span: redefine Frontier in terms of ReadOnlyFrontier

The `ReadOnlyFrontier` interface is a strict subset of `Frontier`
and to reduce duplicate code, this commit redefines `Frontier`
in terms of `ReadOnlyFrontier`.

Release note: None

Author: andyyang890

pkg/util/span/frontier.go

@@ -27,18 +27,14 @@ import (
 // Frontier is not safe for concurrent modification, but MakeConcurrentFrontier
 // can be used to make thread safe frontier.
 type Frontier interface {
+	ReadOnlyFrontier
+
 	// AddSpansAt adds the provided spans to the frontier at the provided timestamp.
 	// If the span overlaps any spans already tracked by the frontier, the tree is adjusted
 	// to hold union of the span and the overlaps, with all entries assigned startAt starting
 	// timestamp.
 	AddSpansAt(startAt hlc.Timestamp, spans ...roachpb.Span) error
 
-	// Frontier returns the minimum timestamp being tracked.
-	Frontier() hlc.Timestamp
-
-	// PeekFrontierSpan returns one of the spans at the Frontier.
-	PeekFrontierSpan() roachpb.Span
-
 	// Forward advances the timestamp for a span. Any part of the span that doesn't
 	// overlap the tracked span set will be ignored. True is returned if the
 	// frontier advanced as a result.
@@ -49,6 +45,16 @@ type Frontier interface {
 	// letting a frontier be GCed is safe in that it won't cause a memory leak,
 	// but it will prevent frontier nodes from being efficiently re-used.
 	Release()
+}
+
+// ReadOnlyFrontier is a subset of Frontier with only the methods
+// that are read-only.
+type ReadOnlyFrontier interface {
+	// Frontier returns the minimum timestamp being tracked.
+	Frontier() hlc.Timestamp
+
+	// PeekFrontierSpan returns one of the spans at the Frontier.
+	PeekFrontierSpan() roachpb.Span
 
 	// Entries returns an iterator over the entries in the frontier.
 	// Updates to the frontier are restricted until iteration is stopped.

pkg/util/span/multi_frontier.go

@@ -241,19 +241,6 @@ func (f *MultiFrontier[P]) String() string {
 	return buf.String()
 }
 
-// ReadOnlyFrontier is a subset of Frontier with only the methods
-// that are read-only.
-type ReadOnlyFrontier interface {
-	Frontier() hlc.Timestamp
-	PeekFrontierSpan() roachpb.Span
-	Entries() iter.Seq2[roachpb.Span, hlc.Timestamp]
-	SpanEntries(span roachpb.Span) iter.Seq2[roachpb.Span, hlc.Timestamp]
-	Len() int
-	String() string
-}
-
-var _ ReadOnlyFrontier = Frontier(nil)
-
 // Frontiers returns an iterator over the sub-frontiers (with read-only access).
 func (f *MultiFrontier[P]) Frontiers() iter.Seq2[P, ReadOnlyFrontier] {
 	return func(yield func(P, ReadOnlyFrontier) bool) {