docs: update assumeutxo.md

Include notes about the `chainstate_snapshot` rename as well as
updates for the included code.

Author: jamesob

doc/design/assumeutxo.md

@@ -3,9 +3,9 @@
 Assumeutxo is a feature that allows fast bootstrapping of a validating bitcoind
 instance with a very similar security model to assumevalid.
 
-The RPC commands `dumptxoutset` and `loadtxoutset` are used to respectively generate
-and load UTXO snapshots. The utility script `./contrib/devtools/utxo_snapshot.sh` may
-be of use.
+The RPC commands `dumptxoutset` and `loadtxoutset` (yet to be merged) are used to
+respectively generate and load UTXO snapshots. The utility script
+`./contrib/devtools/utxo_snapshot.sh` may be of use.
 
 ## General background
 
@@ -22,10 +22,6 @@ be of use.
   chainstate running asynchronously in the background. We also use this flag to control
   which index entries are added to setBlockIndexCandidates during LoadBlockIndex().
 
-- Indexing implementations via BaseIndex can no longer assume that indexation happens
-  sequentially, since background validation chainstates can submit BlockConnected
-  events out of order with the active chain.
-
 - The concept of UTXO snapshots is treated as an implementation detail that lives
   behind the ChainstateManager interface. The external presentation of the changes
   required to facilitate the use of UTXO snapshots is the understanding that there are
@@ -76,9 +72,15 @@ original chainstate remains in use as active.
 
 Once the snapshot chainstate is loaded and validated, it is promoted to active
 chainstate and a sync to tip begins. A new chainstate directory is created in the
-datadir for the snapshot chainstate called `chainstate_snapshot`. When this directory
-is present in the datadir, the snapshot chainstate will be detected and loaded as
-active on node startup (via `DetectSnapshotChainstate()`).
+datadir for the snapshot chainstate called `chainstate_snapshot`.
+
+When this directory is present in the datadir, the snapshot chainstate will be detected
+and loaded as active on node startup (via `DetectSnapshotChainstate()`).
+
+A special file is created within that directory, `base_blockhash`, which contains the
+serialized `uint256` of the base block of the snapshot. This is used to reinitialize
+the snapshot chainstate on subsequent inits. Otherwise, the directory is a normal
+leveldb database.
 
 |    |    |
 | ---------- | ----------- |
@@ -88,7 +90,7 @@ active on node startup (via `DetectSnapshotChainstate()`).
 The snapshot begins to sync to tip from its base block, technically in parallel with
 the original chainstate, but it is given priority during block download and is
 allocated most of the cache (see `MaybeRebalanceCaches()` and usages) as our chief
-consideration is getting to network tip.
+goal is getting to network tip.
 
 **Failure consideration:** if shutdown happens at any point during this phase, both
 chainstates will be detected during the next init and the process will resume.
@@ -107,33 +109,32 @@ sequentially.
 ### Background chainstate hits snapshot base block
 
 Once the tip of the background chainstate hits the base block of the snapshot
-chainstate, we stop use of the background chainstate by setting `m_disabled` (not yet
-committed - see #15606), in `CompleteSnapshotValidation()`, which is checked in
-`ActivateBestChain()`). We hash the background chainstate's UTXO set contents and
-ensure it matches the compiled value in `CMainParams::m_assumeutxo_data`.
-
-The background chainstate data lingers on disk until shutdown, when in
-`ChainstateManager::Reset()`, the background chainstate is cleaned up with
-`ValidatedSnapshotShutdownCleanup()`, which renames the `chainstate_[hash]` datadir as
-`chainstate`.
+chainstate, we stop use of the background chainstate by setting `m_disabled`, in
+`CompleteSnapshotValidation()`, which is checked in `ActivateBestChain()`). We hash the
+background chainstate's UTXO set contents and ensure it matches the compiled value in
+`CMainParams::m_assumeutxo_data`.
 
 |    |    |
 | ---------- | ----------- |
 | number of chainstates | 2 (ibd has `m_disabled=true`) |
 | active chainstate | snapshot |
 
-**Failure consideration:** if bitcoind unexpectedly halts after `m_disabled` is set on
-the background chainstate but before `CompleteSnapshotValidation()` can finish, the
-need to complete snapshot validation will be detected on subsequent init by
-`ChainstateManager::CheckForUncleanShutdown()`.
+The background chainstate data lingers on disk until the program is restarted.
 
 ### Bitcoind restarts sometime after snapshot validation has completed
 
-When bitcoind initializes again, what began as the snapshot chainstate is now
-indistinguishable from a chainstate that has been built from the traditional IBD
-process, and will be initialized as such.
+After a shutdown and subsequent restart, `LoadChainstate()` cleans up the background
+chainstate with `ValidatedSnapshotCleanup()`, which renames the `chainstate_snapshot`
+datadir as `chainstate` and removes the now unnecessary background chainstate data.
 
 |    |    |
 | ---------- | ----------- |
 | number of chainstates | 1 |
-| active chainstate | ibd |
+| active chainstate | ibd (was snapshot, but is now fully validated) |
+
+What began as the snapshot chainstate is now indistinguishable from a chainstate that
+has been built from the traditional IBD process, and will be initialized as such.
+
+A file will be left in `chainstate/base_blockhash`, which indicates that the
+chainstate, even though now fully validated, was originally started from a snapshot
+with the corresponding base blockhash.