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Fix 100% CPU spin when MCP server child exits #1

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Fix 100% CPU spin when MCP server child exits #1

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19 changes: 16 additions & 3 deletions Sources/AgentMCP/StdioConnection.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -24,15 +24,28 @@ final class StdioConnection: @unchecked Sendable, MCPConnection {
self.reader = reader
self.errorReader = errorReader

// Drain stderr to prevent the server from blocking on a full pipe (64 KB OS limit)
// Drain stderr to prevent the server from blocking on a full pipe (64 KB OS limit).
// On EOF (child exited / pipe closed), availableData returns empty Data; if we
// don't clear the handler, the dispatch source fires in a tight loop forever,
// burning ~100% CPU per pipe. Same pattern on the reader below.
errorReader.readabilityHandler = { handle in
_ = handle.availableData
let data = handle.availableData
if data.isEmpty {
handle.readabilityHandler = nil
}
}

// Set up non-blocking read via readabilityHandler
reader.readabilityHandler = { [weak self] handle in
let data = handle.availableData
guard !data.isEmpty, let self else { return }
if data.isEmpty {
handle.readabilityHandler = nil
return
}
guard let self else {
handle.readabilityHandler = nil
return
}

self.lock.lock()
self.buffer.append(data)
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