SCORU/Node: readonly durable RPC

Background

Currently the only way to inspect the state of a running wasm kernel is through the /global/block/<block_id>/state?key=<key> RPC endpoint, which returns the hex-encoded data of the value in the context at the given key.

While it is possible to build a simple view of the kernel state from this, it has some restrictions w.r.t. durable storage:

• Constructing the actual-value from the RPC endpoint requires
  • knowledge of the internals of Chunked_byte_vector and durable's use of the special @ subkey
  • either knowing statically the length of a value, or multiple calls to the endpoint to retrieve both the length of the value, and all its chunks
• There is no means for discovering other values (one can only lookup values that you expect to be there)
• The keys for the context are subtely different compared to the keys for durable storage (e.g. no / prefix) which leads to further confusion

Goal

A new RPC endpoint could be added to give a readonly view of durable storage, e.g. /global/block/<block_id>/durable/<operation>?key=<durable_key> where <operation> could be one of:

• value returns the value stored at <durable_key>, if it exists
• length returns the size of the value stored at <durable_key> if it exists
• subkeys returns the subkeys of <durable_key> if they exist

e.g. once a kernel is running on the rollup, then a request to /global/block/head/durable/value?key=/kernel/boot.wasm would return the hex-encoded WASM of the currently running kernel.

Motivation

Such an RPC would cover a significant number of use-cases for 'kernel-specific' views over durable storage, and allow a kernel-client to produce a reasonable view/interpretation of what the kernel is doing.

e.g. for the prototype tx-client such an endpoint would allow discoverability of other accounts within the TX-kernel rollup, or list tickets which have been recieved into an account - even where the contents of the ticket are not necessarily known.