fix: make session termination idempotent to prevent retry failures

Author: ezynda3

server/streamable_http.go

@@ -1048,8 +1048,11 @@ func (s *InsecureStatefulSessionIdManager) Validate(sessionID string) (isTermina
 }
 
 func (s *InsecureStatefulSessionIdManager) Terminate(sessionID string) (isNotAllowed bool, err error) {
+if _, exists := s.terminated.Load(sessionID); exists {
+return false, nil
+}
 if _, exists := s.sessions.Load(sessionID); !exists {
-return false, fmt.Errorf("session not found: %s", sessionID)
+return false, nil
 }
 s.terminated.Store(sessionID, true)
 s.sessions.Delete(sessionID)

server/streamable_http_test.go

@@ -1254,16 +1254,37 @@ func TestInsecureStatefulSessionIdManager(t *testing.T) {
 }
 })
 
-t.Run("Terminate rejects non-existent session ID", func(t *testing.T) {
+t.Run("Terminate is idempotent for non-existent session ID", func(t *testing.T) {
 manager := &InsecureStatefulSessionIdManager{}
 fakeSessionID := "mcp-session-ffffffff-ffff-ffff-ffff-ffffffffffff"
 
-_, err := manager.Terminate(fakeSessionID)
-if err == nil {
-t.Error("Expected error when terminating non-existent session")
+isNotAllowed, err := manager.Terminate(fakeSessionID)
+if err != nil {
+t.Errorf("Expected no error when terminating non-existent session, got: %v", err)
 }
-if !strings.Contains(err.Error(), "session not found") {
-t.Errorf("Expected 'session not found' error, got: %v", err)
+if isNotAllowed {
+t.Error("Expected isNotAllowed to be false")
+}
+})
+
+t.Run("Terminate is idempotent for already-terminated session", func(t *testing.T) {
+manager := &InsecureStatefulSessionIdManager{}
+sessionID := manager.Generate()
+
+isNotAllowed, err := manager.Terminate(sessionID)
+if err != nil {
+t.Errorf("Expected no error on first termination, got: %v", err)
+}
+if isNotAllowed {
+t.Error("Expected termination to be allowed")
+}
+
+isNotAllowed, err = manager.Terminate(sessionID)
+if err != nil {
+t.Errorf("Expected no error on second termination (idempotent), got: %v", err)
+}
+if isNotAllowed {
+t.Error("Expected termination to be allowed on retry")
 }
 })