fix(pools): do not panic when a rebalance stop finds no rebalance metadata

Signed-off-by: Denis Babchuk <babchukdenis@icloud.com>

cmd/admin-handlers-pools.go

@@ -373,7 +373,9 @@ func (a adminAPIHandlers) RebalanceStop(w http.ResponseWriter, r *http.Request)
 	// Cancel any ongoing rebalance operation
 	globalNotificationSys.StopRebalance(r.Context())
 	writeSuccessResponseHeadersOnly(w)
-	adminLogIf(ctx, pools.saveRebalanceStats(GlobalContext, 0, rebalSaveStoppedAt))
+	if err := pools.saveRebalanceStats(GlobalContext, 0, rebalSaveStoppedAt); !errors.Is(err, errRebalanceNotStarted) {
+		adminLogIf(ctx, err)
+	}
 	globalNotificationSys.LoadRebalanceMeta(ctx, false)
 }
 

cmd/erasure-server-pool-rebalance.go

@@ -826,6 +826,10 @@ func (z *erasureServerPools) saveRebalanceStats(ctx context.Context, poolIdx int
 	// if not found, we store the memory metadata back
 	// when rebalance status changed, will notify all nodes update status to memory, we can treat the memory metadata is the latest status
 	if errors.Is(err, errConfigNotFound) {
+		if z.rebalMeta == nil {
+			// No rebalance has run, e.g. a rebalance stop without one.
+			return errRebalanceNotStarted
+		}
 		r = z.rebalMeta
 	}
 

cmd/erasure-server-pool-rebalance_test.go

@@ -0,0 +1,42 @@
+// Copyright (c) 2026 Denis Babchuk
+//
+// This file is part of Silo Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+package cmd
+
+import (
+	"errors"
+	"testing"
+)
+
+// Without rebalance.bin and without in-memory metadata no rebalance has run,
+// so there is nothing to update: saving must report that instead of
+// dereferencing the missing metadata, and must not create rebalance.bin.
+func TestSaveRebalanceStatsWithoutMeta(t *testing.T) {
+	z, _ := consistencyPools(t)
+	for _, opts := range []rebalSaveOpts{rebalSaveStoppedAt, rebalSaveStats} {
+		if err := z.saveRebalanceStats(t.Context(), 0, opts); !errors.Is(err, errRebalanceNotStarted) {
+			t.Fatalf("opts %d: got %v, want %v", opts, err, errRebalanceNotStarted)
+		}
+		if z.rebalMeta != nil {
+			t.Fatalf("opts %d: in-memory rebalance metadata was set", opts)
+		}
+		r := &rebalanceMeta{}
+		if err := r.load(t.Context(), z.serverPools[0]); !errors.Is(err, errConfigNotFound) {
+			t.Fatalf("opts %d: rebalance.bin after save: %v, want %v", opts, err, errConfigNotFound)
+		}
+	}
+}