Merge bitcoin/bitcoin#27596: assumeutxo (2)

edbed31066e3674ba52b8c093ab235625527f383 chainparams: add signet assumeutxo param at height 160_000 (Sjors Provoost)
b8cafe38713cbf10d15459042f7f911bcc1b1e4e chainparams: add testnet assumeutxo param at height 2_500_000 (Sjors Provoost)
99839bbfa7110c7abf22e587ae2f72c9c57d3c85 doc: add note about confusing HaveTxsDownloaded name (James O'Beirne)
7ee46a755f1d57ce9d51975d3b54dc9ac3d08d52 contrib: add script to demo/test assumeutxo (James O'Beirne)
42cae39356fd20d521aaf99aff1ed85856f3c9f3 test: add feature_assumeutxo functional test (James O'Beirne)
0f64bac6030334d798ae205cd7af4bf248feddd9 rpc: add getchainstates (James O'Beirne)
bb0585779472962f40d9cdd9c6532132850d371c refuse to activate a UTXO snapshot if mempool not empty (James O'Beirne)
ce585a9a158476b0ad3296477b922e79f308e795 rpc: add loadtxoutset (James O'Beirne)
62ac519e718eb7a31dca1102a96ba219fbc7f95d validation: do not activate snapshot if behind active chain (James O'Beirne)
9511fb3616b7bbe1d0d2f54a45ea0a650ba0367b validation: assumeutxo: swap m_mempool on snapshot activation (James O'Beirne)
7fcd21544a333ffdf1910b65c573579860be6a36 blockstorage: segment normal/assumedvalid blockfiles (James O'Beirne)
4c3b8ca35c2e4a441264749bb312df2bd054b5b8 validation: populate nChainTx value for assumedvalid chainstates (James O'Beirne)
49ef778158c43859946a592e11ec34fe1b93a5b6 test: adjust chainstate tests to use recognized snapshot base (James O'Beirne)
1019c399825b0d512c1fd751c376d46fed4992b9 validation: pruning for multiple chainstates (James O'Beirne)
373cf91531b84bfdd06fdf8abf4dca228029ce6b validation: indexing changes for assumeutxo (James O'Beirne)
1fffdd76a1bca908f55d73b64983655b14cf7432 net_processing: validationinterface: ignore some events for bg chain (James O'Beirne)
fbe0a7d7ca680358237b6c2369b3fd2b43221113 wallet: validationinterface: only handle active chain notifications (James O'Beirne)
f073917a9e7ba423643dcae0339776470b628f65 validationinterface: only send zmq notifications for active (James O'Beirne)
4d8f4dcb450d31e4847804e62bf91545b949fa14 validation: pass ChainstateRole for validationinterface calls (James O'Beirne)
1e59acdf17309f567c370885f0cf02605e2baa58 validation: only call UpdatedBlockTip for active chainstate (James O'Beirne)
c6af23c5179cc383f8e6c275373af8d11e6a989f validation: add ChainstateRole (James O'Beirne)
9f2318c76cc6986d48e13831cf5bd8dab194fdf4 validation: MaybeRebalanceCaches when chain leaves IBD (James O'Beirne)
434495a8c1496ca23fe35b84499f3daf668d76b8 chainparams: add blockhash to AssumeutxoData (James O'Beirne)
c711ca186f8d8a28810be0beedcb615ddcf93163 assumeutxo: remove snapshot during -reindex{-chainstate} (James O'Beirne)
c93ef43e4fd4fbc1263cdc9e98ae5856830fe89e bugfix: correct is_snapshot_cs in VerifyDB (James O'Beirne)
b73d3bbd23220857bf17cbb6401275bf58013b72 net_processing: Request assumeutxo background chain blocks (Suhas Daftuar)

Pull request description:

  - Background and FAQ: https://github.com/jamesob/assumeutxo-docs/tree/2019-04-proposal/proposal
  - Prior progress/project: https://github.com/bitcoin/bitcoin/projects/11
  - Replaces https://github.com/bitcoin/bitcoin/pull/15606, which was closed due to Github slowness. Original description and commentary can be found there.

  ---

  This changeset finishes the first phase of the assumeutxo project. It makes UTXO snapshots loadable via RPC (`loadtxoutset`) and adds `assumeutxo` parameters to chainparams. It contains all the remaining changes necessary to both use an assumedvalid snapshot chainstate and do a full validation sync in the background.

  This may look like a lot to review, but note that
  - ~200 lines are a (non-essential) demo shell script
  - Many lines are functional test, documentation, and relatively dilute RPC code.

  So it shouldn't be as burdensome to review as the linecount might suggest.

  - **P2P**: minor changes are made to `init.cpp` and `net_processing.cpp` to make simultaneous IBD across multiple chainstates work.
  - **Pruning**: implement correct pruning behavior when using a background chainstate
  - **Blockfile separation**: to prevent "fragmentation" in blockfile storage, have background chainstates use separate blockfiles from active snapshot chainstates to avoid interleaving heights and impairing pruning.
  - **Indexing**: some `CValidationInterface` events are given with an additional parameter, ChainstateRole, and all indexers ignore events from ChainstateRole::ASSUMEDVALID so that indexation only happens sequentially.
  - Have `-reindex` properly wipe snapshot chainstates.
  - **RPC**: introduce RPC commands `loadtxoutset` and (hidden) `getchainstates`.
  - **Release docs & first assumeutxo commitment**: add notes and a particular assumeutxo hash value for first AU-enabled release.
    - This will complete the project and allow use of UTXO snapshots for faster node bootstrap.

  The next phase, if it were to be pursued, would be coming up with a way to distribute the UTXO snapshots over the P2P network.

  ---

  ### UTXO snapshots

  Create your own with `./contrib/devtools/utxo_snapshot.sh`, e.g.
  ```shell
  ./contrib/devtools/utxo_snapshot.sh 788000 utxo.dat ./src/bitcoin-cli -datadir=$(pwd)/testdata`)
  ```
  or use the pre-generated ones listed below.

  - Testnet: **2'500'000** (Sjors):
    - torrent: `magnet:?xt=urn:btih:511e09f4bf853aefab00de5c070b1e031f0ecbe9&dn=utxo-testnet-2500000.dat&tr=udp%3A%2F%2Ftracker.bitcoin.sprovoost.nl%3A6969`
    - sha256: `79db4b025448cc0ac388d8589a28eab02de53055d181e34eb47391717aa16388`
  - Signet: **160'000** (Sjors):
    - torrent: `magnet:?xt=urn:btih:9da986cb27b3980ea7fd06b21e199b148d486880&dn=utxo-signet-160000.dat&tr=udp%3A%2F%2Ftracker.bitcoin.sprovoost.nl%3A6969`
    - sha256: `eeeca845385ba91e84ef58c09d38f98f246a24feadaad57fe1e5874f3f92ef8c`
  - Mainnet: **800'000** (Sjors):
    - Note: this needs the following commit cherry-picked in: https://github.com/Sjors/bitcoin/commit/24deb2022b822f22fba9fcbee201e37a83225eb2
    - torrent: `magnet:?xt=urn:btih:50ee955bef37f5ec3e5b0df4cf0288af3d715a2e&dn=utxo-800000.dat&tr=udp%3A%2F%2Ftracker.bitcoin.sprovoost.nl%3A6969`

  ### Testing

  #### For fun (~5min)

  If you want to do a quick test, you can run `./contrib/devtools/test_utxo_snapshots.sh` and follow the instructions. This is mostly obviated by the functional tests, though.

  #### For real (longer)

  If you'd like to experience a real usage of assumeutxo, you can do that too.
  I've cut a new snapshot at height 788'000 (http://img.jameso.be/utxo-788000.dat - but you can do it yourself with `./contrib/devtools/utxo_snapshot.sh` if you want). Download that, and then create a datadir for testing:
  ```sh
  $ cd ~/src/bitcoin  # or whatever

  # get the snapshot
  $ curl http://img.jameso.be/utxo-788000.dat > utxo-788000.dat

  # you'll want to do this if you like copy/pasting
  $ export AU_DATADIR=/home/${USER}/au-test # or wherever

  $ mkdir ${AU_DATADIR}
  $ vim ${AU_DATADIR}/bitcoin.conf

  dbcache=8000  # or, you know, something high
  blockfilterindex=1
  coinstatsindex=1
  prune=3000
  logthreadnames=1
  ```
  Obtain this branch, build it, and then start bitcoind:
  ```sh
  $ git remote add jamesob https://github.com/jamesob/bitcoin
  $ git fetch jamesob assumeutxo
  $ git checkout jamesob/assumeutxo

  $ ./configure $conf_args && make  # (whatever you like to do here)

  # start 'er up and watch the logs
  $ ./src/bitcoind -datadir=${AU_DATADIR}
  ```
  Then, in some other window, load the snapshot
  ```sh
  $ ./src/bitcoin-cli -datadir=${AU_DATADIR} loadtxoutset $(pwd)/utxo-788000.dat
  ```

  You'll see some log messages about headers retrieval and waiting to see the snapshot in the headers chain. Once you get the full headers chain, you'll spend a decent amount of time (~10min) loading the snapshot, checking it, and flushing it to disk. After all that happens, you should be syncing to tip in pretty short order, and you'll see the occasional `[background validation]` log message go by.

  In yet another window, you can check out chainstate status with
  ```sh
  $ ./src/bitcoin-cli -datadir=${AU_DATADIR} getchainstates
  ```
  as well as usual favorites like `getblockchaininfo`.

ACKs for top commit:
  achow101:
    ACK edbed31066e3674ba52b8c093ab235625527f383

Tree-SHA512: 6086fb9a38dc7df85fedc76b30084dd8154617a2a91e89a84fb41326d34ef8e7d7ea593107afba01369093bf8cc91770621d98f0ea42a5b3b99db868d2f14dc2

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