fix(rpc): make IO stream lifecycle race-safe

Co-authored-by: naiba/CloudCode <hi+cloudcode@nai.ba>

service/rpc/state_metrics_test.go

@@ -0,0 +1,42 @@
+package rpc
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/nezhahq/nezha/model"
+	"github.com/nezhahq/nezha/pkg/tsdb"
+)
+
+func TestStateMetricsWriterRunsOnlyForCurrentGeneration(t *testing.T) {
+	// Given
+	server := &model.Server{}
+	model.InitServer(server)
+	oldLease := server.AttachStateStream(stateGenerationStream{})
+	newLease := server.AttachStateStream(stateGenerationStream{})
+	oldCalls := 0
+	newCalls := 0
+	oldWriter := writeServerMetrics
+	writeServerMetrics = func(*tsdb.ServerMetrics) error {
+		newCalls++
+		return nil
+	}
+	t.Cleanup(func() { writeServerMetrics = oldWriter })
+
+	// When
+	oldAccepted := server.UpdateStateIfCurrentWithSideEffect(oldLease, &model.HostState{Uptime: 11}, time.Unix(100, 0), func() error {
+		oldCalls++
+		return writeServerMetrics(&tsdb.ServerMetrics{ServerID: 7, Timestamp: time.Unix(100, 0)})
+	})
+	newAccepted := server.UpdateStateIfCurrentWithSideEffect(newLease, &model.HostState{Uptime: 22}, time.Unix(200, 0), func() error {
+		return writeServerMetrics(&tsdb.ServerMetrics{ServerID: 7, Timestamp: time.Unix(200, 0)})
+	})
+
+	// Then
+	require.False(t, oldAccepted)
+	require.True(t, newAccepted)
+	require.Zero(t, oldCalls)
+	require.Equal(t, 1, newCalls)
+}