Filter out blank responses (#270)

* I'm observing cases where we have a non empty suggested cell which
ends up getting deleted.
* It looks like this happens for one of two reasons
* The model responses with an empty code block which doesn't get
filtered out
  * We return a response with no blocks in it

* We should filter these out.
* This means we bias towards generating irrelvant suggestions rather
than no suggestions.
* Right now I think this is helpful because otherwise people might get
confused and wonder whether Foyle is working.

Here's the supporting notebook
https://gist.github.com/jlewi/31857545ec62b36d2949ccd904918d53

app/pkg/agent/agent.go

@@ -244,6 +244,11 @@ func (a *Agent) StreamGenerate(ctx context.Context, stream *connect.BidiStream[v
 					return
 				}
 
+				if dropResponse(response) {
+					log.V(logs.Debug).Info("Dropping response", zap.Object("response", response))
+					continue
+				}
+
 				log.V(logs.Debug).Info("Sending response", zap.Object("response", response))
 				if err := stream.Send(response); err != nil {
 					log.Error(err, "Failed to send response")
@@ -564,6 +569,11 @@ func postProcessBlocks(blocks []*v1alpha1.Block) ([]*v1alpha1.Block, error) {
 		if isOutputTag(block.Contents) {
 			continue
 		}
+
+		// If the block is empty filter it out.
+		if strings.TrimSpace(block.Contents) == "" {
+			continue
+		}
 		results = append(results, block)
 		return results, nil
 	}
@@ -607,3 +617,17 @@ func shouldTrigger(doc *v1alpha1.Doc, selectedIndex int32) bool {
 	selectedCell := doc.Blocks[selectedIndex]
 	return selectedCell.GetKind() == v1alpha1.BlockKind_MARKUP
 }
+
+// dropResponse returns true if the response should be dropped rather than being sent to the client.
+// The reason for doing this is because if a previous generation generated a "good" response we don't want
+// to overwrite it with this one
+func dropResponse(response *v1alpha1.StreamGenerateResponse) bool {
+	if response == nil {
+		return true
+	}
+	// We don't want to send empty cells because that will cause the client to remove any previously suggested cells
+	if len(response.Cells) == 0 {
+		return true
+	}
+	return false
+}

app/pkg/agent/agent_test.go

@@ -361,6 +361,16 @@ func Test_PostProcessBlocks(t *testing.T) {
 			},
 			expected: []*v1alpha1.Block{},
 		},
+		{
+			name: "whitespace-only-block",
+			blocks: []*v1alpha1.Block{
+				{
+					Kind:     v1alpha1.BlockKind_CODE,
+					Contents: "   ",
+				},
+			},
+			expected: []*v1alpha1.Block{},
+		},
 	}
 
 	for _, c := range cases {
@@ -375,3 +385,42 @@ func Test_PostProcessBlocks(t *testing.T) {
 		})
 	}
 }
+
+func Test_dropResponse(t *testing.T) {
+	type testCase struct {
+		name     string
+		response *v1alpha1.StreamGenerateResponse
+		expected bool
+	}
+
+	cases := []testCase{
+		{
+			name: "basic",
+			response: &v1alpha1.StreamGenerateResponse{
+				Cells: []*parserv1.Cell{
+					{
+						Kind:  parserv1.CellKind_CELL_KIND_CODE,
+						Value: "print('Hello, World!')",
+					},
+				},
+			},
+			expected: false,
+		},
+		{
+			name: "empty",
+			response: &v1alpha1.StreamGenerateResponse{
+				Cells: []*parserv1.Cell{},
+			},
+			expected: true,
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			result := dropResponse(c.response)
+			if result != c.expected {
+				t.Errorf("Expected %v; got %v", c.expected, result)
+			}
+		})
+	}
+}