state: serialise IP backfill with node persistence

A persist between the backfill's database write and its NodeStore reload
wrote the old addresses back, silently undoing the backfill.

hscontrol/state/persist_test.go

@@ -5,6 +5,7 @@ import (
 	"net/netip"
 	"slices"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -1090,3 +1091,61 @@ func TestNodeWriteChangeWhenPolicyRefreshFails(t *testing.T) {
 		})
 	}
 }
+
+// TestBackfillNodeIPsSurvivesConcurrentPersist guards the backfill against a
+// node persist landing between its database write and the NodeStore reload:
+// that persist writes the old addresses back and the backfill is lost.
+func TestBackfillNodeIPsSurvivesConcurrentPersist(t *testing.T) {
+	dbPath, s, nodeID := persistTestSetup(t)
+	require.NoError(t, s.Close())
+
+	// Dropping the IPv6 prefix gives the backfill work to do.
+	cfg := persistTestConfig(dbPath)
+	cfg.PrefixV6 = nil
+
+	s, err := NewState(cfg)
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = s.Close() })
+
+	nv, ok := s.GetNodeByID(nodeID)
+	require.True(t, ok)
+	require.True(t, nv.IPv6().Valid(), "precondition: node has an IPv6 address")
+
+	// The backfill reads the nodes table twice: inside its write, then to
+	// reload NodeStore. Persist the node just before the reload.
+	var reads atomic.Int32
+
+	persisted := make(chan struct{})
+
+	require.NoError(t, s.db.DB.Callback().Query().Before("gorm:query").
+		Register("persist_before_reload", func(tx *gorm.DB) {
+			if tx.Statement.Table != "nodes" || reads.Add(1) != 2 {
+				return
+			}
+
+			go func() {
+				_, _ = s.persistNode(nv)
+
+				close(persisted)
+			}()
+
+			// Give an unserialised persist time to land before the reload.
+			select {
+			case <-persisted:
+			case <-time.After(200 * time.Millisecond):
+			}
+		}))
+	t.Cleanup(func() { _ = s.db.DB.Callback().Query().Remove("persist_before_reload") })
+
+	_, _, err = s.BackfillNodeIPs()
+	require.NoError(t, err)
+	<-persisted
+
+	nv, ok = s.GetNodeByID(nodeID)
+	require.True(t, ok)
+	assert.False(t, nv.IPv6().Valid(), "NodeStore kept the removed IPv6 address")
+
+	dbNode, err := s.db.GetNodeByID(nodeID)
+	require.NoError(t, err)
+	assert.Nil(t, dbNode.IPv6, "database kept the removed IPv6 address")
+}