sqlstats: fix race condition in ss_mem_iterator

StmtStatsIterator.Next() copies the next statement stats info into a
new CollectedStatementStatistics object, but the slice fields
of stmtStats.mu.data are not copied explicitly. This makes them
susecptible to race conditions, as seen in #138224.

To fix, all slice fields in the StatementStatistics are
explicitly copied, using slices.Clone.

Fixes: 138224
Epic: none
Release note: none

pkg/sql/sqlstats/ssmemstorage/ss_mem_iterator.go

@@ -6,6 +6,7 @@
 package ssmemstorage
 
 import (
+	"slices"
 	"sort"
 
 	"github.com/cockroachdb/cockroach/pkg/sql/appstatspb"
@@ -77,7 +78,7 @@ func (s *StmtStatsIterator) Next() bool {
 	}
 
 	statementStats.mu.Lock()
-	data := statementStats.mu.data
+	data := copyDataLocked(statementStats)
 	distSQLUsed := statementStats.mu.distSQLUsed
 	vectorized := statementStats.mu.vectorized
 	fullScan := statementStats.mu.fullScan
@@ -105,6 +106,20 @@ func (s *StmtStatsIterator) Next() bool {
 	return true
 }
 
+// copyDataLocked Copies the statement stat's data object under the mutex.
+// This function requires that there is a lock on the stats object.
+func copyDataLocked(stats *stmtStats) appstatspb.StatementStatistics {
+	stats.mu.AssertHeld()
+	data := stats.mu.data
+	data.Nodes = slices.Clone(data.Nodes)
+	data.KVNodeIDs = slices.Clone(data.KVNodeIDs)
+	data.Regions = slices.Clone(data.Regions)
+	data.PlanGists = slices.Clone(data.PlanGists)
+	data.IndexRecommendations = slices.Clone(data.IndexRecommendations)
+	data.Indexes = slices.Clone(data.Indexes)
+	return data
+}
+
 // Cur returns the appstatspb.CollectedStatementStatistics at the current internal
 // counter.
 func (s *StmtStatsIterator) Cur() *appstatspb.CollectedStatementStatistics {