fix(srv/stream): properly remove sessionTool to prevent memory leak

[cryo-zd]

Description

Fix two potential issues in StreamableHTTPServer's func sessionToolsStore that may lead to performance degradation or memory leak over time:

1. Memory leak risk:
   Previously, when a client session was deleted (via handleDelete), we simply set its entry in sessionToolsStore to nil, but did not remove the key from the internal map. Over time, this could cause the internal map to grow unbounded, especially on long-running servers with many short-lived sessions.
2. Redundant allocation for unused sessions:
   Even for sessions that never registered/customized any tools, we would still insert a sessionID → nil entry during deletion, which is unnecessary and wasteful.

Fix

• Added a delete(sessionID string) method to sessionToolsStore that safely removes the key from the internal map.
• optimize this method using double-checked locking to improve performance in common scenarios where most sessions don't register/customize session tools

Proof

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• MCP spec compatibility implementation
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring (no functional changes)
• Performance improvement
• Tests only (no functional changes)
• Other (please describe):

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly

MCP Spec Compliance

• This PR implements a feature defined in the MCP specification
• Link to relevant spec section: Link text
• Implementation follows the specification exactly

Summary by CodeRabbit

• Bug Fixes
  • Improved session deletion to ensure all session data is fully removed when a session is deleted.

[coderabbitai]

Walkthrough

The session deletion logic in the StreamableHTTPServer was updated to fully remove session entries from the sessionToolsStore using a new delete(sessionID) method, replacing the previous approach of setting the session's tools map to nil. The new method ensures complete removal with proper locking.

Changes

File(s)	Change Summary
server/streamable_http.go	Replaced session tools map nullification with a new delete(sessionID) method for full removal; added delete method with locking.

Warning

There were issues while running some tools. Please review the errors and either fix the tool's configuration or disable the tool if it's a critical failure.

🔧 golangci-lint (1.64.8)

Error: you are using a configuration file for golangci-lint v2 with golangci-lint v1: please use golangci-lint v2
Failed executing command with error: you are using a configuration file for golangci-lint v2 with golangci-lint v1: please use golangci-lint v2

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📥 Commits

Reviewing files that changed from the base of the PR and between 97f5bea and 7c15187.

📒 Files selected for processing (1)

• server/streamable_http.go (2 hunks)

🚧 Files skipped from review as they are similar to previous changes (1)

• server/streamable_http.go

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[dugenkui03]

we could keep code below only and remove code above, delete is an idempotent operation.

[cryo-zd]

  Sorry for my delayed response — and you’re absolutely right that delete is idempotent and safe even when the key does not exist.
  That said, the reason we added the double-checked locking is because we noticed that most streamableHttpSessions don’t call SetSessionTools() to customize session tools or override global tools, meaning that in many cases the sessionID won’t be present in sessionToolsStore.tools(map) at all. With this in mind, the early RLock check helps us avoid acquiring an exclusive lock unnecessarily, which could reduce write lock contention and context switching under high server load — especially in a read-heavy environment..
  Do you think this trade-off makes sense? If you feel this is an over-optimization, I’d be happy to simplify the code as you suggested.

[dugenkui03]

I think keeping the code below is good enough, it's very readable. and the pre-check only make sense if the same sessionID is deleted multiple times, but that’s a rare case (or should not happen)

[cryo-zd]

Removed.