Merge #144476

144476: kvserver: metamorphically enable kv.closed_timestamp.lead_for_global_reads_auto_tune.enabled r=arulajmani a=wenyihu6

**kvserver: metamorphically enable kv.closed_timestamp.lead_for_global_reads_auto_tune.enabled**

This commit metamorphically enables
kv.closed_timestamp.lead_for_global_reads_auto_tune.enabled.

Part of: #143888
Release note: none

---

**kvserver: clarify round-trip latency in policy refresher comments**

Previously, it might be unclear to readers unless they trace the behavior back
through rpccontext. This commit updates the comments in the policy refresher to
explicitly state that the latency being referred to is round-trip.

Epic: none
Release note: none


Co-authored-by: wenyihu6 <[email protected]>

Author: craig[bot]

pkg/kv/kvserver/closedts/BUILD.bazel

@@ -13,6 +13,7 @@ go_library(
         "//pkg/kv/kvserver/closedts/ctpb",
         "//pkg/settings",
         "//pkg/util/hlc",
+        "//pkg/util/metamorphic",
     ],
 )
 

pkg/kv/kvserver/closedts/policyrefresher/policy_refresher.go

@@ -40,9 +40,10 @@ type PolicyRefresher struct {
 	// when there is a leaseholder change or when there is a span config change.
 	refreshNotificationCh chan struct{}
 
-	// getNodeLatencies returns a map of node IDs to their measured latencies
-	// from the current node. Replicas use this information to determine
-	// appropriate closed timestamp policies.
+	// getNodeLatencies returns a map of node IDs to their observed round-trip
+	// latencies from the current node. Replicas use this information to determine
+	// appropriate closed timestamp policies. See rpc.RemoteClockMonitor for more
+	// details.
 	getNodeLatencies func() map[roachpb.NodeID]time.Duration
 
 	// latencyCache caches the latency information from getNodeLatencies. This
@@ -85,10 +86,10 @@ func NewPolicyRefresher(
 // Replica defines a thin interface to update closed timestamp policies.
 type Replica interface {
 	// RefreshPolicy informs the replica that it should refresh its closed
-	// timestamp policy. A latency map, which includes observed latency to other
-	// nodes in the system by the PolicyRefresher may be supplied (or may be nil),
-	// in which case it may be used to correctly place a replica in its latency
-	// based global reads bucket.
+	// timestamp policy. A latency map, which includes round-trip observed latency
+	// to other nodes in the system by the PolicyRefresher may be supplied (or may
+	// be nil), in which case it may be used to correctly place a replica in its
+	// latency based global reads bucket.
 	RefreshPolicy(map[roachpb.NodeID]time.Duration)
 }
 
@@ -118,17 +119,19 @@ func (pr *PolicyRefresher) EnqueueReplicaForRefresh(replica Replica) {
 	}
 }
 
-// updateLatencyCache refreshes the cached latency information by fetching fresh
-// measurements from the actual RPC context. This can be expensive, so we only
-// do this periodically based on the configured refresh interval.
+// updateLatencyCache refreshes the cached round-trip latency information by
+// fetching fresh measurements from the actual RPC context. This can be
+// expensive, so we only do this periodically based on the configured refresh
+// interval.
 func (pr *PolicyRefresher) updateLatencyCache() {
 	pr.mu.Lock()
 	defer pr.mu.Unlock()
 	pr.mu.latencyCache = pr.getNodeLatencies()
 }
 
-// getCurrentLatencies returns the current latency information if auto-tuning is
-// enabled and the cluster has been fully upgraded to v25.2, or nil otherwise.
+// getCurrentLatencies returns the current round-trip latency information if
+// auto-tuning is enabled and the cluster has been fully upgraded to v25.2, or
+// nil otherwise.
 func (pr *PolicyRefresher) getCurrentLatencies() map[roachpb.NodeID]time.Duration {
 	// Testing knobs only.
 	if pr.knobs != nil && pr.knobs.InjectedLatencies != nil {

pkg/kv/kvserver/closedts/setting.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/cockroachdb/cockroach/pkg/settings"
+	"github.com/cockroachdb/cockroach/pkg/util/metamorphic"
 )
 
 // NB: These settings are SystemVisible because they need to be read by e.g.
@@ -86,6 +87,6 @@ var LeadForGlobalReadsAutoTuneEnabled = settings.RegisterBoolSetting(
 		"furthest follower will be used to adjust closed timestamp policies for ranges"+
 		"ranges configured to serve global reads. "+
 		"kv.closed_timestamp.lead_for_global_reads_override takes precedence if set.",
-	false,
+	metamorphic.ConstantWithTestBool("kv.closed_timestamp.lead_for_global_reads_auto_tune.enabled", false),
 	settings.WithPublic,
 )

pkg/kv/kvserver/replica.go

@@ -1339,7 +1339,7 @@ func (r *Replica) closedTimestampPolicyRLocked() ctpb.RangeClosedTimestampPolicy
 }
 
 // RefreshPolicy updates the replica's cached closed timestamp policy based on
-// span configurations and provided node latencies.
+// span configurations and provided node round-trip latencies.
 func (r *Replica) RefreshPolicy(latencies map[roachpb.NodeID]time.Duration) {
 	policy := func() ctpb.RangeClosedTimestampPolicy {
 		desc, conf := r.DescAndSpanConfig()