fix(server/mcp): scope defer span.End inside loop iteration

[marlonbarreto-git]

Description

In readInputStream(), defer span.End() is called inside a for loop. Since defer schedules execution when the enclosing function returns (not the current loop iteration), all spans created per message accumulate without ever being ended for the lifetime of the stdio session.

For a long-lived stdio server, this means:

• Memory leak: span objects accumulate indefinitely
• Incorrect traces: spans are never properly closed, producing misleading telemetry data

Changes

Wrap the per-message processing logic in an immediately invoked function expression (IIFE), so defer span.End() correctly fires at the end of each iteration. This follows the same pattern already used in InitializeConfigs (server.go lines 85-98) for scoping spans inside loops.

Test

All existing tests pass. The TestStdioSession test exercises readLine and write on the stdio session. The readInputStream loop is covered by the existing MCP endpoint tests that exercise the full message processing pipeline.

Fixes #2549

[gemini-code-assist]

Summary of Changes

Hello @marlonbarreto-git, 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 issue in the MCP server's stdio session where OpenTelemetry spans were not being closed correctly. By introducing an immediately invoked function expression (IIFE) within the message processing loop, the defer span.End() call is now properly scoped to each iteration, preventing memory leaks from accumulating span objects and ensuring accurate tracing data for long-lived stdio sessions.

Highlights

• Fixes OpenTelemetry Span Leak: Corrected an issue where OpenTelemetry spans were not properly closed in the readInputStream function of the MCP server's stdio session.
• Prevents Memory Leaks and Incorrect Traces: The previous behavior led to indefinite accumulation of span objects and misleading telemetry data due to spans never being properly ended.
• Introduces IIFE for Scope Management: The message processing logic within the loop is now wrapped in an immediately invoked function expression (IIFE) to ensure defer span.End() executes at the end of each loop iteration.

Changelog

• internal/server/mcp.go
  • Wrapped message processing logic in an IIFE to correctly scope defer span.End().

Activity

• No activity has been recorded for this pull request yet.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

Footnotes

1. Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

The pull request effectively addresses the issue of defer span.End() not being scoped correctly within the readInputStream() loop. By wrapping the message processing logic in an immediately invoked function expression (IIFE), each OpenTelemetry span is now properly ended at the conclusion of its respective message processing, preventing potential memory leaks and ensuring accurate tracing data. This change aligns with established Go idioms for handling defer in loops and maintains consistency with existing patterns in the codebase.

[marlonbarreto-git]

@anubhav756 this PR is ready for review — single commit, CLA signed, conventional commit format.

Could you please trigger the CI by applying the tests: run label or commenting /gcbrun? As an external contributor I can't trigger the Cloud Build checks myself.

All tests pass locally:

$ go test ./internal/server/... -v
--- PASS: TestMcpEndpointWithoutInitialized (0.02s)
--- PASS: TestMcpEndpoint (0.03s)
--- PASS: TestStdioSession (0.00s)
PASS
ok  github.com/googleapis/genai-toolbox/internal/server  0.344s

[averikitsch]

/gcbrun

[duwenxin99]

/gcbrun

[averikitsch]

/gcbrun

[averikitsch]

/gcbrun