Fix: Memory leak from unclosed sessions in Server

[devcrocod]

Motivation and Context

Sessions were added to the sessions list on connect but never removed when closed, causing a memory leak in long-running server processes. The closed sessions and their closures would accumulate indefinitely, preventing garbage collection.

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[Copilot]

Pull Request Overview

This PR adds automatic cleanup of closed sessions from the server's active sessions list. The change ensures that when a session closes, it is automatically removed from the internal sessions list maintained by the Server class.

• Registers an onClose callback on each session to handle cleanup when the session terminates

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[Copilot]

Pull Request Overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/Server.kt:180

• The onClose handler is registered before the session is added to the sessions list, creating a race condition. If the session closes during or immediately after session.connect(transport) (line 178), the cleanup handler will attempt to remove a session that hasn't been added yet, and the session will be added to the list (line 180) after it's already closed. This can cause closed sessions to remain in the sessions list indefinitely. Move the session addition (line 180) before registering the onClose handler (lines 173-176) to ensure the session is in the list before the cleanup handler can execute.

        // Register cleanup handler to remove session from list when it closes
        session.onClose {
            logger.debug { "Removing closed session from active sessions list" }
            sessions.update { list -> list - session }
        }
        logger.debug { "Server session connecting to transport" }
        session.connect(transport)
        logger.debug { "Server session successfully connected to transport" }
        sessions.update { it.add(session) }

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[kpavlov]

LGTM