test: Check that RPCs do not time out, even under load #34927

It turns out there is no test currently to check that the RPC server does not time out under load. With "load" I mean a flood of trivial payloads. That is, the only work needed is JSON encoding and decoding of (let's say) a block of data of 2 MB or so. This may take a few milliseconds, but should never take more than a few seconds.

So add a test for this.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/34927.

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Conflicts

No conflicts as of last run.

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Draft for now, because the test fails :(

Edit: It fails in CI, but it passes locally for me:

./bld-cmake/test/functional/test_runner.py -j 16 $( printf 'rpc_echo_payload.py %.0s' {1..2000} )

Timed out after 40 minutes in https://github.com/bitcoin/bitcoin/actions/runs/23606118115/job/68749250793?pr=34927#step:11:3195:

2026-03-26T16:46:08.3589405Z Remaining jobs: [interface_ipc_mining.py, rpc_echo_payload.py]
2026-03-26T16:47:46.9801923Z ......................................................................................................................................................................................................
2026-03-26T16:47:47.4805641Z                                                                                                                                                                                                       
2026-03-26T16:47:47.4806053Z Remaining jobs: [rpc_echo_payload.py]
2026-03-26T17:27:36.9551875Z ##[error]The operation was canceled.

cc @dergoegge Can you copy-paste the Python test into Antithesis to see if it fails there?

Can this be tested with 120 commits or so?

Can this be tested with 120 commits or so?

Sorry, turns out I was wrong, and I spun up a new connection while disabling Nagle's. Obviously spinning up a new connection is already known to work around the issue and Nagle's didn't seem to affect it.

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Guix builds (on x86_64) [untrusted test-only build, possibly unsafe, not for production use]

Concept ACK

Turning into draft while CI is red, but the code should be correct, reviewable, and mergeable after libevent is nuked.