Add Stderr() Method to StdioMCPClient

[mashiike]

This PR adds a Stderr() method to StdioMCPClient, allowing the parent process to read from the standard error output (stderr) of the spawned subprocess.

In many cases, the subprocess writes logs or error messages to stderr. By exposing stderr via the Stderr() method, the parent process can now handle this output — for example, for log collection, monitoring, or display purposes.

The Stderr() method returns an io.Reader that allows access to the subprocess’s stderr stream.

Summary by CodeRabbit

• New Features
  • Enhanced logging: The application now captures structured diagnostic messages and error logs from background processes, providing clear, real-time feedback during startup and runtime. Users will see more transparent status updates—including informative launch messages—that improve monitoring and troubleshooting.

[coderabbitai]

Walkthrough

The pull request introduces comprehensive support for handling standard error output. In the production code (client/stdio.go), a new stderr field and associated methods are added to capture and manage subprocess error streams, with corresponding updates to initialization and cleanup operations. The test code (client/stdio_test.go) now captures JSON log records from the standard error via a goroutine, using synchronization mechanisms to ensure safe concurrent access, and includes a new test case validating a launch message. Additionally, structured JSON logging is added in the mock server (testdata/mockstdio_server.go) to emit an informational log.

Changes

Files	Change Summary
client/stdio.go	Added a stderr field (io.ReadCloser) to StdioMCPClient; updated NewStdioMCPClient to create the stderr pipe with error handling; modified the Close method to also close the stderr pipe; added a new Stderr() method returning an io.Reader for accessing standard error output.
client/stdio_test.go, testdata/mockstdio_server.go	In tests, implemented a log capturing mechanism using a slice, mutex, and wait group to safely collect JSON log records from the client's stderr, with a new "CheckLogs" test case validating a specific launch message; in the mock server, introduced structured logging using log/slog with a JSON handler that logs an informational launch message to stderr.

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📜 Recent review details

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Review profile: CHILL
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📥 Commits

Reviewing files that changed from the base of the PR and between d3dc35c and c314786.

📒 Files selected for processing (3)

• client/stdio.go (3 hunks)
• client/stdio_test.go (3 hunks)
• testdata/mockstdio_server.go (2 hunks)

🔇 Additional comments (9)

testdata/mockstdio_server.go (1)

29-30: LGTM! Clean implementation of structured logging.

The implementation correctly sets up a JSON logger that writes to stderr and logs a well-formatted launch message. This works well with the new Stderr() method introduced in the StdioMCPClient.

client/stdio.go (5)

26-26: LGTM! Good addition of stderr field.

Adding the stderr field to the StdioMCPClient struct is a clean approach to capture the subprocess's standard error output.

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62-65: LGTM! Proper error handling for stderr pipe.

The implementation correctly creates a pipe for stderr and includes appropriate error handling, following the same pattern used for stdin and stdout.

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70-70: LGTM! Proper initialization.

The stderr field is correctly initialized in the client struct.

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98-100: LGTM! Proper resource cleanup.

The implementation correctly closes the stderr pipe during cleanup, with proper error handling.

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104-108: LGTM! Well-documented public method.

The Stderr() method is well-documented and follows the interface design pattern, returning an io.Reader which is the appropriate interface for reading operations.

client/stdio_test.go (3)

44-60: LGTM! Well-implemented concurrent log capture.

The implementation properly sets up a goroutine to continuously read JSON log records from stderr, with appropriate synchronization using a mutex to protect access to the shared logRecords slice. The use of a wait group ensures proper cleanup.

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261-263: LGTM! Clean resource management.

Properly closes the client and waits for the log reading goroutine to finish, ensuring all resources are cleaned up correctly and all logs are processed before verification.

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264-280: LGTM! Good test coverage for the new feature.

The test case correctly verifies that:

1. Exactly one log record is captured
2. The log record has a message key
3. The message content matches the expected value

This provides good validation of the end-to-end functionality of the new Stderr() method.

✨ Finishing Touches

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