BMv2 returns UNIMPLEMENTED on register read

Symptom

A call to sw.readRegister("...") (or readRegisterAsync) throws:

io.github.zhh2001.jp4.exceptions.P4OperationException:
    UNIMPLEMENTED: Register reads are not supported yet
    (operationType=READ)

Counter, meter, table, action-profile, multicast, and clone reads against the same device work normally. Only register reads hit this.

Reason

The P4Runtime specification allows a server to refuse a Read RPC with the UNIMPLEMENTED gRPC status code if the entity type is not implemented. BMv2 simple_switch_grpc version 1.15.1 (the version pinned in the jp4 CI matrix and the version most BMv2 docker images currently ship with) does not implement RegisterEntry Read; it returns UNIMPLEMENTED for any Read request whose entity oneof selects register_entry.

This is a device-side limitation, not a jp4 bug. jp4's readRegister surface is exercised end-to-end by the unit test P4SwitchReadRegisterTest against an in-process gRPC fake, and is correct on both the request-build and response-parse paths.

Fix

• In integration tests against BMv2: catch the P4OperationException, check for errorCode() == UNIMPLEMENTED, and skip the assertion through JUnit's Assumptions.assumeTrue(false, ...). The pattern is used by jp4's own CountersMetersRegistersGroupsIntegrationTest — see the Assumptions.assumeTrue block in that file.
• In production controllers: catch P4OperationException, inspect errorCode(), and if it's UNIMPLEMENTED fall back to a different observability mechanism (a packet probe, a counter alongside the register write, etc.) for the specific BMv2 deployment.
• For canonical-bytestring-aware controllers using a non-BMv2 server: register reads work on any P4Runtime server that implements them (spec-compliant Tofino / Stratum builds).

Background

The P4Runtime spec's tolerance of UNIMPLEMENTED for read RPCs reflects that not every device backend supports the same entity types. The jp4 surface is target-agnostic; the device-side runtime decides which entity reads it can answer.

The Read RPC is one of the few P4Runtime RPCs where UNIMPLEMENTED is a legitimate spec-conformant response, alongside OK (with rows) and OK (empty). It's not a transient error to retry — the device is telling you the entity type itself isn't supported.

See also

• Reading counters, meters, registers, and action-profile entries — the recipe; carries this same caveat in its BMv2 note.
• v1.3 → v1.4 migration guide — the surface introduction, also notes this BMv2 limitation.
• Error handling — how P4OperationException carries the gRPC errorCode().