fix(mcp): prevent panic when SSE session is missing

[vishnukannaujia]

Summary

• fix sseManager.get to only update lastActive when the session exists
• return (nil, false) cleanly for unknown sessionId lookups
• add a regression test for unknown session retrieval

Why

Issue #2548 reports a nil pointer dereference in sseManager.get() when sessionId is unknown. The map lookup can return nil, and unguarded field assignment panics.

Fix

Guard the lastActive update behind ok, preserving existing behavior for valid sessions and avoiding panic for invalid IDs.

Testing

• Added TestSseManagerGetUnknownSession in internal/server/mcp_test.go
• Local go test could not run in this environment because go is not installed (zsh: command not found: go)

Fixes #2548

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the SSE session manager that could lead to a nil pointer dereference when attempting to retrieve a non-existent session. The changes enhance the robustness of session handling by safely managing unknown session lookups and include a dedicated test to ensure the stability of this behavior.

Highlights

• Nil Pointer Dereference Fix: Implemented a guard in sseManager.get to prevent a nil pointer dereference when attempting to update lastActive for a non-existent SSE session.
• Robust Session Retrieval: Ensured that sseManager.get now correctly returns (nil, false) for unknown session IDs, maintaining existing behavior for valid sessions.
• Regression Test Added: Introduced a new test case, TestSseManagerGetUnknownSession, to validate the fix and prevent future regressions for unknown session lookups.

Changelog

• internal/server/mcp.go
  • Guarded the session.lastActive update with an if ok check to prevent panics when a session is not found.
• internal/server/mcp_test.go
  • Added TestSseManagerGetUnknownSession to verify that sseManager.get correctly handles requests for non-existent sessions by returning (nil, false).

Activity

• The author added a regression test for unknown session retrieval.
• The author noted that local go test could not be run due to go not being installed in their environment.

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[gemini-code-assist]

Code Review

The pull request correctly fixes a potential panic in sseManager.get by adding a check before accessing a potentially nil session. The accompanying regression test is also well-written and covers the fixed scenario. The included suggestion aims to slightly improve the test's failure diagnostics.

[gemini-code-assist]

For better test failure diagnostics, you could combine the checks for the two return values. This provides a more comprehensive error message if the test fails, showing both returned values at once.

Suggested change

	if ok {
	t.Fatal("expected unknown session lookup to return ok=false")
	}
	if session != nil {
	t.Fatal("expected unknown session lookup to return nil session")
	}
	if ok || session != nil {
	t.Fatalf("manager.get() with unknown id returned (%v, %v), want (nil, false)", session, ok)
	}

[vishnukannaujia]

Addressed review feedback from Gemini:

• updated TestSseManagerGetUnknownSession assertion to report both return values in a single diagnostic

Change pushed to the same branch: fix-2548-ssemanager-get-nil-panic (commit c7059bc229).

I will proceed with any additional maintainer feedback once CLA is complete.

[averikitsch]

Thanks for the contribution. We have merged this fix in https://github.com/googleapis/genai-toolbox/pull/2557/changes. Thanks again!