Add semantic configuration profile comparison

internal/config/pipeline_composition.go

@@ -46,7 +46,20 @@ type pipelineProfileDeclaration struct {
 	overlay string
 }
 
-func loadComposedPipeline(path string, opts PipelineLoadOptions) (*PipelineConfig, error) {
+// pipelineCompositionSources retains one validated root source set. Each
+// selected profile is resolved from a cloned base document so callers can
+// safely compare or otherwise resolve multiple profiles without rereading or
+// mutating the source set.
+type pipelineCompositionSources struct {
+	rootPath       string
+	envelope       pipelineCompositionEnvelope
+	imports        []loadedPipelineImport
+	overlays       []loadedPipelineProfileOverlay
+	overlaysLoaded bool
+	base           *compositionDocument
+}
+
+func loadPipelineCompositionSources(path string) (*pipelineCompositionSources, error) {
 	rootPath, err := filepath.Abs(path)
 	if err != nil {
 		return nil, fmt.Errorf("resolve root pipeline path %q: %w", path, err)
@@ -72,14 +85,6 @@ func loadComposedPipeline(path string, opts PipelineLoadOptions) (*PipelineConfi
 	if err != nil {
 		return nil, err
 	}
-	selection, err := selectPipelineProfile(envelope, opts)
-	if err != nil {
-		return nil, err
-	}
-	overlays, err := loadPipelineProfileOverlays(rootPath, envelope.profiles, imports)
-	if err != nil {
-		return nil, err
-	}
 	documents := make([]*compositionDocument, 0, len(imports)+1)
 	documents = append(documents, baseRoot)
 	for _, imported := range imports {
@@ -89,8 +94,47 @@ func loadComposedPipeline(path string, opts PipelineLoadOptions) (*PipelineConfi
 	if err != nil {
 		return nil, err
 	}
+	return &pipelineCompositionSources{
+		rootPath: rootPath,
+		envelope: envelope,
+		imports:  imports,
+		base:     merged,
+	}, nil
+}
+
+func (sources *pipelineCompositionSources) loadOverlays() error {
+	if sources == nil {
+		return fmt.Errorf("pipeline composition sources are required")
+	}
+	if sources.overlaysLoaded {
+		return nil
+	}
+	overlays, err := loadPipelineProfileOverlays(sources.rootPath, sources.envelope.profiles, sources.imports)
+	if err != nil {
+		return err
+	}
+	sources.overlays = overlays
+	sources.overlaysLoaded = true
+	return nil
+}
+
+func (sources *pipelineCompositionSources) resolve(opts PipelineLoadOptions) (*PipelineConfig, error) {
+	if sources == nil || sources.base == nil {
+		return nil, fmt.Errorf("pipeline composition sources are required")
+	}
+	if !sources.overlaysLoaded {
+		return nil, fmt.Errorf("pipeline profile overlays have not been loaded")
+	}
+	selection, err := selectPipelineProfile(sources.envelope, opts)
+	if err != nil {
+		return nil, err
+	}
+	merged := &compositionDocument{
+		root:    cloneCompositionNode(sources.base.root),
+		sources: append([]string(nil), sources.base.sources...),
+	}
 	if selection != nil {
-		overlay, ok := loadedProfileOverlay(overlays, selection.name)
+		overlay, ok := loadedProfileOverlay(sources.overlays, selection.name)
 		if !ok {
 			return nil, fmt.Errorf("selected profile %q overlay was not loaded", selection.name)
 		}
@@ -106,7 +150,7 @@ func loadComposedPipeline(path string, opts PipelineLoadOptions) (*PipelineConfi
 		return nil, err
 	}
 	var cfg PipelineConfig
-	if err := decodeStrictYAMLFromReader("pipeline", rootPath, strings.NewReader(string(rendered)), &cfg); err != nil {
+	if err := decodeStrictYAMLFromReader("pipeline", sources.rootPath, strings.NewReader(string(rendered)), &cfg); err != nil {
 		return nil, fmt.Errorf("assembled pipeline sources %s: %w", formatCompositionSources(merged.sources), err)
 	}
 	records, err := merged.semanticRecords()
@@ -114,11 +158,11 @@ func loadComposedPipeline(path string, opts PipelineLoadOptions) (*PipelineConfi
 		return nil, err
 	}
 	metadata := &pipelineResolutionMetadata{
-		rootPath:        rootPath,
+		rootPath:        sources.rootPath,
 		sources:         append([]string(nil), merged.sources...),
 		selectedProfile: selection,
 	}
-	for _, imported := range imports {
+	for _, imported := range sources.imports {
 		metadata.imports = append(metadata.imports, imported.path)
 	}
 	for _, record := range records {