internal/mcp/protocol: featureSet[T]

What the spec calls "lists" of features (tools, prompts and so on)
are actually sets. Define a common data structure we can use to
implement all of them, and use it for server tools and prompts.

Change-Id: I257d0776b87a746ff23cd49ebcc7b317f4ea65e1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/671359
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/features.go

@@ -0,0 +1,73 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package mcp
+
+import (
+	"iter"
+
+	"github.com/modelcontextprotocol/go-sdk/internal/util"
+)
+
+// This file contains implementations that are common to all features.
+// A feature is an item provided to a peer. In the 2025-03-26 spec,
+// the features are prompt, tool, resource and root.
+
+// A featureSet is a collection of features of type T.
+// Every feature has a unique ID, and the spec never mentions
+// an ordering for the List calls, so what it calls a "list" is actually a set.
+type featureSet[T any] struct {
+	uniqueID func(T) string
+	features map[string]T
+}
+
+// newFeatureSet creates a new featureSet for features of type T.
+// The argument function should return the unique ID for a single feature.
+func newFeatureSet[T any](uniqueIDFunc func(T) string) *featureSet[T] {
+	return &featureSet[T]{
+		uniqueID: uniqueIDFunc,
+		features: make(map[string]T),
+	}
+}
+
+// add adds each feature to the set if it is not present,
+// or replaces an existing feature.
+func (s *featureSet[T]) add(fs ...T) {
+	for _, f := range fs {
+		s.features[s.uniqueID(f)] = f
+	}
+}
+
+// remove removes all features with the given uids from the set if present,
+// and returns whether any were removed.
+// It is not an error to remove a nonexistent feature.
+func (s *featureSet[T]) remove(uids ...string) bool {
+	changed := false
+	for _, uid := range uids {
+		if _, ok := s.features[uid]; ok {
+			changed = true
+			delete(s.features, uid)
+		}
+	}
+	return changed
+}
+
+// get returns the feature with the given uid.
+// If there is none, it returns zero, false.
+func (s *featureSet[T]) get(uid string) (T, bool) {
+	t, ok := s.features[uid]
+	return t, ok
+}
+
+// all returns an iterator over of all the features in the set
+// sorted by unique ID.
+func (s *featureSet[T]) all() iter.Seq[T] {
+	return func(yield func(T) bool) {
+		for _, f := range util.Sorted(s.features) {
+			if !yield(f) {
+				return
+			}
+		}
+	}
+}