feat: Add merged skill source proxy. (#747)

This proxy allows combining multiple skill sources into a single source.

Author: foxfrikses

tool/skilltoolset/skill/merged_source.go

@@ -0,0 +1,107 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package skill
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"slices"
+)
+
+type mergedSource struct {
+	sources []Source
+}
+
+// NewMergedSource creates a Source that combines multiple underlying
+// Source implementations. The sources are queried in the order they are
+// provided.
+func NewMergedSource(sources ...Source) Source {
+	return &mergedSource{sources: slices.Clone(sources)}
+}
+
+func (m *mergedSource) ListFrontmatters(ctx context.Context) ([]*Frontmatter, error) {
+	var allFrontmatters []*Frontmatter
+	names := make(map[string]bool) // Track skill names to detect duplicates.
+
+	for _, source := range m.sources {
+		frontmatters, err := source.ListFrontmatters(ctx)
+		if err != nil {
+			return nil, err
+		}
+		for _, fm := range frontmatters {
+			if names[fm.Name] {
+				return nil, fmt.Errorf("%w: %q", ErrDuplicateSkill, fm.Name)
+			}
+			names[fm.Name] = true
+		}
+		allFrontmatters = append(allFrontmatters, frontmatters...)
+	}
+
+	return allFrontmatters, nil
+}
+
+func (m *mergedSource) ListResources(ctx context.Context, name, subpath string) ([]string, error) {
+	for _, source := range m.sources {
+		res, err := source.ListResources(ctx, name, subpath)
+		if err == nil {
+			return res, nil
+		}
+		if !errors.Is(err, ErrSkillNotFound) {
+			return nil, err
+		}
+	}
+	return nil, ErrSkillNotFound
+}
+
+func (m *mergedSource) LoadFrontmatter(ctx context.Context, name string) (*Frontmatter, error) {
+	for _, source := range m.sources {
+		frontmatter, err := source.LoadFrontmatter(ctx, name)
+		if err == nil {
+			return frontmatter, nil
+		}
+		if !errors.Is(err, ErrSkillNotFound) {
+			return nil, err
+		}
+	}
+	return nil, ErrSkillNotFound
+}
+
+func (m *mergedSource) LoadInstructions(ctx context.Context, name string) (string, error) {
+	for _, source := range m.sources {
+		instructions, err := source.LoadInstructions(ctx, name)
+		if err == nil {
+			return instructions, nil
+		}
+		if !errors.Is(err, ErrSkillNotFound) {
+			return "", err
+		}
+	}
+	return "", ErrSkillNotFound
+}
+
+func (m *mergedSource) LoadResource(ctx context.Context, name, resourcePath string) (io.ReadCloser, error) {
+	for _, source := range m.sources {
+		resource, err := source.LoadResource(ctx, name, resourcePath)
+		if err == nil {
+			return resource, nil
+		}
+		if !errors.Is(err, ErrSkillNotFound) {
+			return nil, err
+		}
+	}
+	return nil, ErrSkillNotFound
+}