A proposal to use block_version to control schema evolution

text/0024-block-version-based-protocol-evolution.md

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+- Feature Name: Block-Version-based Protocol Evolution
+- Start Date: 2022-02-05
+- RFC PR: [mobilecoinfoundation/mcips#0024](https://github.com/mobilecoinfoundation/mcips/pull/0024)
+- MobileCoin Epic: None
+
+# Summary
+[summary]: #summary
+
+Since inception, MobileCoin has not yet made any changes to the ledger format.
+
+This document proposes a general approach to rolling out changes to the ledger format
+efficiently while carefully maintaining backwards compatibility.
+
+# Motivation
+[motivation]: #motivation
+
+Conventional web applications today are usually developed using data interchange
+formats that support schema evolution with forward and backwards compatibility.
+A good example of this is protobuf -- protobuf allows that new fields can be
+added to schema, and new programs will be able to deserialize old payloads, and
+old programs will be able to deserialize the new payloads (ignoring the new fields).
+
+As long as changes are strictly additive then they can be rolled out without
+breaking old clients. Eventually the old clients are sunsetted and only the new
+clients are supported.
+
+In blockchains, this is made more complicated for two reasons:
+* Older data payloads exist forever in the blockchain and clients have to be able to handle them forever
+* Frequently, clients and nodes need to agree about the hash of of a piece of data, so that
+  clients that sync the blockchain compute the correct block id, and clients that submit merkle proofs
+  of membership compute correct proofs.
+
+Because of the need for everyone to compute hashes correctly, it may not be enough for
+an old client to successfully deserialize a new payload by ignoring the fields it doesn't
+understand -- if those fields are ignored, then it will compute a different hash for
+this object from the new clients.
+
+To make this concrete, this means that if we want to roll out changes that add new
+features to `TxOut`'s, the new clients and old clients probably cannot coexist the way
+they might in a more conventional application.
+
+If a new client sends a `TxOut` to a user using an old client, which contains a `TxOut`
+feature that they don't recognize, then:
+1. They may fail to compute the correct block id
+2. If they submit a transaction spending the TxOut, the merkle proof of membership
+   may be wrong if the hash doesn't incorporate the new field.
+3. Even if they are a fog client, and (new) fog is building the proof of membership for them,
+   if the client strips out the fields it doesn't understand that fog gives them, then consensus
+   will likely reject the transaction. (There are some protobuf implementations that support "preserving"
+   unknown fields, but PROST, which is the only rust implementation which is enclave-compatible, does not do this.)
+
+This proposal describes a way to navigate these difficulties:
+* Schema evolution can be accompanied by bumps to `block_version` (which has never been bumped before)
+* Clients can be built that support multiple values of `block_version`
+* The clients all switch to using a newer `block_version` at the same time, preventing breakage.
+
+Thus, new transactions do not begin happening immediately when new clients are shipped -- instead, backwards-compatible
+clients are shipped, and new behavior is triggered everywhere at once after a suitable deprecation window for the
+old clients. Transaction validation enforces that new fields do not appear (which could break the old clients) until
+the appropriate time.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The `block_version` is an existing field in the MobileCoin block header. It currently always has the value 1.
+
+When new fields are added to the blockchain, e.g. to the `TxOut` object, the following principles should be followed:
+  * The changes must be additive (in the sense of protobuf)
+  * A new value of `block_version` must be announced, (in the MCIP that introduced the changes)
+  * The new fields are only accepted by consensus when the `block_version` on chain has incremented
+  * Before the block version can be increased (and the new feature activated), clients must be shipped which are `block_version` aware, and should support at least two block versions,
+    an "old" version and a "new" version.
+  * When building transactions, clients will learn the "current" block version either from the ledger, or from Fog, and will
+    build "old" or "new" transactions as appropriate. (The transaction builder will be modified to make this easy.)
+  * The enclaves will support a run-time determination of `block_version`, and transaction validation will be `block_version` aware.
+  * A mechanism will be described which triggers the increase of the block version. This mechanism may vary from MCIP to MCIP but should
+    be governed by the consensus of the node operators.
+
+We also propose a `block_version` increment for the `0003-encrypted-memos` MCIP, and propose
+a `block_version` increment for the (nascent) `0023-confidential-token-ids` MCIP.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The `block_version` number already exists in the MobileCoin block header, and as a constant in `mc-transaction-core`.
+It has never been modified.
+
+There is a constant called `BLOCK_VERSION` in `mc-transaction-core` which has value 1.
+This constant will be renamed `MAX_BLOCK_VERSION`, and the meaning of the constant will be, the largest value of
+`block_version` which this version of `mc-transaction-core` has support for.
+
+This proposal largely is about establishing norms for server and client implementations, particularly, that they
+should both be `block_version` aware and support multiple block versions when an upgrade occurs.
+
+* The `block_version` of the blockchain is the `block_version` of the latest block.
+* The block version of a block can never be less than the `block_version` of its parent.
+* We propose that the encrypted memo field from `0003-encrypted-memos` MCIP should be associated to a bump to `block_version = 2`.
+  That is, consensus will reject `Tx` that contain any `TxOut` that have memos until `block_version = 2`.
+* We propose that the `masked_token_id` field from the  `0023-confidential-token-ids` MCIP should be associated to a bump to `block_version = 3`.
+  That is, consensus will reject `Tx` that contain any `TxOut` that have this field until `block_version = 3`.
+* We propose that `block_version = 3` may be triggered by the acceptance of the first minting transaction to the blockchain.
+* FIXME: We do not at this point have a proposal for how to trigger `block_version = 2`.
+
+We'll attempt to describe code changes that would be needed in mobilecoin software to implement this:
+
+The current block version can be obtained from the `mc-ledger-db` LMDB ledger interface by getting
+the block header of the latest block, so clients like `mobilecoind` and `full-service` can already access this number.
+
+The changes that are required are:
+* Fog should start telling mobile clients about the latest block version, similarly to how it currently advertises the
+"highest known block count". Both fog-view and fog-ledger should do this.
+* The standard transaction builder should become `block_version` aware. Clients should pass the latest block version to it.
+* Transaction validation should become `block_version` aware.
+* The consensus enclave should support dynamic `block_version`.
+
+Mobile clients SHOULD use the `latest_block_version` number to indicate to the users that an upgrade is required. The constant
+`mc_transaction_core::MAX_BLOCK_VERSION` will indicate the largest block version that this version of the `TransactionBuilder` object supports.
+
+Desktop clients, which use the lmdb database `mc-ledger-db`, already work by comparing the block version of new blocks to the
+`mc_transaction_core::BLOCK_VERSION` constant. They should continue doing this, but the constant should be renamed `MAX_BLOCK_VERSION`.
+
+We do not at this time propose that the `Tx` object itself should contain a `block_version`,
+instead, transactions are valid if they conform to the rules for the current block version.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+This adds complexity to the transaction validation code, which now must support multiple versions of the rules.
+
+This adds complexity to the transaction building code, which also must support multiple versions of the transactions.
+
+This MCIP proposes ad-hoc mechanisms to trigger increases to the block version.
+
+This MCIP proposes that `0003-encrypted-memos` requires a bump to block version, but does not propose a mechanism to trigger
+the bump. If a suitable mechanism is not proposed, then as an alternative there could be one bump to block version for both the encrypted memos
+changes and the confidential token ids change. Another alternative may be to implement `0007-Protocol-version-statement-type` MCIP.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+* For encrypted memos, we had earlier proposed that there would be a period of time when both transactions with memos
+  and without memos would be accepted, and eventually only transactions with memos would be accepted. However,
+  further thought shows that this may lead to breakage. For example, if a new client sends a TxOut with a memo to
+  an old client, and the old client attempts to spend that TxOut, their transaction may be rejected because their
+  proof of membership is invalid (they ignore the memos when they compute the hashes, but consensus does not.)
+  Even the fog clients, which get merkle proofs of membership from fog-ledger, may fail to build valid transactions,
+  because when they deserialize the protobufs from fog ledger, they strip out the memos (since they don't know about them),
+  which causes the hashes not to match the data submitted to consensus.
+
+* Since it seems that in general, any additive change to a TxOut which could change its hash can cause clients to break,
+  no "rolling release" of upgrades to the transactions can work, because if new clients and old clients are coexisting,
+  then new TxOut's and old clients are coexisting, and that leads to the breakage.
+
+* If such a "rolling release" strategy cannot work, then we must instead do releases where something "triggers" the new
+  behavior. There must be a source of truth for this trigger, and the logical place for this is in the blockchain.
+  The `block_version` value seems a perfectly suitable thing to use for this purpose.
+
+* An alternative may be to use feature flags instead of a version number, so that the software supports turning on
+  individual blockchain features in any order.
+  It seems to the author that this has many drawbacks:
+  * More complexity, since some features may depend on other features.
+  * More testing surface area
+  * More code in more places
+
+  Since in real-life, features will turn on in some scheduled order, it will simplify the code if the code assumes the
+  order the features will turn on. We may be able to simplify the code though by adding predicates like
+  ```
+  fn feature_memos_is_supported(block_version: u32) -> bool { block_version >= 2 }
+  fn feature_masked_token_id_is_supported(block_version: u32) -> bool { block_version >= 3 }
+  ```
+  and using these in the transaction validation and construction routines, which will get us most of the benefits
+  of a feature-flag based system from a programming point of view.
+
+# Prior art
+[prior-art]: #prior-art
+
+This proposal relates to several earlier MCIP proposals:
+
+* 0007-Protocol-version-statement-type
+* 0008-SCP-based-hard-forks
+* 0009-Dual-rule-blocks
+
+In particular, `0007-protocol-version-statement-type` proposes a way that node operators
+could trigger `block_version` to increase by voting for this and reaching consensus on it using SCP itself.
+
+This proposal would resolve some of the drawbacks in this proposal around ad-hoc mechanisms, and may strengthen this proposal.
+The main concern would be the risk and complexity around adding new ballot types to SCP as envisioned in the proposal.
+
+The `0008-SCP-based-hard-forks` proposal would also normalize the process around triggering increases to the `block_version`.
+
+The `0009-Dual-rule-blocks` proposal envisions that nodes may accept both "new" transactions and "old" transactions simultaneously
+for a period of time. However, as we have seen in this proposal, this will lead to breakage of old clients in some
+situations, for both the `0023-confidential-token-ids` changes and the `0003-encrypted-memos` changes.
+This proposal can be seen as modifying `0009-Dual-rule-blocks` to avoid this breakage.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+* Is there an elegant way to trigger the encrypted-memos version bump?
+  It would be nice not to have to do these two changes simultaneously.
+* Should we attempt `0007-Protocol-version-statement-type` before attempting this.
+* Should we split this MCIP into a "process" part and an "implementation" part.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+None at this time.