fix(client): drain piped stderr so unread pipe cannot deadlock - #2788
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fix(client): drain piped stderr so unread pipe cannot deadlock#2788tiagovilasboas wants to merge 4 commits into
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Put the stderr PassThrough in flowing mode after piping so a chatty child cannot fill the unread pipe and hang the session. Listeners attached before start() still receive chunks. Fixes modelcontextprotocol#2776 Co-authored-by: Tiago Vilas Boas <tcarvalhovb@gmail.com>
Assert listTools() completes when stderr is piped with no reader, that a pre-start listener still receives chunks, and that a late listener does not deadlock and sees only post-attach output. Co-authored-by: Tiago Vilas Boas <tcarvalhovb@gmail.com>
Co-authored-by: Tiago Vilas Boas <tcarvalhovb@gmail.com>
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@modelcontextprotocol/client
@modelcontextprotocol/codemod
@modelcontextprotocol/core
@modelcontextprotocol/server
@modelcontextprotocol/server-legacy
@modelcontextprotocol/express
@modelcontextprotocol/fastify
@modelcontextprotocol/hono
@modelcontextprotocol/node
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Avoid racing stdout completion against flowing-mode stderr delivery, and keep flood size large enough to fill a paused pipe without dumping megabytes into assertion output. Co-authored-by: Tiago Vilas Boas <tcarvalhovb@gmail.com>
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v1.x backport of modelcontextprotocol#2788. `StdioClientTransport` with `stderr: 'pipe'` (or `'overlapped'`) pipes child stderr into a PassThrough that stays paused until a reader attaches. A server that logs a normal amount fills that buffer and the OS pipe, blocks on write(2), stops reading stdin, and the session hangs with no error, no rejection and no timeout. After piping, the PassThrough is put in flowing mode so unread chunks drain; listeners attached before start() still receive every chunk, a late listener sees only post-attach data. Fixes modelcontextprotocol#2776 on v1.x.
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Problem
StdioClientTransportwithstderr: 'pipe'(or'overlapped') pipes child stderr into aPassThroughthat stays paused until a reader attaches. A server that logs a normal amount to stderr fills that buffer (16 KiBhighWaterMark, then the OS pipe), blocks onwrite(2), stops reading stdin, and the session hangs — noonerror, no rejection.await client.listTools()never returns.Solution
Issue option (1): after
child.stderr.pipe(passThrough), callpassThrough.resume()so the stream is in flowing mode. Unread chunks are drained; listeners attached beforestart()/connect()still receive every chunk.Late listeners (after
connect()) see only post-attach data — lost log lines beat a hung session. Documented onStdioServerParameters.stderrand thestderrgetter, including that paused-mode.read()is not supported afterstart().Test plan
stderr: 'pipe', no reader →listTools()completes (timeout = fail)start()still receives startup + post-request chunkspnpm --filter @modelcontextprotocol/client exec vitest run test/client/stdio.test.ts— 9/9 passed locally (1.21s)Follow-up
v1.x backport (
src/client/stdio.ts) is out of scope for this PR.Fixes #2776