Fix inconsistencies related to best chain order in RFC and state impl (#1267)

Prior to this PR we realized that the RFC had been drafted with the assumption that chains would be ordered from best to worst in `NonFinalizedState`. This assumption was incorrect, since `BTreeSet` only ever orders values in ascending order. This discrepancy was noticed and fixed in the code, but there were still some inconsistencies that needed to be cleaned up.

This PR updates all the incorrect or confusing comments about chain ordering in the RFC and code.

book/src/dev/rfcs/0005-state-updates.md

@@ -124,11 +124,12 @@ pub enum Request {
 ```
 
 `zebra-state` breaks down its requests into two categories and provides
-different guarantees for each category: requests that modify the state, and requests that
-do not. Requests that update the state are guaranteed to run sequentially and
-will never race against each other. Requests that read state are done
-asynchronously and are guaranteed to read at least the state present at the
-time the request was processed by the service, or a later state present at the time the request future is executed. The state service avoids
+different guarantees for each category: requests that modify the state, and
+requests that do not. Requests that update the state are guaranteed to run
+sequentially and will never race against each other. Requests that read state
+are done asynchronously and are guaranteed to read at least the state present
+at the time the request was processed by the service, or a later state
+present at the time the request future is executed. The state service avoids
 race conditions between the read state and the written state by doing all
 contextual verification internally.
 
@@ -231,7 +232,7 @@ time the request was processed, or a later state.
 At a high level, the in-memory data structures store a collection of chains,
 each rooted at the highest finalized block. Each chain consists of a map from
 heights to blocks. Chains are stored using an ordered map from cumulative work to
-chains, so that the map ordering is the ordering of best to worst chains.
+chains, so that the map ordering is the ordering of worst to best chains.
 
 ### The `Chain` type
 [chain-type]: #chain-type
@@ -343,10 +344,10 @@ Remove the highest height block of the non-finalized portion of a chain.
 
 #### `Ord`
 
-The `Chain` type implements `Ord` for reorganizing chains. First chains
-are compared by their `partial_cumulative_work`. Ties are then broken by
-comparing `block::Hash`es of the tips of each chain. (This tie-breaker
-means that all `Chain`s in the `ChainSet` must have at least one block.)
+The `Chain` type implements `Ord` for reorganizing chains. First chains are
+compared by their `partial_cumulative_work`. Ties are then broken by
+comparing `block::Hash`es of the tips of each chain. (This tie-breaker means
+that all `Chain`s in the `NonFinalizedState` must have at least one block.)
 
 **Note**: Unlike `zcashd`, Zebra does not use block arrival times as a
 tie-breaker for the best tip. Since Zebra downloads blocks in parallel,
@@ -617,7 +618,7 @@ Zcash structures are encoded using `ZcashSerialize`/`ZcashDeserialize`.
 - The `hash_by_height` and `height_by_hash` trees provide a bijection between
   block heights and block hashes.  (Since the Sled state only stores finalized
   state, they are actually a bijection).
-  
+
 - The `block_by_height` tree provides a bijection between block heights and block
   data. There is no corresponding `height_by_block` tree: instead, hash the block,
   and use `height_by_hash`. (Since the Sled state only stores finalized state,
@@ -668,7 +669,7 @@ check that `block`'s parent hash is `null` (all zeroes) and its height is `0`.
 
     (Due to a [bug in zcashd](https://github.com/ZcashFoundation/zebra/issues/559),
     genesis block anchors and transactions are ignored during validation.)
-    
+
 4.  Update the `sprout_anchors` and `sapling_anchors` trees with the Sprout and
     Sapling anchors.
 

zebra-state/src/service/memory_state/non_finalized_state.rs

@@ -93,6 +93,7 @@ impl NonFinalizedState {
     pub fn any_chain_contains(&self, hash: &block::Hash) -> bool {
         self.chain_set
             .iter()
+            .rev()
             .any(|chain| chain.height_by_hash.contains_key(hash))
     }