rditer: add a filter for just lock table spans

This patch adds a new `ReplicatedSpansFilter` called
`ReplicatedSpansLocksOnly`, which can be used to select just lock table
spans for a given range. While here, I also wrapped some comments over
80 chars that were bothering me.

Epic: none

Release note: None

pkg/kv/kvserver/rditer/select.go

@@ -15,20 +15,30 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
 )
 
+// ReplicatedSpansFilter is used to declare filters when selecting replicated
+// spans.
+//
+// TODO(arul): we could consider using a bitset here instead. Note that a lot of
+// the fields here are mutually exclusive (e.g. ReplicatedSpansExcludeLocks and
+// ReplicatedSpansLocksOnly), so we'd need some sort of validation here.
 type ReplicatedSpansFilter int
 
 const (
 	// ReplicatedSpansAll includes all replicated spans, including user keys,
 	// range descriptors, and lock keys.
 	ReplicatedSpansAll ReplicatedSpansFilter = iota
-	// ReplicatedSpansExcludeUser includes all replicated spans except for user keys.
+	// ReplicatedSpansExcludeUser includes all replicated spans except for user
+	// keys.
 	ReplicatedSpansExcludeUser
 	// ReplicatedSpansUserOnly includes just user keys, and no other replicated
 	// spans.
 	ReplicatedSpansUserOnly
-	// ReplicatedSpansExcludeLockTable includes all replicated spans except for the
+	// ReplicatedSpansExcludeLocks includes all replicated spans except for the
 	// lock table.
 	ReplicatedSpansExcludeLocks
+	// ReplicatedSpansLocksOnly includes just spans for the lock table, and no
+	// other replicated spans.
+	ReplicatedSpansLocksOnly
 )
 
 // SelectOpts configures which spans for a Replica to return from Select.
@@ -70,26 +80,29 @@ func Select(rangeID roachpb.RangeID, opts SelectOpts) []roachpb.Span {
 	}
 
 	if !opts.ReplicatedBySpan.Equal(roachpb.RSpan{}) {
-		// r1 "really" only starts at LocalMax. But because we use a StartKey of RKeyMin
-		// for r1, we actually do anchor range descriptors (and their locks and txn records)
-		// at RKeyMin as well. On the other hand, the "user key space" cannot start at RKeyMin
-		// because then it encompasses the special-cased prefix \x02... (/Local/).
-		// So awkwardly for key-based local keyspace we must not call KeySpan, for
-		// user keys we have to.
+		// r1 "really" only starts at LocalMax. But because we use a StartKey of
+		// RKeyMin for r1, we actually do anchor range descriptors (and their locks
+		// and txn records) at RKeyMin as well. On the other hand, the "user key
+		// space" cannot start at RKeyMin because then it encompasses the
+		// special-cased prefix \x02... (/Local/). So awkwardly for key-based local
+		// keyspace we must not call KeySpan, for user keys we have to.
 		//
 		// See also the comment on KeySpan.
 		in := opts.ReplicatedBySpan
 		adjustedIn := in.KeySpan()
 		if opts.ReplicatedSpansFilter != ReplicatedSpansUserOnly {
-			sl = append(sl, makeRangeLocalKeySpan(in))
+			if opts.ReplicatedSpansFilter != ReplicatedSpansLocksOnly {
+				sl = append(sl, makeRangeLocalKeySpan(in))
+			}
 
 			// Lock table.
 			if opts.ReplicatedSpansFilter != ReplicatedSpansExcludeLocks {
 				// Handle doubly-local lock table keys since range descriptor key
 				// is a range local key that can have a replicated lock acquired on it.
 				startRangeLocal, _ := keys.LockTableSingleKey(keys.MakeRangeKeyPrefix(in.Key), nil)
 				endRangeLocal, _ := keys.LockTableSingleKey(keys.MakeRangeKeyPrefix(in.EndKey), nil)
-				// Need adjusted start key to avoid overlapping with the local lock span right above.
+				// Need adjusted start key to avoid overlapping with the local lock span
+				// right above.
 				startGlobal, _ := keys.LockTableSingleKey(adjustedIn.Key.AsRawKey(), nil)
 				endGlobal, _ := keys.LockTableSingleKey(adjustedIn.EndKey.AsRawKey(), nil)
 				sl = append(sl, roachpb.Span{
@@ -101,8 +114,10 @@ func Select(rangeID roachpb.RangeID, opts SelectOpts) []roachpb.Span {
 				})
 			}
 		}
-		if opts.ReplicatedSpansFilter != ReplicatedSpansExcludeUser {
-			// Adjusted span because r1's "normal" keyspace starts only at LocalMax, not RKeyMin.
+		if opts.ReplicatedSpansFilter != ReplicatedSpansExcludeUser &&
+			opts.ReplicatedSpansFilter != ReplicatedSpansLocksOnly {
+			// Adjusted span because r1's "normal" keyspace starts only at LocalMax,
+			// not RKeyMin.
 			sl = append(sl, adjustedIn.AsRawSpanWithNoLocals())
 		}
 	}

pkg/kv/kvserver/rditer/select_test.go

@@ -76,6 +76,14 @@ func TestSelect(t *testing.T) {
 			},
 			filter: ReplicatedSpansExcludeLocks,
 		},
+		{
+			name: "r2_locksonly",
+			sp: roachpb.RSpan{
+				Key:    roachpb.RKey("a"),
+				EndKey: roachpb.RKey("c"),
+			},
+			filter: ReplicatedSpansLocksOnly,
+		},
 		{
 			name: "r3",
 			sp: roachpb.RSpan{

pkg/kv/kvserver/rditer/testdata/TestSelect/r2_locksonly

@@ -0,0 +1,18 @@
+echo
+----
+Select({ReplicatedBySpan:{a-c} ReplicatedSpansFilter:4 ReplicatedByRangeID:false UnreplicatedByRangeID:false}):
+  /Local/Lock/Local/Range"{a"-c"}
+  /Local/Lock"{a"-c"}
+Select({ReplicatedBySpan:{a-c} ReplicatedSpansFilter:4 ReplicatedByRangeID:false UnreplicatedByRangeID:true}):
+  /Local/RangeID/123/{u""-v""}
+  /Local/Lock/Local/Range"{a"-c"}
+  /Local/Lock"{a"-c"}
+Select({ReplicatedBySpan:{a-c} ReplicatedSpansFilter:4 ReplicatedByRangeID:true UnreplicatedByRangeID:false}):
+  /Local/RangeID/123/{r""-s""}
+  /Local/Lock/Local/Range"{a"-c"}
+  /Local/Lock"{a"-c"}
+Select({ReplicatedBySpan:{a-c} ReplicatedSpansFilter:4 ReplicatedByRangeID:true UnreplicatedByRangeID:true}):
+  /Local/RangeID/123/{r""-s""}
+  /Local/RangeID/123/{u""-v""}
+  /Local/Lock/Local/Range"{a"-c"}
+  /Local/Lock"{a"-c"}