http: throttle send buffer when client stops draining - #36174
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A client streaming pipelined requests into a busy connection (or any connection whose replies are slower than the sender) could grow server memory without limit, up to remote OOM. Stop selecting RecvEvent for clients whose request is being processed; pipelined data then backs up in the kernel socket buffer, applying TCP backpressure to the sender. One request per connection is in flight at a time. Functional test streams pipelined submitblock requests into a connection blocked on waitforblockheight. Unpatched builds continue draining the socket buffer indefinitely, patched builds will stall.
Prevents a memory exhaustion case where a misbehaving client refuses to read responses and drain the socket buffer. Instead of packing more data on to the server-side m_send_buffer, stop dispatching requests from the client to workers
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2026-09-05 11:50:10 |
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This is a follow-up to #36123 and applies a small tweak to the read-side throttle mechanism in addition to applying a second mechanism to the send-side. If a misbehaving client refuses to read data from the socket, the server will now stop processing requests instead of packing more and more responses to
m_send_bufferwithout bound.This is a solution to https://github.com/bitcoin-security-council/bitcoin/pull/59 (maybe private, but adding for attestation).
After we parse a complete request from a client, before we dispatch it to a worker, we quickly lock and check the size of
m_send_buffer. If there's already 32MiB of data there (reusingMAX_BODY_SIZEhere, open for bikeshedding...) we do not dispatch the request to a worker, leaving it in place asm_req. This is where the tweak to #36123 comes in. In that PR we throttle reads unlessm_reqis present, assuming it is incomplete and needs more data. This commit changes that assumption and so!= State::Completemust be checked in addition.