fix(stats): clear a moved username from its previous holder's rows

internal/modules/stats/startup_mongo_test.go

@@ -138,3 +138,8 @@ func setupMongoStatsTest(t *testing.T) (context.Context, storage.Collection, sto
 	provider := storage.NewMongoProvider(db)
 	return ctx, provider.Collection("stats"), provider.Collection(systemstate.CollectionName)
 }
+
+func TestInc_MongoUsernameMoveClearsOldHolder(t *testing.T) {
+	_, statsColl, _ := setupMongoStatsTest(t)
+	assertUsernameMoveClearsOldHolder(t, statsColl)
+}

internal/modules/stats/stats_test.go

@@ -497,3 +497,36 @@ func tail(s string) string {
 	}
 	return s[len(s)-n:]
 }
+
+// assertUsernameMoveClearsOldHolder checks that when a Telegram username moves
+// to another account, the previous holder's rows drop it, so /stats user
+// <name> resolves to the new owner alone.
+func assertUsernameMoveClearsOldHolder(t *testing.T, coll storage.Collection) {
+	t.Helper()
+	ctx := context.Background()
+	c := newCounter(coll)
+	docs := storage.Typed[usageEntry](coll)
+
+	c.Inc(ctx, "ping", updateFrom(42, "bob"))
+	c.Inc(ctx, "wordle", updateFrom(42, "bob"))
+	c.Inc(ctx, "lol", updateFrom(77, "bob"))
+
+	old, _, err := docs.Get(ctx, usageKey("ping", 42))
+	if err != nil {
+		t.Fatalf("ping:42: %v", err)
+	}
+	if old.Username != "" || old.N != 1 {
+		t.Errorf("old holder row = %+v, want username cleared and count kept", old)
+	}
+	rows, found, err := c.store.CommandsByUser(ctx, "bob", 10)
+	if err != nil || !found {
+		t.Fatalf("CommandsByUser(bob) found=%v err=%v", found, err)
+	}
+	if len(rows) != 1 || rows[0].display != "/lol" {
+		t.Errorf("CommandsByUser(bob) = %+v, want only the new holder's /lol", rows)
+	}
+}
+
+func TestInc_UsernameMoveClearsOldHolder(t *testing.T) {
+	assertUsernameMoveClearsOldHolder(t, storage.NewMemoryProvider().Collection("stats"))
+}

internal/modules/stats/usage_store.go

@@ -106,16 +106,25 @@ func (s *docUsageStore) Increment(ctx context.Context, cmd string, user usageUse
 	return nil
 }
 
+// refreshUsernameLocked rewrites the username on every row of user.ID, so a
+// Telegram username change carries the user's whole history with it and
+// /stats user <new name> finds all of it. It also clears that username from
+// other users' rows: Telegram usernames can move to a new account, and a stale
+// copy would make lookups by name ambiguous. The caller must hold s.mu.
 func (s *docUsageStore) refreshUsernameLocked(ctx context.Context, user usageUser) error {
 	entries, err := s.loadEntriesLocked(ctx)
 	if err != nil {
 		return err
 	}
 	for _, e := range entries {
-		if e.UserID != user.ID || e.Username == user.Username {
+		switch {
+		case e.UserID == user.ID && e.Username != user.Username:
+			e.Username = user.Username
+		case e.UserID != user.ID && user.Username != "" && e.Username == user.Username:
+			e.Username = ""
+		default:
 			continue
 		}
-		e.Username = user.Username
 		if err := s.docs.Put(ctx, usageKey(e.Cmd, e.UserID), e); err != nil {
 			return fmt.Errorf("stats refresh username %d: %w", user.ID, err)
 		}
@@ -255,6 +264,11 @@ type mongoUsageStore struct {
 	coll *mongo.Collection
 }
 
+// Increment upserts the row with $inc, so concurrent increments need no lock.
+// Unsetting "deleted" revives a soft-deleted row that is being used again; the
+// retired command is filtered out before that can happen. A follow-up
+// UpdateMany propagates a username change to the user's other rows, and a
+// second one clears the username from any other account that held it before.
 func (s *mongoUsageStore) Increment(ctx context.Context, cmd string, user usageUser, hasUser bool) error {
 	if isRetiredCommand(cmd) {
 		return nil
@@ -301,6 +315,21 @@ func (s *mongoUsageStore) Increment(ctx context.Context, cmd string, user usageU
 	); err != nil {
 		return fmt.Errorf("mongo stats refresh username %d: %w", user.ID, err)
 	}
+	if user.Username == "" {
+		return nil
+	}
+	// The username may have moved from another account; drop the stale copy so
+	// lookups by name resolve to one user.
+	if _, err := s.coll.UpdateMany(ctx,
+		bson.M{"uid": bson.M{"$ne": user.ID}, "user": user.Username},
+		bson.M{
+			"$unset":       bson.M{"user": ""},
+			"$inc":         bson.M{"version": int64(1)},
+			"$currentDate": bson.M{"updatedAt": true},
+		},
+	); err != nil {
+		return fmt.Errorf("mongo stats clear moved username %s: %w", user.Username, err)
+	}
 	return nil
 }