backport: assumeutxo M6 — hardening (bitcoin#29370 + A4 batch)

doc/assumeutxo.md

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+# Assumeutxo Usage
+
+Assumeutxo is a feature that allows fast bootstrapping of a validating dashd
+instance.
+
+For notes on the design of Assumeutxo, please refer to [the design doc](/doc/design/assumeutxo.md).
+
+## Loading a snapshot
+
+There is currently no canonical source for snapshots, but any downloaded snapshot
+will be checked against a hash that's been hardcoded in source code. If there is
+no source for the snapshot you need, you can generate it yourself using
+`dumptxoutset` on another node that is already synced (see
+[Generating a snapshot](#generating-a-snapshot)).
+
+Once you've obtained the snapshot, you can use the RPC command `loadtxoutset` to
+load it.
+
+```
+$ dash-cli loadtxoutset /path/to/input
+```
+
+After the snapshot has loaded, the syncing process of both the snapshot chain
+and the background IBD chain can be monitored with the `getchainstates` RPC.
+
+### Pruning
+
+A pruned node can load a snapshot. To save space, it's possible to delete the
+snapshot file as soon as `loadtxoutset` finishes.
+
+The minimum `-prune` setting is 550 MiB, but this functionality ignores that
+minimum and uses at least 1100 MiB.
+
+As the background sync continues there will be temporarily two chainstate
+directories, each multiple gigabytes in size (likely growing larger than the
+downloaded snapshot).
+
+### Indexes
+
+Indexes work but don't take advantage of this feature. They always start building
+from the genesis block and can only apply blocks in order. Once the background
+validation reaches the snapshot block, indexes will continue to build all the
+way to the tip.
+
+
+For indexes that support pruning, note that these indexes only allow blocks that
+were already indexed to be pruned. Blocks that are not indexed yet will also
+not be pruned.
+
+This means that, if the snapshot is old, then a lot of blocks after the snapshot
+block will need to be downloaded, and these blocks can't be pruned until they
+are indexed, so they could consume a lot of disk space until indexing catches up
+to the snapshot block.
+
+## Generating a snapshot
+
+The RPC command `dumptxoutset` can be used to generate a snapshot for the current
+tip (using type "latest") or a recent height (using type "rollback"). A generated
+snapshot from one node can then be loaded
+on any other node. However, keep in mind that the snapshot hash needs to be
+listed in the chainparams to make it usable. If there is no snapshot hash for
+the height you have chosen already, you will need to change the code there and
+re-compile.
+
+Using the type parameter "rollback", `dumptxoutset` can also be used to verify the
+hardcoded snapshot hash in the source code by regenerating the snapshot and
+comparing the hash.
+
+Example usage:
+
+```
+$ dash-cli -rpcclienttimeout=0 dumptxoutset /path/to/output rollback
+```
+
+For most of the duration of `dumptxoutset` running the node is in a temporary
+state that does not actually reflect reality, i.e. blocks are marked invalid
+although we know they are not invalid. Because of this it is discouraged to
+interact with the node in any other way during this time to avoid inconsistent
+results and race conditions, particularly RPCs that interact with blockstorage.
+This inconsistent state is also why network activity is temporarily disabled,
+causing us to disconnect from all peers.
+
+`dumptxoutset` takes some time to complete, independent of hardware and
+what parameter is chosen. Because of that it is recommended to increase the RPC
+client timeout value (use `-rpcclienttimeout=0` for no timeout).