private key: string, private introspection: GraphQLIntrospection) {
+		super(`${key} is not defined in your ${introspection.apiNamespace} datasource`);
+		Object.setPrototypeOf(this, MissingKeyError.prototype);
+	}
+}
+
 export const resolveGraphqlIntrospectionHeaders = (headers?: { [key: string]: HTTPHeader }): Record<string, string> => {
 	const baseHeaders: Record<string, string> = {
 		'Content-Type': 'application/json',
@@ -83,13 +90,19 @@ export const introspectGraphql = async (introspection: GraphQLIntrospection): Pr
 		const federationEnabled = isFederationService(schema);
 		const upstreamSchema = cleanupSchema(schema, introspection);
 		const { schemaSDL, customScalarTypeFields } = transformSchema.replaceCustomScalars(upstreamSchema, introspection);
-		const serviceSDL = !federationEnabled
-			? undefined
-			: introspection.loadSchemaFromString
-			? loadFile(introspection.loadSchemaFromString)
-			: await fetchFederationServiceSDL(resolveVariable(introspection.url), introspectionHeaders, {
+		let serviceSDL: string | undefined;
+		if (federationEnabled) {
+			if (introspection.loadSchemaFromString) {
+				serviceSDL = loadFile(introspection.loadSchemaFromString);
+			} else {
+				if (!introspection.url) {
+					throw new MissingKeyError('url', introspection);
+				}
+				serviceSDL = await fetchFederationServiceSDL(resolveVariable(introspection.url), introspectionHeaders, {
 					apiNamespace: introspection.apiNamespace,
-			  });
+				});
+			}
+		}
 		const serviceDocumentNode = serviceSDL !== undefined ? parse(serviceSDL) : undefined;
 		const schemaDocumentNode = parse(schemaSDL);
 		const graphQLSchema = buildSchema(schemaSDL);
@@ -215,6 +228,12 @@ const introspectGraphQLAPI = async (
 	};
 
 	if (introspection.mTLS) {
+		if (!introspection.mTLS.key) {
+			throw new MissingKeyError('mTLS.key', introspection);
+		}
+		if (!introspection.mTLS.cert) {
+			throw new MissingKeyError('mTLS.cert', introspection);
+		}
 		opts.httpsAgent = new https.Agent({
 			key: resolveVariable(introspection.mTLS.key),
 			cert: resolveVariable(introspection.mTLS.cert),
@@ -224,6 +243,9 @@ const introspectGraphQLAPI = async (
 
 	let res: AxiosResponse | undefined;
 	try {
+		if (!introspection.url) {
+			throw new MissingKeyError('url', introspection);
+		}
 		res = await Fetcher().post(resolveVariable(introspection.url), data, opts);
 	} catch (e: any) {
 		throw new Error(