CachingJestRuntime should not do extra work if not watching

The benefits of `CachingJestRuntime` only apply if in `watch` mode.
If not, then we're doing FS calls but not getting benefit from them.

This is a small optimization that will hopefully make CI happier with
our caching runtime, if/when it's used.

Signed-off-by: Mitchell Hentges <mhentges@spotify.com>

.changeset/curly-rats-itch.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Tweak the Jest Caching loader to only operate when in `watch` mode

packages/cli/config/jest.js

@@ -25,6 +25,11 @@ const envOptions = {
   enableSourceMaps: Boolean(process.env.ENABLE_SOURCE_MAPS),
 };
 
+if (envOptions.nextTests) {
+  // Needed so that, at import-time, it can hook into Jest's internals.
+  require('./jestCachingModuleLoader');
+}
+
 const transformIgnorePattern = [
   '@material-ui',
   'ajv',

packages/cli/config/jestCachingModuleLoader.js

@@ -21,6 +21,7 @@ const fileTransformCache = new Map();
 const scriptTransformCache = new Map();
 
 let runtimeGeneration = 0;
+let isWatchMode;
 
 module.exports = class CachingJestRuntime extends JestRuntime {
   // Each Jest run creates a new runtime, including when rerunning tests in
@@ -28,6 +29,10 @@ module.exports = class CachingJestRuntime extends JestRuntime {
   __runtimeGeneration = runtimeGeneration++;
 
   transformFile(filename, options) {
+    if (!isWatchMode) {
+      return super.transformFile(filename, options);
+    }
+
     const entry = fileTransformCache.get(filename);
     if (entry) {
       // Only check modification time if it's from a different runtime generation
@@ -79,3 +84,11 @@ module.exports = class CachingJestRuntime extends JestRuntime {
     return script;
   }
 };
+
+// Inject hook into createHasteMap, as it's the only way that we can
+// determine (from our scope here) if we're in "watch mode" or not.
+const originalCreateHasteMap = JestRuntime.createHasteMap;
+JestRuntime.createHasteMap = (config, options = undefined) => {
+  isWatchMode = options && options.watch;
+  return originalCreateHasteMap(config, options);
+};