fix: truncate existing JSON export files

cmd/dive/cli/internal/command/adapter/exporter.go

@@ -3,13 +3,14 @@ package adapter
 import (
 	"context"
 	"fmt"
+	"os"
+
 	"github.com/spf13/afero"
 	"github.com/wagoodman/dive/cmd/dive/cli/internal/command/export"
 	"github.com/wagoodman/dive/dive/image"
 	"github.com/wagoodman/dive/internal/bus"
 	"github.com/wagoodman/dive/internal/bus/event/payload"
 	"github.com/wagoodman/dive/internal/log"
-	"os"
 )
 
 type Exporter interface {
@@ -49,7 +50,7 @@ func (e *jsonExporter) ExportTo(ctx context.Context, analysis *image.Analysis, p
 		mon.SetCompleted()
 	}
 
-	file, err := e.filesystem.OpenFile(path, os.O_RDWR|os.O_CREATE, 0644)
+	file, err := e.filesystem.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
 	if err != nil {
 		return fmt.Errorf("cannot open export file: %w", err)
 	}

cmd/dive/cli/internal/command/adapter/exporter_test.go

@@ -0,0 +1,33 @@
+package adapter
+
+import (
+	"context"
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"github.com/spf13/afero"
+	"github.com/wagoodman/dive/dive/image"
+)
+
+func TestJSONExporterTruncatesExistingFile(t *testing.T) {
+	fs := afero.NewMemMapFs()
+	const path = "dive.json"
+
+	if err := afero.WriteFile(fs, path, []byte(strings.Repeat("x", 4096)), 0o644); err != nil {
+		t.Fatalf("failed to seed export file: %v", err)
+	}
+
+	analysis := &image.Analysis{Image: "example:latest"}
+	if err := NewExporter(fs).ExportTo(context.Background(), analysis, path); err != nil {
+		t.Fatalf("failed to export analysis: %v", err)
+	}
+
+	contents, err := afero.ReadFile(fs, path)
+	if err != nil {
+		t.Fatalf("failed to read export file: %v", err)
+	}
+	if !json.Valid(contents) {
+		t.Fatalf("export file contains trailing data: %q", contents)
+	}
+}