devbox: clean up profile history after sync

After syncing the flake packages to the profile, remove all old
generations of the Nix profile. This allows the Nix garbage collector to
eventually remove any old packages.

Opted for a Go implementation instead of calling `nix profile
wipe-history` because deleting the history is pretty simple and this is
faster.

Author: gcurtis

internal/devbox/nixprofile.go

@@ -5,6 +5,8 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"os"
+	"path/filepath"
 	"strings"
 
 	"github.com/samber/lo"
@@ -84,5 +86,42 @@ func (d *Devbox) syncNixProfileFromFlake(ctx context.Context) error {
 			return fmt.Errorf("error installing packages in nix profile %s: %w", add, err)
 		}
 	}
+	if len(add) > 0 || len(remove) > 0 {
+		err := wipeProfileHistory(profilePath)
+		if err != nil {
+			// Log the error, but nothing terrible happens if this
+			// fails.
+			slog.DebugContext(ctx, "error cleaning up profile history", "err", err)
+		}
+	}
+	return nil
+}
+
+// wipeProfileHistory removes all old generations of a Nix profile, similar to
+// nix profile wipe-history. profile should be a path to the "default" symlink,
+// like .devbox/nix/profile/default.
+func wipeProfileHistory(profile string) error {
+	link, err := os.Readlink(profile)
+	if errors.Is(err, os.ErrNotExist) {
+		return nil
+	}
+	if err != nil {
+		return err
+	}
+
+	dir := filepath.Dir(profile)
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return err
+	}
+	for _, dent := range entries {
+		if dent.Name() == "default" || dent.Name() == link {
+			continue
+		}
+		err := os.Remove(filepath.Join(dir, dent.Name()))
+		if err != nil && !errors.Is(err, os.ErrNotExist) {
+			return err
+		}
+	}
 	return nil
 }