mvcc: use GaugeFunc metric to load db size when requested

Relying on mvcc to set the db size metric can cause it to
miss size changes when a txn commits after the last write
completes before a quiescent period. Instead, load the
db size on demand.

Fixes etcd-io#8146

mvcc/kvstore.go

@@ -251,6 +251,11 @@ func (s *store) Restore(b backend.Backend) error {
 }
 
 func (s *store) restore() error {
+	reportDbTotalSizeInBytesMu.Lock()
+	b := s.b
+	reportDbTotalSizeInBytes = func() float64 { return float64(b.Size()) }
+	reportDbTotalSizeInBytesMu.Unlock()
+
 	min, max := newRevBytes(), newRevBytes()
 	revToBytes(revision{main: 1}, min)
 	revToBytes(revision{main: math.MaxInt64, sub: math.MaxInt64}, max)
@@ -261,8 +266,6 @@ func (s *store) restore() error {
 	tx := s.b.BatchTx()
 	tx.Lock()
 
-	dbTotalSize.Set(float64(s.b.Size()))
-
 	_, finishedCompactBytes := tx.UnsafeRange(metaBucketName, finishedCompactKeyName, nil, 0)
 	if len(finishedCompactBytes) != 0 {
 		s.compactMainRev = bytesToRev(finishedCompactBytes[0]).main

mvcc/kvstore_txn.go

@@ -105,7 +105,6 @@ func (tw *storeTxnWrite) End() {
 	if len(tw.changes) != 0 {
 		tw.s.revMu.Unlock()
 	}
-	dbTotalSize.Set(float64(tw.s.b.Size()))
 	tw.s.mu.RUnlock()
 }
 

mvcc/metrics.go

@@ -15,6 +15,8 @@
 package mvcc
 
 import (
+	"sync"
+
 	"github.com/prometheus/client_golang/prometheus"
 )
 
@@ -129,12 +131,21 @@ var (
 			Buckets: prometheus.ExponentialBuckets(100, 2, 14),
 		})
 
-	dbTotalSize = prometheus.NewGauge(prometheus.GaugeOpts{
+	dbTotalSize = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
 		Namespace: "etcd_debugging",
 		Subsystem: "mvcc",
 		Name:      "db_total_size_in_bytes",
 		Help:      "Total size of the underlying database in bytes.",
-	})
+	},
+		func() float64 {
+			reportDbTotalSizeInBytesMu.RLock()
+			defer reportDbTotalSizeInBytesMu.RUnlock()
+			return reportDbTotalSizeInBytes()
+		},
+	)
+	// overridden by mvcc initialization
+	reportDbTotalSizeInBytesMu sync.RWMutex
+	reportDbTotalSizeInBytes   func() float64 = func() float64 { return 0 }
 )
 
 func init() {