fix: use lstatSync in generatePosixLink (nodejs#767)

Using `fs.promises.realpath` in `generatePosixLink` returns the fully resolved, absolute path of the symlink target.

However, the symlink variable used in the comparison is a relative path calculated by `path.relative`. This results in a comparison between an absolute path and a relative path, causing the `currentSymlink !== symlink` check to always evaluate to true.

Author: polyomino24

sources/commands/Enable.ts

@@ -81,20 +81,22 @@ export class EnableCommand extends Command<Context> {
   async generatePosixLink(installDirectory: string, distFolder: string, binName: string) {
     const file = path.join(installDirectory, binName);
     const symlink = path.relative(installDirectory, path.join(distFolder, `${binName}.js`));
+    const stats = fs.lstatSync(file, {throwIfNoEntry: false});
 
-    if (fs.existsSync(file)) {
-      const currentSymlink = await fs.promises.realpath(file);
+    if (stats) {
+      if (stats.isSymbolicLink()) {
+        const currentSymlink = await fs.promises.readlink(file);
 
-      if (binName.includes(`yarn`) && corepackUtils.isYarnSwitchPath(currentSymlink)) {
-        console.warn(`${binName} is already installed in ${file} and points to a Yarn Switch install - skipping`);
-        return;
-      }
+        if (binName.includes(`yarn`) && corepackUtils.isYarnSwitchPath(await fs.promises.realpath(file))) {
+          console.warn(`${binName} is already installed in ${file} and points to a Yarn Switch install - skipping`);
+          return;
+        }
 
-      if (currentSymlink !== symlink) {
-        await fs.promises.unlink(file);
-      } else {
-        return;
+        if (currentSymlink === symlink) {
+          return;
+        }
       }
+      await fs.promises.unlink(file);
     }
 
     await fs.promises.symlink(symlink, file);

tests/Enable.test.ts

@@ -1,6 +1,7 @@
 import {Filename, ppath, xfs, npath}                                    from '@yarnpkg/fslib';
 import {delimiter}                                                      from 'node:path';
 import process                                                          from 'node:process';
+import {setTimeout}                                                     from 'node:timers/promises';
 import {describe, beforeEach, it, expect, test}                         from 'vitest';
 
 import {Engine}                                                         from '../sources/Engine';
@@ -92,7 +93,6 @@ describe(`EnableCommand`, () => {
     await xfs.mktempPromise(async cwd => {
       await xfs.writeFilePromise(ppath.join(cwd, `yarn`), `hello`);
 
-      process.env.PATH = `${npath.fromPortablePath(cwd)}${delimiter}${process.env.PATH}`;
       await expect(runCli(cwd, [`enable`, `--install-directory`, npath.fromPortablePath(cwd)])).resolves.toMatchObject({
         stdout: ``,
         stderr: ``,
@@ -114,7 +114,6 @@ describe(`EnableCommand`, () => {
         ppath.join(cwd, `yarn`),
       );
 
-      process.env.PATH = `${npath.fromPortablePath(cwd)}${delimiter}${process.env.PATH}`;
       await expect(runCli(cwd, [`enable`, `--install-directory`, npath.fromPortablePath(cwd)])).resolves.toMatchObject({
         stdout: ``,
         stderr: expect.stringMatching(/^yarn is already installed in .+ and points to a Yarn Switch install - skipping\n$/),
@@ -125,4 +124,46 @@ describe(`EnableCommand`, () => {
       expect(file).toBe(`hello`);
     });
   });
+
+  test.skipIf(process.platform === `win32`)(`should not re-link if binaries are already correct`, async () => {
+    await xfs.mktempPromise(async cwd => {
+      await makeBin(cwd, `corepack` as Filename);
+
+      await expect(runCli(cwd, [`enable`, `--install-directory`, npath.fromPortablePath(cwd)])).resolves.toMatchObject({
+        stdout: ``,
+        stderr: ``,
+        exitCode: 0,
+      });
+      const yarnStat1 = await xfs.lstatPromise(ppath.join(cwd, `yarn`));
+
+      await setTimeout(10);
+
+      await expect(runCli(cwd, [`enable`, `--install-directory`, npath.fromPortablePath(cwd)])).resolves.toMatchObject({
+        stdout: ``,
+        stderr: ``,
+        exitCode: 0,
+      });
+      const yarnStat2 = await xfs.lstatPromise(ppath.join(cwd, `yarn`));
+
+      expect(yarnStat2.mtimeMs).toBe(yarnStat1.mtimeMs);
+    });
+  });
+
+  test.skipIf(process.platform === `win32`)(`should overwrite existing symlinks if they are incorrect`, async () => {
+    await xfs.mktempPromise(async cwd => {
+      await makeBin(cwd, `corepack` as Filename);
+
+      await xfs.writeFilePromise(ppath.join(cwd, `dummy-target`), `hello`);
+      await xfs.symlinkPromise(ppath.join(cwd, `dummy-target`), ppath.join(cwd, `yarn`));
+
+      await expect(runCli(cwd, [`enable`, `--install-directory`, npath.fromPortablePath(cwd)])).resolves.toMatchObject({
+        stdout: ``,
+        stderr: ``,
+        exitCode: 0,
+      });
+
+      const newLink = await xfs.readlinkPromise(ppath.join(cwd, `yarn`));
+      expect(newLink).toContain(`yarn.js`);
+    });
+  });
 });