Documentation improvements for assumeutxo

Author: ryanofsky

doc/design/assumeutxo.md

@@ -17,10 +17,9 @@ respectively generate and load UTXO snapshots. The utility script
 
 - A new block index `nStatus` flag is introduced, `BLOCK_ASSUMED_VALID`, to mark block
   index entries that are required to be assumed-valid by a chainstate created
-  from a UTXO snapshot. This flag is mostly used as a way to modify certain
+  from a UTXO snapshot. This flag is used as a way to modify certain
   CheckBlockIndex() logic to account for index entries that are pending validation by a
-  chainstate running asynchronously in the background. We also use this flag to control
-  which index entries are added to setBlockIndexCandidates during LoadBlockIndex().
+  chainstate running asynchronously in the background.
 
 - The concept of UTXO snapshots is treated as an implementation detail that lives
   behind the ChainstateManager interface. The external presentation of the changes

src/chain.h

@@ -113,10 +113,10 @@ enum BlockStatus : uint32_t {
     BLOCK_VALID_TRANSACTIONS =    3,
 
     //! Outputs do not overspend inputs, no double spends, coinbase output ok, no immature coinbase spends, BIP30.
-    //! Implies all parents are also at least CHAIN.
+    //! Implies all parents are either at least VALID_CHAIN, or are ASSUMED_VALID
     BLOCK_VALID_CHAIN        =    4,
 
-    //! Scripts & signatures ok. Implies all parents are also at least SCRIPTS.
+    //! Scripts & signatures ok. Implies all parents are either at least VALID_SCRIPTS, or are ASSUMED_VALID.
     BLOCK_VALID_SCRIPTS      =    5,
 
     //! All validity bits.
@@ -134,10 +134,18 @@ enum BlockStatus : uint32_t {
     BLOCK_OPT_WITNESS        =   128, //!< block data in blk*.dat was received with a witness-enforcing client
 
     /**
-     * If set, this indicates that the block index entry is assumed-valid.
-     * Certain diagnostics will be skipped in e.g. CheckBlockIndex().
-     * It almost certainly means that the block's full validation is pending
-     * on a background chainstate. See `doc/design/assumeutxo.md`.
+     * If ASSUMED_VALID is set, it means that this block has not been validated
+     * and has validity status less than VALID_SCRIPTS. Also that it may have
+     * descendant blocks with VALID_SCRIPTS set, because they can be validated
+     * based on an assumeutxo snapshot.
+     *
+     * When an assumeutxo snapshot is loaded, the ASSUMED_VALID flag is added to
+     * unvalidated blocks at the snapshot height and below. Then, as the background
+     * validation progresses, and these blocks are validated, the ASSUMED_VALID
+     * flags are removed. See `doc/design/assumeutxo.md` for details.
+     *
+     * This flag is only used to implement checks in CheckBlockIndex() and
+     * should not be used elsewhere.
      */
     BLOCK_ASSUMED_VALID      =   256,
 };