feat: subscribe() returns request id, monotonic message id, protobuf via C API

[Thecave3]

Summary

• Surface the assigned E3-MessageID from subscribe() so a dApp can correlate a SubscriptionResponse (which echoes request_id) back to the originating request by id, instead of relying on FIFO ordering that desyncs on a dropped/reordered response. The id is threaded via an out-param queue_subscription_request → E3Agent::subscribe → the SWIG DAppSession::subscribe, which now returns the positive id on success / negative ErrorCode on failure.
• Make generate_message_id() a lock-free monotonic counter wrapping into the ASN.1 E3-MessageID (1..1000) range, replacing the random draw that could collide within the in-flight window and defeat that correlation.
• Let protobuf reach the wire from C-API consumers: widen the e3_config_t.encoding clamp in c_api.cpp from 0..1 to 0..2 (it silently dropped PROTOBUF=2), and document it in c_api.h.
• build_libe3: add --enable-json/--disable-json and default JSON on — JSON previously had no flag and defaulted off, so build_libe3 --enable-swig produced a library/binding without the JSON encoder (a JSON peer then failed at start() with INTERNAL_ERROR). ASN.1+JSON now build by default; protobuf stays opt-in.

Type of change

• New feature / enhancement
• Test / CI / packaging
• Other

Linked issue

No standalone tracking issue — this is coordinated with paired changes in the twin repos (see Twin-repo coordination). Happy to file one if preferred.

Mandatory test checklist

• ./build_libe3 -c -d build -j $(nproc) -r -t passes (Release build + tests)
• ./build_libe3 -c -d build -j $(nproc) -g -t passes (Debug build + tests)
• cd build && ctest --output-on-failure is clean (18/18)
• MPMC queue benchmark shows no regression vs main (this PR does not touch mpmc_queue.hpp)
• VERSION bumped per SemVer (0.0.9 → 0.0.10; ABI change — subscribe() return contract)
• Public headers under include/ touched; ./build_libe3 --docs renders with no new Doxygen warnings vs main
• No new build dependencies added
• libe3.pc interface unchanged; downstream consumers still link cleanly (verified: the OAI gNB twin builds and runs against this)

CI checklist

• Unit Tests workflow is green (Debug + Release matrix) — will run on this PR
• Commit policy workflow is green — commits carry Assisted-by: and no Co-authored-by/Signed-off-by; verified locally with scripts/check_commit_trailers.py
• MPMC Queue Benchmark — not triggered (mpmc_queue.hpp unchanged)

Twin-repo coordination

This PR changes the public ABI (subscribe() return contract) and the C-API encoding acceptance, so it is released together with paired changes in the twins:

• dApp-library (dapps): consumes the returned request id for id-based subscription correlation, and adds the protobuf E3SM codec path.
• dApp-openairinterface5g: the gNB uses encoding = PROTOBUF through the C API and adds a protobuf-c SM payload codec.

Validated end-to-end: the OAI gNB (rfsim, core-less) driving the real spectrum dApp passes for asn1, json, and protobuf.

• A paired change exists in each affected twin repo (coordinated release).

Paired PR(s): prepared in the dApp-library and dApp-openairinterface5g twins; released alongside this PR.

Workflow confirmation

• My branch is a linear, fast-forward-able descendant of main, no merge commits.
• Every commit builds and passes tests on its own (2 atomic commits).
• I have read and followed CONTRIBUTING.md.

[github-actions]

⏱️ Full-loop Latency Benchmark (commit 35a3141)

Full-loop latency benchmark (N=1044 after 50 warmup)

All values in microseconds (us). Link: zmq, transport: ipc, encoding: ASN.1 APER.

Phase	mean	p50	p99	max
1. Collect indication data	0.16	0.14	0.30	7.27
2. Create & encode indication	0.74	0.67	1.70	8.90
3. Deliver indication (RAN -> dApp)	92.87	87.92	162.21	183.45
4. Decode indication	0.60	0.56	1.20	8.23
5. Process data	0.04	0.03	0.04	9.16
6. Create & encode control	0.30	0.27	0.66	8.72
7. Deliver control (dApp -> RAN)	108.81	109.47	178.82	552.91
8. Decode & handle control	0.53	0.47	1.11	14.18
Total round-trip	204.06	195.52	330.84	594.45

Benchmarked on ubuntu-latest, Release build, ZMQ + IPC, ASN.1 APER.

[github-actions]

🔄 E2E dApp Integration Results (commit 35a3141)

posix/ipc

• dApp exit: 0
• Indications received: 7

posix/tcp

• dApp exit: 0
• Indications received: 7

zmq/ipc

• dApp exit: 0
• Indications received: 7

zmq/tcp

• dApp exit: 0
• Indications received: 7

example_simple_agent + example_simple_dapp on ubuntu-latest, Release build

[github-actions]

🔀 E2E Topologies — multi-dApp / multi-RAN (commit 35a3141)

zmq/ipc

• ✅ 1 RAN - 1 dApp: indications=5
  • dapp peer=t11 ran=ran-solo sub=1 indications=5 seq=[0..4] dropped=0 (0%) age_ms(avg=0 max=0 @seq=0) hist[<=1:5 2-5:0 6-10:0 >10:0]
• ✅ 1 RAN - 2 dApps: dApp#1 ind=5 sub=1, dApp#2 ind=6 sub=2, RAN saw 2 dApps
  • dapp1 peer=t12 ran=ran-shared sub=1 indications=5 seq=[0..4] dropped=0 (0%) age_ms(avg=1 max=1 @seq=0) hist[<=1:5 2-5:0 6-10:0 >10:0]
  • dapp2 peer=t12 ran=ran-shared sub=2 indications=6 seq=[0..5] dropped=0 (0%) age_ms(avg=1 max=1 @seq=0) hist[<=1:6 2-5:0 6-10:0 >10:0]
• ✅ 2 RANs - 1 dApp: from ran-a ind=5, from ran-b ind=5
  • dapp peer=t2a ran=ran-a sub=1 indications=5 seq=[0..4] dropped=0 (0%) age_ms(avg=0.2 max=1 @seq=0) hist[<=1:5 2-5:0 6-10:0 >10:0]
  • dapp peer=t2b ran=ran-b sub=1 indications=5 seq=[0..4] dropped=0 (0%) age_ms(avg=0 max=0 @seq=0) hist[<=1:5 2-5:0 6-10:0 >10:0]

zmq/tcp

• ✅ 1 RAN - 1 dApp: indications=5
  • dapp peer=default ran=ran-solo sub=1 indications=5 seq=[0..4] dropped=0 (0%) age_ms(avg=0 max=0 @seq=0) hist[<=1:5 2-5:0 6-10:0 >10:0]
• ✅ 1 RAN - 2 dApps: dApp#1 ind=5 sub=1, dApp#2 ind=6 sub=2, RAN saw 2 dApps
  • dapp1 peer=default ran=ran-shared sub=1 indications=5 seq=[0..4] dropped=0 (0%) age_ms(avg=0 max=0 @seq=0) hist[<=1:5 2-5:0 6-10:0 >10:0]
  • dapp2 peer=default ran=ran-shared sub=2 indications=6 seq=[0..5] dropped=0 (0%) age_ms(avg=0 max=0 @seq=0) hist[<=1:6 2-5:0 6-10:0 >10:0]
• ✅ 2 RANs - 1 dApp: from ran-a ind=5, from ran-b ind=5
  • dapp peer=default ran=ran-a sub=1 indications=5 seq=[0..4] dropped=0 (0%) age_ms(avg=0 max=0 @seq=0) hist[<=1:5 2-5:0 6-10:0 >10:0]
  • dapp peer=off100 ran=ran-b sub=1 indications=5 seq=[0..4] dropped=0 (0%) age_ms(avg=0.2 max=1 @seq=0) hist[<=1:5 2-5:0 6-10:0 >10:0]

example_simple_agent + example_simple_dapp on ubuntu-latest, Release. Indication age is report-only.