test: Avoid interface_ipc.py Duplicate ID errors #33420

This change should fix issue #33417 reported by zaidmstrr. It's possible to reproduce the mp/proxy.capnp:0: failed: Duplicate ID [@0xcc316e3f71a040fb](/bitcoin-bitcoin/contributor/0xcc316e3f71a040fb/) error by installing libmultiprocess system-wide, or to one of the locations listed in the python test's imports list before the local libmultiprocess subtree, and then running the test.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33420.

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LLM Linter (✨ experimental)

Possible typos and grammar issues:

• and there are not "failed: Duplicate ID @0xcc316e3f71a040fb" errors. -> and that there are no "failed: Duplicate ID @0xcc316e3f71a040fb" errors. [the clause needs "that" and "no" to be grammatical and clear]

^{drahtbot_id_5_m}

Tested ACK e9c5227

This fixes the issue and is also mentioned here.

Concept ACK

That said, I'm unable to reproduce the issue on macOS. I installed libmultiprocess using (sudo) make install (https://github.com/bitcoin-core/libmultiprocess/blob/master/doc/install.md). capnp is installed via Homebrew.

I also haven't reproduced the issue, using the steps provided. i.e installing libmulitprocess system-wide in a new VM, and then building Core & running interface_ipc.py. Can someone list the steps to reach the failure, using a clean VM.

Here are the steps to reproduce the error in Ubuntu based systems:

1. Install libmultiprocess library system using sudo make install
2. Then install capnp system-wide. You can refer this for the installation. After succesfully downloading the capnp library you can check using the which capnp command, which outputs the default location of the installed binary. In most of the cases the location was /usr/local/bin/capnp.
3. Now run the interface_ipc.py test.

Thanks, repro'd:

Remaining jobs: [interface_ipc.py]
1/1 - interface_ipc.py failed, Duration: 1 s

stdout:
2025-09-18T10:18:46.257357Z TestFramework (INFO): PRNG seed is: 5267162394658679552
2025-09-18T10:18:46.257836Z TestFramework (INFO): Initializing test directory /tmp/test_runner_₿_🏃_20250918_101846/interface_ipc_0


stderr:
terminate called after throwing an instance of 'kj::ExceptionImpl'
  what():  mp/proxy.capnp:0: failed: Duplicate ID [@0xcc316e3f71a040fb](/bitcoin-bitcoin/contributor/0xcc316e3f71a040fb/).



TEST             | STATUS    | DURATION

interface_ipc.py | ✖ Failed  | 1 s

ALL              | ✖ Failed  | 1 s (accumulated) 
Runtime: 1 s

Backported to 30.x in #33424.

I also haven't reproduced the issue, using the steps provided. i.e installing libmulitprocess system-wide in a new VM, and then building Core & running interface_ipc.py.

Yeah, an additional requirement for the bug to happen is that cap'nproto and libmultiprocess need to be installed in the same prefix. If you are using a system provided cap'n proto installed in /usr and a locally build libmultiprocess installed in /usr/local the bug won't happen. They both need to be installed in /usr or /usr/local or other prefix.