kv: remove dependency on ticks from maybeDropMsgApp

Related to #73838.

In d77bee9, we stopped ticking uninitialized replicas, so we can no longer use
ticks as a proxy for the age of a replica in the escape hatch of `maybeDropMsgApp`.
Instead, we now use the age of the replica directly. We hit the escape hatch for
any replica that is older than 20s, which corresponds to the 100 ticks we used
before.

pkg/kv/kvserver/replica.go

@@ -214,6 +214,9 @@ type Replica struct {
 	// The writes to this key happen in Replica.setStartKeyLocked.
 	startKey roachpb.RKey
 
+	// creationTime is the time that the Replica struct was initially constructed.
+	creationTime time.Time
+
 	store     *Store
 	abortSpan *abortspan.AbortSpan // Avoids anomalous reads after abort
 

pkg/kv/kvserver/replica_init.go

@@ -71,6 +71,7 @@ func newUnloadedReplica(
 	r := &Replica{
 		AmbientContext: store.cfg.AmbientCtx,
 		RangeID:        desc.RangeID,
+		creationTime:   timeutil.Now(),
 		store:          store,
 		abortSpan:      abortspan.New(desc.RangeID),
 		concMgr: concurrency.NewManager(concurrency.Config{

pkg/kv/kvserver/split_trigger_helper.go

@@ -13,23 +13,26 @@ package kvserver
 import (
 	"context"
 	"fmt"
+	"time"
 
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
 	"github.com/cockroachdb/cockroach/pkg/util/log"
+	"github.com/cockroachdb/cockroach/pkg/util/timeutil"
 	"go.etcd.io/etcd/raft/v3/raftpb"
 )
 
-const maxDelaySplitTriggerTicks = 100
+const maxDelaySplitTriggerDur = 20 * time.Second
 
 type replicaMsgAppDropper Replica
 
-func (rd *replicaMsgAppDropper) Args() (initialized bool, ticks int) {
+func (rd *replicaMsgAppDropper) Args() (initialized bool, age time.Duration) {
 	r := (*Replica)(rd)
 	r.mu.RLock()
 	initialized = r.isInitializedRLocked()
-	ticks = r.mu.ticks
+	creationTime := r.creationTime
 	r.mu.RUnlock()
-	return initialized, ticks
+	age = timeutil.Since(creationTime)
+	return initialized, age
 }
 
 func (rd *replicaMsgAppDropper) ShouldDrop(
@@ -44,7 +47,7 @@ func (rd *replicaMsgAppDropper) ShouldDrop(
 }
 
 type msgAppDropper interface {
-	Args() (initialized bool, ticks int)
+	Args() (initialized bool, age time.Duration)
 	ShouldDrop(ctx context.Context, key roachpb.RKey) (fmt.Stringer, bool)
 }
 
@@ -71,7 +74,7 @@ func maybeDropMsgApp(
 	// message via msg.Context. Check if this replica might be waiting for a
 	// split trigger. The first condition for that is not knowing the key
 	// bounds, i.e. not being initialized.
-	initialized, ticks := r.Args()
+	initialized, age := r.Args()
 
 	if initialized {
 		return false
@@ -135,16 +138,16 @@ func maybeDropMsgApp(
 	if verbose {
 		log.Infof(ctx, "start key is contained in replica %v", lhsRepl)
 	}
-	if ticks > maxDelaySplitTriggerTicks {
+	if age > maxDelaySplitTriggerDur {
 		// This is an escape hatch in case there are other scenarios (missed in
 		// the above analysis) in which a split trigger just isn't coming. If
 		// there are, the idea is that we notice this log message and improve
 		// the heuristics.
 		log.Warningf(
 			ctx,
 			"would have dropped incoming MsgApp to wait for split trigger, "+
-				"but allowing due to %d (>%d) ticks",
-			ticks, maxDelaySplitTriggerTicks)
+				"but allowing because uninitialized replica was created %s (>%s) ago",
+			age, maxDelaySplitTriggerDur)
 		return false
 	}
 	if verbose {

pkg/kv/kvserver/split_trigger_helper_test.go

@@ -14,6 +14,7 @@ import (
 	"context"
 	"fmt"
 	"testing"
+	"time"
 
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
 	"github.com/cockroachdb/cockroach/pkg/testutils"
@@ -26,14 +27,14 @@ import (
 
 type testMsgAppDropper struct {
 	initialized bool
-	ticks       int
+	age         time.Duration
 	lhs         bool
 
 	startKey string // set by ShouldDrop
 }
 
-func (td *testMsgAppDropper) Args() (initialized bool, ticks int) {
-	return td.initialized, td.ticks
+func (td *testMsgAppDropper) Args() (initialized bool, age time.Duration) {
+	return td.initialized, td.age
 }
 
 func (td *testMsgAppDropper) ShouldDrop(
@@ -58,9 +59,9 @@ func TestMaybeDropMsgApp(t *testing.T) {
 		// Drop message to wait for trigger.
 		{initialized: false, lhs: true}: true,
 		// Drop message to wait for trigger.
-		{initialized: false, lhs: true, ticks: maxDelaySplitTriggerTicks}: true,
+		{initialized: false, lhs: true, age: maxDelaySplitTriggerDur}: true,
 		// Escape hatch fires.
-		{initialized: false, lhs: true, ticks: maxDelaySplitTriggerTicks + 1}: false,
+		{initialized: false, lhs: true, age: maxDelaySplitTriggerDur + 1}: false,
 	}
 
 	msgHeartbeat := &raftpb.Message{