readme

compatibility/src/README.md

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+# Backwards Compatibility
+
+## TL;DR
+The encoding of the bytes in libra uses `BCS` which is a de/serialization implementation using `serde`. Libra Version Six and beyond cannot decode transaction logs or state snapshots from V5 without these tools.
+
+# Explain
+
+V6 was a kitchen sink upgrade with a new genesis, since there were upgrades throughout the stack that would have created a discontinuity in blocks anyhow.
+
+The bytes present in prior db, logs, and backups prior to V6 had different memory layouts. For every K-V structure the keys had different hashes, and the values had different encoding layouts.
+
+Also, looking up data in K-V representations of bytes is done with byte encoded access_paths. Since the Move language address format and data structure names have changed, nothing can be found, and you will receive a `remaining input` error. I gift you this koan.
+
+This compatibility library ports the some V5 Rust code so that certain elemental types (StructTag, TypeTag, HashValue, AccountAddress), use the correct layout.
+
+# Principal PITAs
+
+1. Backup Manifests have changed layout. State Snapshot Manifests have changed ever so slightly, they previously did not include "epoch" keys. Reading V5 backup archive manifests would fail with V6+ tooling.
+
+1. `AccountStateBlob` stored bytes are not what they seem: In the State Snapshot backup files, each chunk is represented by a tuple of `(HashedValue, AccountStateBlob)`. For clarity we added a definition of `AccountStateBlobChunkV5`.
+
+1. `HashValue` is evil: The HashValue layout has not changed, but it invokes loup garou vodoo, and the custom deserializer of HashedValue uses a different intermediary representation for the byte layout
+```
+#[derive(::serde::Deserialize)]
+#[serde(rename = "HashValue")]
+struct Value<'a>(&'a [u8]);
+
+let value = Value::deserialize(deserializer)?;
+Self::from_slice(value.0).map_err(<D::Error as ::serde::de::Error>::custom)
+```
+1. `AccountAddress` makes everything fail: fixed lengths have changed, from V5 to V6 the addresses doubled in size (32 to 64 bits). No KV lookup will work because the byte-encoded key always has the Core Code Address, (0x1) which changed from being prepended with 16 zeros, to 32 zeros. So all language_storage.rs structs are changed to use `LegacyAddressV5`.
+
+# Tests
+The principal tests to run are in `state_snapshot_v5.rs`, where we try to sanity test the encoding and decoding of structs for the v5 language elements.
+
+
+# References
+
+```
+$ >
+$ > Loop Garoo
+Goin' down to junk anew?
+$ > Loop Garoo
+Goin' put my hook to you
+```