kv: don't embed RangeDescriptor and Lease in EvictionToken

This commit switches the handling of the RangeDescriptor and Lease
fields in EvictionToken to be stored by reference, pointing into an
immutable `CacheEntry` object instead of copying and storing these
fields by value. This change is made because storing the fields by value
was causing the object to escape to the heap all over the place. It was
too easy to Call `Desc()` or `Leaseholder()` and then have this
reference escape, which meant that we were allocating a single
`EvictionToken` multiple times throughout the DistSender stack.

A secondary benefit of this change is that it removes memory copies and
shrinks the size of the EvictionToken from 224 bytes to 40 bytes.

pkg/kv/kvclient/rangecache/range_cache.go

@@ -211,8 +211,8 @@ type EvictionToken struct {
 	// methods re-synchronize with the cache. However, it it changes, the
 	// descriptor only changes to other "compatible" descriptors (same range id
 	// and key bounds).
-	desc     roachpb.RangeDescriptor
-	lease    roachpb.Lease
+	desc     *roachpb.RangeDescriptor
+	lease    *roachpb.Lease
 	closedts roachpb.RangeClosedTimestampPolicy
 
 	// speculativeDesc, if not nil, is the descriptor that should replace desc if
@@ -250,8 +250,8 @@ func (rc *RangeCache) makeEvictionToken(
 	}
 	return EvictionToken{
 		rdc:             rc,
-		desc:            entry.desc,
-		lease:           entry.lease,
+		desc:            entry.Desc(),
+		lease:           entry.leaseEvenIfSpeculative(),
 		closedts:        entry.closedts,
 		speculativeDesc: speculativeDesc,
 	}
@@ -288,7 +288,7 @@ func (et EvictionToken) Desc() *roachpb.RangeDescriptor {
 	if !et.Valid() {
 		return nil
 	}
-	return &et.desc
+	return et.desc
 }
 
 // Leaseholder returns the cached leaseholder. If the cache didn't have any
@@ -297,7 +297,7 @@ func (et EvictionToken) Desc() *roachpb.RangeDescriptor {
 // If a leaseholder is returned, it will correspond to one of the replicas in
 // et.Desc().
 func (et EvictionToken) Leaseholder() *roachpb.ReplicaDescriptor {
-	if et.lease.Empty() {
+	if !et.Valid() || et.lease == nil {
 		return nil
 	}
 	return &et.lease.Replica
@@ -309,6 +309,9 @@ func (et EvictionToken) LeaseSeq() roachpb.LeaseSequence {
 	if !et.Valid() {
 		panic("invalid LeaseSeq() call on empty EvictionToken")
 	}
+	if et.lease == nil {
+		return 0
+	}
 	return et.lease.Sequence
 }
 
@@ -334,8 +337,8 @@ func (et *EvictionToken) syncRLocked(
 		et.clear()
 		return false, nil, nil
 	}
-	et.desc = cachedEntry.desc
-	et.lease = cachedEntry.lease
+	et.desc = cachedEntry.Desc()
+	et.lease = cachedEntry.leaseEvenIfSpeculative()
 	return true, cachedEntry, rawEntry
 }
 
@@ -382,8 +385,8 @@ func (et *EvictionToken) UpdateLease(ctx context.Context, l *roachpb.Lease) bool
 		return false
 	}
 	if newEntry != nil {
-		et.desc = newEntry.desc
-		et.lease = newEntry.lease
+		et.desc = newEntry.Desc()
+		et.lease = newEntry.leaseEvenIfSpeculative()
 	} else {
 		// newEntry == nil means the lease is not compatible with the descriptor.
 		et.clear()
@@ -424,7 +427,7 @@ func (et *EvictionToken) EvictLease(ctx context.Context) {
 	et.rdc.rangeCache.Lock()
 	defer et.rdc.rangeCache.Unlock()
 
-	if et.lease.Empty() {
+	if et.lease == nil {
 		log.Fatalf(ctx, "attempting to clear lease from cache entry without lease")
 	}
 
@@ -437,8 +440,8 @@ func (et *EvictionToken) EvictLease(ctx context.Context) {
 	if !ok {
 		return
 	}
-	et.desc = newEntry.desc
-	et.lease = newEntry.lease
+	et.desc = newEntry.Desc()
+	et.lease = newEntry.leaseEvenIfSpeculative()
 	et.rdc.swapEntryLocked(ctx, rawEntry, newEntry)
 }
 
@@ -524,7 +527,15 @@ func (rc *RangeCache) Lookup(ctx context.Context, key roachpb.RKey) (CacheEntry,
 	if err != nil {
 		return CacheEntry{}, err
 	}
-	return CacheEntry{tok.desc, tok.lease, tok.closedts}, nil
+	var e CacheEntry
+	if tok.desc != nil {
+		e.desc = *tok.desc
+	}
+	if tok.lease != nil {
+		e.lease = *tok.lease
+	}
+	e.closedts = tok.closedts
+	return e, nil
 }
 
 // GetCachedOverlapping returns all the cached entries which overlap a given
@@ -1118,6 +1129,16 @@ func (e *CacheEntry) Lease() *roachpb.Lease {
 	return &e.lease
 }
 
+// leaseEvenIfSpeculative is like Lease, except it returns a Lease object even
+// if that lease is speculative. Returns nil if no speculative or non-speculative
+// lease is known.
+func (e *CacheEntry) leaseEvenIfSpeculative() *roachpb.Lease {
+	if e.lease.Empty() {
+		return nil
+	}
+	return &e.lease
+}
+
 // ClosedTimestampPolicy returns the cached understanding of the range's closed
 // timestamp policy. If no policy is known, LAG_BY_CLUSTER_SETTING is returned.
 func (e *CacheEntry) ClosedTimestampPolicy() roachpb.RangeClosedTimestampPolicy {