fix(core): revert changes made to the Manifest by set version

Author: merceyz

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

@@ -246,5 +246,21 @@ describe(`Protocols`, () => {
         },
       ),
     );
+
+    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(process.execPath, [process.argv[1], `set`, `version`, `classic`, `--only-if-needed`], {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`);