make --language optional to autobuild and detect dominant language

src/autobuild.ts

@@ -3,16 +3,36 @@ import * as config_utils from './config-utils';
 import { isTracedLanguage, Language } from './languages';
 import { Logger } from './logging';
 
-export async function runAutobuild(
-  language: Language,
-  tmpDir: string,
-  logger: Logger) {
+export function determineAutobuildLanguage(
+  config: config_utils.Config,
+  logger: Logger
+): Language | undefined {
 
-  if (!isTracedLanguage(language)) {
-    throw new Error(`Cannot build "${language}" as it is not a traced language`);
+  // Attempt to find a language to autobuild
+  // We want pick the dominant language in the repo from the ones we're able to build
+  // The languages are sorted in order specified by user or by lines of code if we got
+  // them from the GitHub API, so try to build the first language on the list.
+  const autobuildLanguages = config.languages.filter(isTracedLanguage);
+  const language = autobuildLanguages[0];
+
+  if (!language) {
+    logger.info("None of the languages in this project require extra build steps");
+    return undefined;
   }
 
-  const config = await config_utils.getConfig(tmpDir, logger);
+  logger.debug(`Detected dominant traced language: ${language}`);
+
+  if (autobuildLanguages.length > 1) {
+    logger.warning(`We will only automatically build ${language} code. If you wish to scan ${autobuildLanguages.slice(1).join(' and ')}, you must replace this call with custom build steps.`);
+  }
+
+  return language;
+}
+
+export async function runAutobuild(
+  language: Language,
+  config: config_utils.Config,
+  logger: Logger) {
 
   logger.startGroup(`Attempting to automatically build ${language} code`);
   const codeQL = getCodeQL(config.codeQLCmd);