Abort sl root call if output resembles a steam locomotive (#15053)

Jest detects whether a repository is a sapling repo by calling the `sl`
binary, and getting the output. If `sl` (steam locomotive) is installed,
the output of `sl root` 1) takes forever to get and 2) is not the root,
but a moving image of a steam locomotive. This change monitors the
stdout stream, and aborts the `sl` call if the first character is an
escape character, which indicates that the terminal is being cleared to
make way for a train to come through.

See also: #14046

CHANGELOG.md

@@ -40,6 +40,7 @@
 - `[babel-plugin-jest-hoist]` Use `denylist` instead of the deprecated `blacklist` for Babel 8 support ([#14109](https://github.com/jestjs/jest/pull/14109))
 - `[expect]` Check error instance type for `toThrow/toThrowError` ([#14576](https://github.com/jestjs/jest/pull/14576))
 - `[jest-changed-files]` Print underlying errors when VCS commands fail ([#15052](https://github.com/jestjs/jest/pull/15052))
+- `[jest-changed-files]` Abort `sl root` call if output resembles a steam locomotive ([#15053](https://github.com/jestjs/jest/pull/15053))
 - `[jest-circus]` [**BREAKING**] Prevent false test failures caused by promise rejections handled asynchronously ([#14315](https://github.com/jestjs/jest/pull/14315))
 - `[jest-circus]` Replace recursive `makeTestResults` implementation with iterative one ([#14760](https://github.com/jestjs/jest/pull/14760))
 - `[jest-circus]` Omit `expect.hasAssertions()` errors if a test already has errors ([#14866](https://github.com/jestjs/jest/pull/14866))

packages/jest-changed-files/src/sl.ts

@@ -16,6 +16,9 @@ import type {SCMAdapter} from './types';
  */
 const env = {...process.env, HGPLAIN: '1'};
 
+// Whether `sl` is a steam locomotive or not
+let isSteamLocomotive = false;
+
 const adapter: SCMAdapter = {
   findChangedFiles: async (cwd, options) => {
     const includePaths = options.includePaths ?? [];
@@ -42,8 +45,29 @@ const adapter: SCMAdapter = {
   },
 
   getRoot: async cwd => {
+    if (isSteamLocomotive) {
+      return null;
+    }
+
     try {
-      const result = await execa('sl', ['root'], {cwd, env});
+      const subprocess = execa('sl', ['root'], {cwd, env});
+
+      // Check if we're calling sl (steam locomotive) instead of sl (sapling)
+      // by looking for the escape character in the first chunk of data.
+      if (subprocess.stdout) {
+        subprocess.stdout.once('data', (data: Buffer | string) => {
+          data = Buffer.isBuffer(data) ? data.toString() : data;
+          if (data.codePointAt(0) === 27) {
+            subprocess.cancel();
+            isSteamLocomotive = true;
+          }
+        });
+      }
+
+      const result = await subprocess;
+      if (result.killed && isSteamLocomotive) {
+        return null;
+      }
 
       return result.stdout;
     } catch {