Enhance Nix flake installer to remove old profile entries before installation and update unit tests accordingly.

Includes:
- Added best-effort removal of existing 'package-manager' Nix profile entry.
- Updated install logic to avoid file conflicts.
- Extended unit tests to verify removal behavior and SystemExit-safe handling.

Conversation reference: https://chatgpt.com/share/69332bc4-a128-800f-a69c-fdc24c4cc7fe

pkgmgr/installers/nix_flake.py

@@ -6,6 +6,12 @@ Installer for Nix flakes.
 
 If a repository contains flake.nix and the 'nix' command is available, this
 installer will try to install profile outputs from the flake.
+
+Behavior:
+  - If flake.nix is present and `nix` exists on PATH:
+      * First remove any existing `package-manager` profile entry (best-effort).
+      * Then install the flake outputs (pkgmgr, default) via `nix profile install`.
+  - Any failure in `nix profile install` is treated as fatal (SystemExit).
 """
 
 import os
@@ -20,6 +26,7 @@ class NixFlakeInstaller(BaseInstaller):
     """Install Nix flake profiles for repositories that define flake.nix."""
 
     FLAKE_FILE = "flake.nix"
+    PROFILE_NAME = "package-manager"
 
     def supports(self, ctx: RepoContext) -> bool:
         """
@@ -32,11 +39,39 @@ class NixFlakeInstaller(BaseInstaller):
         flake_path = os.path.join(ctx.repo_dir, self.FLAKE_FILE)
         return os.path.exists(flake_path)
 
+    def _ensure_old_profile_removed(self, ctx: RepoContext) -> None:
+        """
+        Best-effort removal of an existing profile entry.
+
+        This handles the "already provides the following file" conflict by
+        removing previous `package-manager` installations before we install
+        the new one.
+
+        Any error in `nix profile remove` is intentionally ignored, because
+        a missing profile entry is not a fatal condition.
+        """
+        if shutil.which("nix") is None:
+            return
+
+        # We do NOT use run_command here, because we explicitly want to ignore
+        # the failure of `nix profile remove` (e.g. entry not present).
+        # Using `|| true` makes this idempotent.
+        cmd = f"nix profile remove {self.PROFILE_NAME} || true"
+        # This will still respect preview mode inside run_command.
+        try:
+            run_command(cmd, cwd=ctx.repo_dir, preview=ctx.preview)
+        except SystemExit:
+            # Ignore any error here: if the profile entry does not exist,
+            # that's fine and not a fatal condition.
+            pass
+
     def run(self, ctx: RepoContext) -> None:
         """
         Install Nix flake profile outputs (pkgmgr, default).
 
         Any failure in `nix profile install` is treated as fatal (SystemExit).
+        The "already installed / file conflict" situation is avoided by
+        removing the existing profile entry beforehand.
         """
         flake_path = os.path.join(ctx.repo_dir, self.FLAKE_FILE)
         if not os.path.exists(flake_path):
@@ -46,7 +81,11 @@ class NixFlakeInstaller(BaseInstaller):
             print("Warning: flake.nix found but 'nix' command not available. Skipping flake setup.")
             return
 
-        print("Nix flake detected, attempting to install profile output...")
+        print("Nix flake detected, attempting to install profile outputs...")
+
+        # Handle the "already installed" case up-front:
+        self._ensure_old_profile_removed(ctx)
+
         for output in ("pkgmgr", "default"):
             cmd = f"nix profile install {ctx.repo_dir}#{output}"
             try:

tests/unit/pkgmgr/installers/test_nix_flake.py

@@ -40,7 +40,17 @@ class TestNixFlakeInstaller(unittest.TestCase):
     @patch("os.path.exists", return_value=True)
     @patch("shutil.which", return_value="/usr/bin/nix")
     @mock.patch("pkgmgr.installers.nix_flake.run_command")
-    def test_run_tries_pkgmgr_then_default(self, mock_run_command, mock_which, mock_exists):
+    def test_run_removes_old_profile_and_installs_outputs(
+        self,
+        mock_run_command,
+        mock_which,
+        mock_exists,
+    ):
+        """
+        Ensure that run():
+          - first tries to remove the old 'package-manager' profile entry
+          - then installs both 'pkgmgr' and 'default' outputs.
+        """
         cmds = []
 
         def side_effect(cmd, cwd=None, preview=False, *args, **kwargs):
@@ -51,14 +61,45 @@ class TestNixFlakeInstaller(unittest.TestCase):
 
         self.installer.run(self.ctx)
 
-        self.assertIn(
-            f"nix profile install {self.ctx.repo_dir}#pkgmgr",
-            cmds,
-        )
-        self.assertIn(
-            f"nix profile install {self.ctx.repo_dir}#default",
-            cmds,
+        remove_cmd = f"nix profile remove {self.installer.PROFILE_NAME} || true"
+        install_pkgmgr_cmd = f"nix profile install {self.ctx.repo_dir}#pkgmgr"
+        install_default_cmd = f"nix profile install {self.ctx.repo_dir}#default"
+
+        # Mindestens diese drei Kommandos müssen aufgerufen worden sein
+        self.assertIn(remove_cmd, cmds)
+        self.assertIn(install_pkgmgr_cmd, cmds)
+        self.assertIn(install_default_cmd, cmds)
+
+        # Optional: sicherstellen, dass der remove-Aufruf zuerst kam
+        self.assertEqual(cmds[0], remove_cmd)
+
+    @patch("shutil.which", return_value="/usr/bin/nix")
+    @mock.patch("pkgmgr.installers.nix_flake.run_command")
+    def test_ensure_old_profile_removed_ignores_systemexit(
+        self,
+        mock_run_command,
+        mock_which,
+    ):
+        """
+        _ensure_old_profile_removed() must not propagate SystemExit, even if
+        'nix profile remove' fails (e.g. profile entry does not exist).
+        """
+
+        def side_effect(cmd, cwd=None, preview=False, *args, **kwargs):
+            raise SystemExit(1)
+
+        mock_run_command.side_effect = side_effect
+
+        # Should not raise, SystemExit is swallowed internally.
+        self.installer._ensure_old_profile_removed(self.ctx)
+
+        remove_cmd = f"nix profile remove {self.installer.PROFILE_NAME} || true"
+        mock_run_command.assert_called_with(
+            remove_cmd,
+            cwd=self.ctx.repo_dir,
+            preview=self.ctx.preview,
         )
 
+
 if __name__ == "__main__":
     unittest.main()