colblk: add KeyCount, UnsafeBoundaryKeys to KeyspanBlockWriter

Add KeyCount and UnsafeBoundaryKeys methods to the KeyspanBlockWriter. The
KeyCount will be used by the sstable writer to determine whether a range
deletion or range key block needs to be serialized at all. The
UnsafeBoundaryKeys method will be used to determine the smallest and largest
rangedel/rangekey keys within a table.

sstable/colblk/keyspan.go

@@ -82,7 +82,7 @@ func (w *KeyspanBlockWriter) Reset() {
 
 // AddSpan appends a new Span to the pending block. Spans must already be
 // fragmented (non-overlapping) and added in sorted order.
-func (w *KeyspanBlockWriter) AddSpan(s *keyspan.Span) {
+func (w *KeyspanBlockWriter) AddSpan(s keyspan.Span) {
 	// When keyspans are fragmented, abutting spans share a user key. One span's
 	// end key is the next span's start key.  Check if the previous user key
 	// equals this span's start key, and avoid encoding it again if so.
@@ -109,6 +109,26 @@ func (w *KeyspanBlockWriter) AddSpan(s *keyspan.Span) {
 	}
 }
 
+// KeyCount returns the count of keyspan.Keys written to the writer.
+func (w *KeyspanBlockWriter) KeyCount() int {
+	return w.keyCount
+}
+
+// UnsafeBoundaryKeys returns the smallest and largest keys written to the
+// keyspan block so far. The returned internal keys have user keys that point
+// directly into the block writer's memory and must not be mutated.
+func (w *KeyspanBlockWriter) UnsafeBoundaryKeys() (smallest, largest base.InternalKey) {
+	if w.keyCount == 0 {
+		return smallest, largest
+	}
+	smallest.UserKey = w.boundaryUserKeys.UnsafeGet(0)
+	smallest.Trailer = base.InternalKeyTrailer(w.trailers.Get(0))
+	largest.UserKey = w.boundaryUserKeys.UnsafeGet(w.boundaryUserKeys.rows - 1)
+	largest.Trailer = base.MakeTrailer(base.SeqNumMax,
+		base.InternalKeyTrailer(w.trailers.Get(w.keyCount-1)).Kind())
+	return smallest, largest
+}
+
 // Size returns the size of the pending block.
 func (w *KeyspanBlockWriter) Size() int {
 	off := blockHeaderSize(keyspanColumnCount, keyspanHeaderSize)

sstable/colblk/keyspan_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/cockroachdb/datadriven"
 	"github.com/cockroachdb/pebble/internal/base"
 	"github.com/cockroachdb/pebble/internal/keyspan"
+	"github.com/cockroachdb/pebble/internal/testkeys"
 	"golang.org/x/exp/rand"
 )
 
@@ -35,12 +36,13 @@ func TestKeyspanBlock(t *testing.T) {
 			return buf.String()
 		case "add":
 			for _, line := range strings.Split(td.Input, "\n") {
-				s := keyspan.ParseSpan(line)
-				w.AddSpan(&s)
+				w.AddSpan(keyspan.ParseSpan(line))
 			}
 			fmt.Fprint(&buf, &w)
 			return buf.String()
 		case "finish":
+			sm, la := w.UnsafeBoundaryKeys()
+			fmt.Fprintf(&buf, "Boundaries: %s — %s\n", sm.Pretty(testkeys.Comparer.FormatKey), la.Pretty(testkeys.Comparer.FormatKey))
 			block := w.Finish()
 			kr.Init(block)
 			fmt.Fprint(&buf, kr.DebugString())
@@ -90,7 +92,7 @@ func benchmarkKeyspanBlockRangeDeletions(b *testing.B, numSpans, keysPerSpan, ke
 			t := base.MakeTrailer(base.SeqNum(j), base.InternalKeyKindRangeDelete)
 			s.Keys = append(s.Keys, keyspan.Key{Trailer: t})
 		}
-		w.AddSpan(&s)
+		w.AddSpan(s)
 	}
 	block := w.Finish()
 	avgRowSize := float64(w.Size()) / float64(numSpans*keysPerSpan)

sstable/colblk/testdata/keyspan_block

@@ -49,6 +49,7 @@ size=73:
 
 finish
 ----
+Boundaries: a#0,RANGEDEL — e#inf,RANGEDEL
 # keyspan block header
 00-04: x 05000000 # user key count: 5
 # columnar block header
@@ -226,6 +227,7 @@ size=70:
 
 finish
 ----
+Boundaries: b#4,RANGEKEYSET — d#inf,RANGEKEYSET
 # keyspan block header
 00-04: x 02000000         # user key count: 2
 # columnar block header
@@ -311,6 +313,7 @@ size=80:
 
 finish
 ----
+Boundaries: b#4,RANGEKEYSET — g#inf,RANGEKEYSET
 # keyspan block header
 00-04: x 04000000       # user key count: 4
 # columnar block header