Address PR feedback

op-node/rollup/derive/channel.go

@@ -72,16 +72,12 @@ func (ch *Channel) AddFrame(frame Frame, l1InclusionBlock eth.L1BlockRef) error
 	// Prune frames with a number higher than the closing frame number when we receive a closing frame
 	if frame.IsLast && ch.endFrameNumber < ch.highestFrameNumber {
 		// Do a linear scan over saved inputs instead of ranging over ID numbers
-		var idsToPrune []uint64
 		for id, prunedFrame := range ch.inputs {
 			if id >= uint64(ch.endFrameNumber) {
-				idsToPrune = append(idsToPrune, id)
+				delete(ch.inputs, id)
 			}
 			ch.size -= frameSize(prunedFrame)
 		}
-		for _, id := range idsToPrune {
-			delete(ch.inputs, id)
-		}
 		ch.highestFrameNumber = ch.endFrameNumber
 	}
 	// Update highest seen frame number after pruning
@@ -141,7 +137,7 @@ func (ch *Channel) Reader() io.Reader {
 		if !ok {
 			panic("dev error in channel.Reader. Must be called after the channel is ready.")
 		}
-		readers = append(readers, bytes.NewBuffer(frame.Data))
+		readers = append(readers, bytes.NewReader(frame.Data))
 	}
 	return io.MultiReader(readers...)
 }

op-node/rollup/derive/channel_test.go

@@ -61,6 +61,14 @@ func TestFrameValidity(t *testing.T) {
 			shouldErr: []bool{false, true},
 			sizes:     []uint64{204, 204},
 		},
+		{
+			name: "duplicate closing frames",
+			frames: []Frame{
+				{ID: id, FrameNumber: 2, IsLast: true, Data: []byte("four")},
+				{ID: id, FrameNumber: 2, IsLast: true, Data: []byte("seven__")}},
+			shouldErr: []bool{false, true},
+			sizes:     []uint64{204, 204},
+		},
 		{
 			name: "frame past closing",
 			frames: []Frame{
@@ -77,6 +85,14 @@ func TestFrameValidity(t *testing.T) {
 			shouldErr: []bool{false, false},
 			sizes:     []uint64{207, 204},
 		},
+		{
+			name: "multiple valid frames",
+			frames: []Frame{
+				{ID: id, FrameNumber: 10, Data: []byte("seven__")},
+				{ID: id, FrameNumber: 2, Data: []byte("four")}},
+			shouldErr: []bool{false, false},
+			sizes:     []uint64{207, 411},
+		},
 	}
 
 	for _, tc := range testCases {