Warn and short-circuit instead of erroring

pkg/composableschemadsl/compiler/importer.go

@@ -16,7 +16,7 @@ type importContext struct {
 	pathSegments []string
 	sourceFolder string
 	names        *mapz.Set[string]
-	visitedFiles        *mapz.Set[string]
+	visitedFiles *mapz.Set[string]
 }
 
 const SchemaFileSuffix = ".zed"
@@ -25,35 +25,44 @@ func importFile(importContext importContext) (*CompiledSchema, error) {
 	relativeFilepath := constructFilePath(importContext.pathSegments)
 	filePath := path.Join(importContext.sourceFolder, relativeFilepath)
 
-	currentVisitedFiles := importContext.visitedFiles.Copy()
-	if ok := currentVisitedFiles.Add(filePath); !ok {
-		return nil, fmt.Errorf("circular import: file %s has already been visited", filePath)
-	}
-
 	newSourceFolder := filepath.Dir(filePath)
 
-	var schemaBytes []byte
+	if ok := importContext.visitedFiles.Add(filePath); !ok {
+		// If the file has already been visited, we short-circuit the import process
+		// by not reading the schema file in and compiling a schema with an empty string.
+		// This prevents duplicate definitions from ending up in the output, as well
+		// as preventing circular imports.
+		log.Warn().Str("filepath", filePath).Msg("possible circular import: file %s has already been visited")
+		return compileImpl(InputSchema{
+			Source:       input.Source(filePath),
+			SchemaString: "",
+		},
+			compilationContext{
+				existingNames: importContext.names,
+				visitedFiles:  importContext.visitedFiles,
+			},
+			AllowUnprefixedObjectType(),
+			SourceFolder(newSourceFolder),
+		)
+	}
+
 	schemaBytes, err := os.ReadFile(filePath)
 	if err != nil {
 		return nil, fmt.Errorf("failed to read schema file: %w", err)
 	}
 	log.Trace().Str("schema", string(schemaBytes)).Str("file", filePath).Msg("read schema from file")
 
-	compiled, err := compileImpl(InputSchema{
+	return compileImpl(InputSchema{
 		Source:       input.Source(filePath),
 		SchemaString: string(schemaBytes),
 	},
 		compilationContext{
 			existingNames: importContext.names,
-			visitedFiles: currentVisitedFiles,
+			visitedFiles:  importContext.visitedFiles,
 		},
 		AllowUnprefixedObjectType(),
 		SourceFolder(newSourceFolder),
 	)
-	if err != nil {
-		return nil, err
-	}
-	return compiled, nil
 }
 
 func constructFilePath(segments []string) string {