Supported/Unsupported V1 Externs

[stp59]

I've delimited the externs I intend to support for the upcoming deadline, subject to change if deemed appropriate.

Supported:

• counters (and relevant methods)
• registers (and relevant methods)
• random
• mark_to_drop (but not the deprecated version)
• hash*
• verify_checksum*
• update_checksum*
• verify_checksum_with_payload*
• update_checksum_with_payload*
• truncate
• assert
• assume

Unsupported:

• direct counters (and relevant methods)
• meters (and relevant methods)
• direct meters (and relevant methods)
• action profiles
• digest
• action selectors
• checksum16 (and relevant methods)
• resubmit
• recirculate
• clone
• clone3
• log_msg

Support for most of these would involve non-trivial modifications to the architecture of the target interface (such as direct counters), or wouldn't make sense for the scope of petr4 (digest sends to the control plane). Eventually we need target-dependent table evaluation for custom table attributes introduced by architecture, but this is probably not feasible by the deadline.

*these externs have a wide range of support for various hashing algorithms, not all of which we need to support

[hackedy]

I'm not sure we have tests for verify_checksum_with_payload, update_checksum_with_payload, truncate, assert, or assume--why don't we leave those off?

[hackedy]

I grepped around and there's one test that uses update_checksum_with_payload, but it passes right now since it's only a typechecker test.

[stp59]

In my understanding, the with payloads will be trivial to implement once the without payload ones are done, so I included them for good measure. I kept assert and assume and implemented them by raising a new exception. I thought it might be useful for debugging. I have yet to look further into truncate.

[stp59]

Moving truncate to unsupported

[hackedy]

Just need to pick hash algorithms now