Revert "fix(mcp): ensure MCP transport is closed to prevent memory leaks"

[skeshive]

Reverts #18054

This is causing #18738 since not all errors sent through onerror should be treated as fatal connection-closing issues

[gemini-cli]

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[gemini-code-assist]

Summary of Changes

Hello @skeshive, 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 reverts a prior change that introduced logic to close the MCP transport whenever an error occurred. The original intent was to prevent memory leaks, but it inadvertently caused a regression by treating all errors as critical, leading to premature connection closures. This revert ensures that only truly fatal errors lead to connection termination, resolving the reported issue.

Highlights

• Reverted MCP Transport Closure: The pull request reverts a previous change that automatically closed the MCP transport upon any error, which was causing unintended connection terminations for non-fatal issues.
• Addressed Regression: This change resolves a regression where the MCP client was prematurely disconnecting due to errors that should not have been treated as connection-closing events.
• Simplified MCP Client Connection Logic: The transport object is no longer returned or managed directly by connectToMcpServer, connectWithSSETransport, and retryWithOAuth functions, simplifying the connection handling.
• Updated Error Handling: The onerror handlers in McpClient and connectAndDiscover were modified to be synchronous and no longer attempt to close the transport, aligning with the revert's intent.

Changelog

• packages/core/src/tools/mcp-client.test.ts
  • Updated StdioClientTransport mock to return an empty object instead of one with a close function.
  • Modified expect statements to directly assert on the client object instead of client.client.
• packages/core/src/tools/mcp-client.ts
  • Removed the transport property from the McpClient class.
  • Removed logic to close the transport in the onerror handlers of McpClient and connectAndDiscover.
  • Changed the return type of connectToMcpServer from { client: Client; transport: Transport } to Client.
  • Updated connectWithSSETransport and retryWithOAuth functions to return void instead of Transport.
  • Adjusted calls to connectToMcpServer to directly assign the returned client.
  • Modified onerror handler signatures to be synchronous ((error) => {}) instead of asynchronous (async (error) => {}).

Activity

• No human activity (comments, reviews, etc.) has been recorded for this pull request yet.

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[github-actions]

Size Change: -810 B (0%)

Total Size: 23.9 MB

ℹ️ View Unchanged

Filename	Size	Change
./bundle/gemini.js	23.9 MB	-810 B (0%)
./bundle/sandbox-macos-permissive-closed.sb	1.03 kB	0 B
./bundle/sandbox-macos-permissive-open.sb	890 B	0 B
./bundle/sandbox-macos-permissive-proxied.sb	1.31 kB	0 B
./bundle/sandbox-macos-restrictive-closed.sb	3.29 kB	0 B
./bundle/sandbox-macos-restrictive-open.sb	3.36 kB	0 B
./bundle/sandbox-macos-restrictive-proxied.sb	3.56 kB	0 B

_{compressed-size-action}

[gemini-code-assist]

Code Review

This pull request reverts a previous change that aimed to fix a memory leak by closing the MCP transport on error, as the original fix was too aggressive. However, this revert re-introduces a high-severity resource leak vulnerability by removing all transport-closing logic from the onerror handlers while still marking connections as DISCONNECTED. This results in orphaned transports and child processes running indefinitely, leading to leaked resources in McpClient and connectAndDiscover when errors occur and the client is not properly cleaned up on reconnect. The feedback highlights the need to ensure resources are explicitly released when a connection transitions to a disconnected state.

[gemini-code-assist]

Similar to the issue in McpClient.connect, this onerror handler re-introduces a high-severity resource leak vulnerability. It updates the server status to DISCONNECTED but fails to close the mcpClient or its transport. Since mcpClient is local to connectAndDiscover, this leads to orphaned transports and potential process leaks if an error occurs. To remediate, mcpClient.close() should be called within the onerror handler to ensure resources are properly released when the connection is marked as disconnected.

     mcpClient.onerror = (error) => {
      coreEvents.emitFeedback('error', `MCP ERROR (${mcpServerName}):`, error);
      updateMCPServerStatus(mcpServerName, MCPServerStatus.DISCONNECTED);
      // Close the client to prevent resource leaks.
      mcpClient?.close().catch(err => {
        coreEvents.emitFeedback('error', `Error closing errored MCP client: ${mcpServerName}`, err);
      });
    };

[skeshive]

/patch preview

[github-actions]

✅ Patch workflow(s) dispatched successfully!

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[github-actions]

🚀 Patch PR Created!

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• Environment: prod
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• Commit: 9590a092ae7465f732e7bfe6031773748767db07
• Hotfix Branch: hotfix/v0.29.0-preview.4/0.29.0-preview.5/preview/cherry-pick-9590a09/pr-18771
• Hotfix PR: #19274

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[github-actions]

🚀 Patch Release Started!

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✅ Patch Release Complete!

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• Version: 0.29.0-preview.5
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