mcp: add optional issuer validator for pre-registered client validation (SEP-2352) (#946)

Fixes #978

internal/authutil/util.go

@@ -0,0 +1,13 @@
+// Copyright 2026 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by the license
+// that can be found in the LICENSE file.
+
+package authutil
+
+import "strings"
+
+// IssuersEqual reports whether two OAuth 2.0 authorization server issuer
+// identifiers refer to the same server comparing them without the final trailing slash.
+func IssuersEqual(a, b string) bool {
+	return strings.TrimSuffix(a, "/") == strings.TrimSuffix(b, "/")
+}

internal/authutil/util_test.go

@@ -0,0 +1,29 @@
+// Copyright 2026 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by the license
+// that can be found in the LICENSE file.
+
+package authutil
+
+import "testing"
+
+func TestIssuersEqual(t *testing.T) {
+	tests := []struct {
+		a, b string
+		want bool
+	}{
+		{"https://issuer.example.com", "https://issuer.example.com", true},
+		{"https://issuer.example.com/", "https://issuer.example.com", true},
+		{"https://issuer.example.com", "https://issuer.example.com/", true},
+		{"https://issuer.example.com/", "https://issuer.example.com/", true},
+		{"https://issuer.example.com/tenant", "https://issuer.example.com/tenant", true},
+		{"https://issuer.example.com/tenant/", "https://issuer.example.com/tenant", true},
+		{"https://issuer.example.com", "https://other.example.com", false},
+		{"https://issuer.example.com/a", "https://issuer.example.com/b", false},
+		{"", "", true},
+	}
+	for _, tt := range tests {
+		if got := IssuersEqual(tt.a, tt.b); got != tt.want {
+			t.Errorf("IssuersEqual(%q, %q) = %v, want %v", tt.a, tt.b, got, tt.want)
+		}
+	}
+}