linkNodeModulesHook: replace symlinks atomically (#499199)

      // Don't unlink this file, we just wrote it.

      managed.delete(file);

      // Link file

      // No need to replace the symlink if it already points to the right target

      try {

        await fs.promises.symlink(sourcePath, targetPath);

        if (await fs.promises.readlink(targetPath) === sourcePath) {

          return

        }

      } catch (err) {

        // If the target file already exists remove it and try again

        if (err.code !== "EEXIST") {

        // If the symlink doesn't already exist, keep going. If we get a

        // different error trying to stat it, fail.

        if (err.code !== "ENOENT") {

          throw err;

        }

        await fs.promises.unlink(targetPath);

        await fs.promises.symlink(sourcePath, targetPath);

      }

      // Replace the link, if it exists, atomically.

      const tmpPath = `${targetPath}.tmp.${process.pid}`

      await fs.promises.symlink(sourcePath, tmpPath);

      await fs.promises.rename(tmpPath, targetPath);

    })

  );