Cache requested source maps

This fixes #1245 [1].

[1] #1245

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 All notable changes to this project will be documented in this file.
 
+## Unreleased
+
+- Cache requested source maps, fixes [#1245](https://github.com/badeball/cypress-cucumber-preprocessor/discussions/1245).
+
 ## v21.0.1
 
 - Support config file locations other than project root, fixes [#1243](https://github.com/badeball/cypress-cucumber-preprocessor/discussions/1243).

features/issues/1245.feature

@@ -0,0 +1,30 @@
+Feature: cached source maps
+
+  Ideally, I would have liked to test this in a more behavior driven way, like counting the number
+  of times that the bundler, whichever was configured, was invoked. However, it turns out that
+  Cypress does in fact cache [1]. Unfortunately, this cache is dead slow and requires higher-order
+  caching. The only way to verify that Cypress' cache isn't invoked, is by interpreting stderr.
+
+  [1] https://github.com/cypress-io/cypress/blob/v13.15.0/packages/server/lib/plugins/preprocessor.js#L94-L98
+
+  Scenario:
+    Given a file named "cypress/e2e/a.feature" with:
+      """
+      Feature: a feature name
+        Scenario: a scenario name
+          Given a step
+      """
+    And a file named "cypress/support/step_definitions/steps.ts" with:
+      """
+      import { Given } from "@badeball/cypress-cucumber-preprocessor";
+      Given("a step", function(this: Mocha.Context) {});
+      for (let i = 0; i < 10; i++) {
+        Given(`an unused step (${i + 1})`, function(this: Mocha.Context) {});
+      };
+      """
+    When I run cypress with environment variables
+      | name  | value                       |
+      | DEBUG | cypress:server:preprocessor |
+    Then it passes
+    # Why two? Who knows. Cypress requests this file twice and the library once.
+    And I should see exactly 2 instances of "headless and already processed" in stderr

features/step_definitions/cli_steps.ts

@@ -249,3 +249,14 @@ Then(
     assert.match(expectLastRun(this).stdout, pendingScenarioExpr(scenarioName));
   },
 );
+
+Then(
+  "I should see exactly {int} instance(s) of {string} in stderr",
+  function (this: ICustomWorld, expectedOccurences: number, output: string) {
+    const actualyOccurences =
+      expectLastRun(this).stderr.match(new RegExp(rescape(output), "g"))
+        ?.length ?? 0;
+
+    assert.equal(actualyOccurences, expectedOccurences);
+  },
+);

lib/helpers/source-map.ts

@@ -21,6 +21,8 @@ function sourceMapWarn(message: string) {
   isSourceMapWarned = true;
 }
 
+const cache = new Map<string, string | undefined>();
+
 /**
  * Taken from https://github.com/evanw/node-source-map-support/blob/v0.5.21/source-map-support.js#L148-L177.
  */
@@ -61,14 +63,24 @@ export function retrieveSourceMapURL(source: string) {
   return lastMatch[1];
 }
 
+export function cachedRetrieveSourceMapURL(source: string): string | undefined {
+  if (cache.has(source)) {
+    return cache.get(source);
+  } else {
+    const result = retrieveSourceMapURL(source);
+    cache.set(source, result);
+    return result;
+  }
+}
+
 export function maybeRetrievePositionFromSourceMap(): Position | undefined {
   const stack = ErrorStackParser.parse(new Error());
 
   if (stack[0].fileName == null) {
     return;
   }
 
-  const sourceMappingURL = retrieveSourceMapURL(stack[0].fileName);
+  const sourceMappingURL = cachedRetrieveSourceMapURL(stack[0].fileName);
 
   if (!sourceMappingURL) {
     return;