Economic protocol sandboxing

In the currently active protocol, as in any sound future protocols, updates are approved by voting. That way, the responsibility of switching to a new protocol code is the responsibility of voters, and one could argue that it is up to them to check that the code does not call, for instance, unsafe array access functions.

Yet, we decided to introduce a minimum level of machine checks, by compiling with a specific compiler that checks that no known-unsafe function is used. This static form of sandboxing is performed by the OCaml typechecker: we simply compile protocols in a restricted set of modules with restricted interfaces that hide any unsafe, non wanted feature.

Another goal of that specific environment is maintaining a stable OCaml API for protocol development. Imagine that at some point, the OCaml standard library changes (a function is added or removed, a type is changed), then we will be able to upgrade to the new OCaml while still remaining compatible with past protocols, by providing an adapter layer.

Here is a quick description of each file in this environment, all located under src/lib_protocol_environment/:

  • Files in the sigs/vX/ directories declare the interface that the protocols can be compiled against. These interfaces are repeats of other library interfaces with some unsafe functions removed. These interfaces are frozen and cannot be changed: changing them can break previous protocols. These files are assembled into a single interface file VX.mli by a helper program located in s_packer/.

  • Special cases: the files context.mli, fitness.mli, and updater.mli are interfaces that the protocol exposes to the shell.

  • Files in the structs/vX/ directory declare compatibility layers for old protocol environments. E.g., structs/V0/error_monad_traversors.ml contain the code of error-monad-compatible list traversers that used to be part of the Error Monad at the time of the environment V0. These implementations are meant to evolve alongside the ecosystem: when an incompatibility is introduced in a library an implementation needs to be implemented or amended in order to provide a backwards compatible overlay to the protocol. These implementations are assembled into a single implementation file VX.ml by a helper program located in s_packer/.

Environment versions

The environment interfaces are frozen, providing forever an immutable interface to the protocol. And so when a protocol needs new functions, types, or values, this protocol must use a different environment. This is why the environments are versioned.

A protocol’s manifest (e.g., src/proto_alpha/lib_protocol/TEZOS_PROTOCOL) includes a field named expected_env_version. This field specifies the environment used to compile the protocol.

Protocol environments can only ever increase in version. Specifically, from within a protocol P1 built on environment version X, the activation of a protocol P2 built on environment Y fails unless X ≤ Y.