fix(core): use the current CLI to prepare external Yarn classic projects

.yarn/versions/8fe58483.yml

@@ -0,0 +1,34 @@
+releases:
+  "@yarnpkg/cli": patch
+  "@yarnpkg/core": patch
+
+declined:
+  - "@yarnpkg/plugin-compat"
+  - "@yarnpkg/plugin-constraints"
+  - "@yarnpkg/plugin-dlx"
+  - "@yarnpkg/plugin-essentials"
+  - "@yarnpkg/plugin-exec"
+  - "@yarnpkg/plugin-file"
+  - "@yarnpkg/plugin-git"
+  - "@yarnpkg/plugin-github"
+  - "@yarnpkg/plugin-http"
+  - "@yarnpkg/plugin-init"
+  - "@yarnpkg/plugin-interactive-tools"
+  - "@yarnpkg/plugin-link"
+  - "@yarnpkg/plugin-nm"
+  - "@yarnpkg/plugin-npm"
+  - "@yarnpkg/plugin-npm-cli"
+  - "@yarnpkg/plugin-pack"
+  - "@yarnpkg/plugin-patch"
+  - "@yarnpkg/plugin-pnp"
+  - "@yarnpkg/plugin-pnpm"
+  - "@yarnpkg/plugin-stage"
+  - "@yarnpkg/plugin-typescript"
+  - "@yarnpkg/plugin-version"
+  - "@yarnpkg/plugin-workspace-tools"
+  - "@yarnpkg/builder"
+  - "@yarnpkg/doctor"
+  - "@yarnpkg/extensions"
+  - "@yarnpkg/nm"
+  - "@yarnpkg/pnpify"
+  - "@yarnpkg/sdks"

CHANGELOG.md

@@ -57,6 +57,7 @@ The following changes only affect people writing Yarn plugins:
 ### Bugfixes
 
 - `yarn dlx` will no longer report false-positive `UNUSED_PACKAGE_EXTENSION` warnings
+- When Corepack is enabled Yarn will now use the current CLI to prepare external Yarn classic projects, matching the behaviour of when Corepack is disabled.
 
 ## 3.2.2
 

packages/acceptance-tests/pkg-tests-specs/sources/protocols/git.test.js

@@ -4,6 +4,7 @@ const {
 } = require(`pkg-tests-core`);
 const {parseSyml} = require(`@yarnpkg/parsers`);
 const {execUtils, semverUtils} = require(`@yarnpkg/core`);
+const {npath} = require(`@yarnpkg/fslib`);
 
 const TESTED_URLS = {
   // We've picked util-deprecate because it doesn't have any dependency, and
@@ -221,5 +222,45 @@ describe(`Protocols`, () => {
         },
       ),
     );
+
+    test(
+      `it should not use Corepack to fetch Yarn Classic`,
+      makeTemporaryEnv(
+        {
+          dependencies: {
+            [`yarn-1-project`]: startPackageServer().then(url => `${url}/repositories/yarn-1-project.git`),
+          },
+        },
+        async ({path, run, source}) => {
+          // This checks that the `set version classic` part of `scriptUtils.prepareExternalProject` doesn't use Corepack.
+          // The rest of the install will fail though.
+          await expect(run(`install`, {
+            env: {
+              COREPACK_ROOT: npath.join(npath.fromPortablePath(path), `404`),
+              YARN_ENABLE_INLINE_BUILDS: `true`,
+            },
+          })).rejects.toMatchObject({
+            code: 1,
+            stdout: expect.stringContaining(`Saving the new release`),
+          });
+        },
+      ),
+    );
+
+    test(
+      `it should not add a 'packageManager' field to a Yarn classic project`,
+      makeTemporaryEnv(
+        {
+          dependencies: {
+            [`yarn-1-project`]: startPackageServer().then(url => `${url}/repositories/yarn-1-project.git`),
+          },
+        },
+        async ({path, run, source}) => {
+          await expect(run(`install`)).resolves.toBeTruthy();
+
+          await expect(source(`require('yarn-1-project/package.json').packageManager`)).resolves.toBeUndefined();
+        },
+      ),
+    );
   });
 });

packages/yarnpkg-core/sources/scriptUtils.ts

@@ -250,11 +250,20 @@ export async function prepareExternalProject(cwd: PortablePath, outputPath: Port
               ? [`workspace`, workspace]
               : [];
 
+            // `set version` will update the Manifest to contain a `packageManager` field with the latest
+            // Yarn version which causes the results to change depending on when this command was run,
+            // therefore we revert any change made to it.
+            const manifestPath = ppath.join(cwd, Filename.manifest);
+            const manifestBuffer = await xfs.readFilePromise(manifestPath);
+
             // Makes sure that we'll be using Yarn 1.x
-            const version = await execUtils.pipevp(`yarn`, [`set`, `version`, `classic`, `--only-if-needed`], {cwd, env, stdin, stdout, stderr, end: execUtils.EndStrategy.ErrorCode});
+            const version = await execUtils.pipevp(process.execPath, [process.argv[1], `set`, `version`, `classic`, `--only-if-needed`, `--yarn-path`], {cwd, env, stdin, stdout, stderr, end: execUtils.EndStrategy.ErrorCode});
             if (version.code !== 0)
               return version.code;
 
+            // Revert any changes made to the Manifest by `set version`.
+            await xfs.writeFilePromise(manifestPath, manifestBuffer);
+
             // Otherwise Yarn 1 will pack the .yarn directory :(
             await xfs.appendFilePromise(ppath.join(cwd, `.npmignore` as PortablePath), `/.yarn\n`);