Merge bitcoin/bitcoin#29215: test: assumeutxo: spend coin from snapshot chainstate after loading

931575418e test: assumeutxo: spend coin from snapshot chainstate after loading (Sebastian Falbesoner)

Pull request description:

  This PR extends the AssumeUTXO functional test by submitting a spending transaction for an UTXO that is only available in a the snapshot chainstate (after loading via `loadtxoutset`), i.e. it hasn't been seen in a block before. With that we can verify that snapshot coins are visible to the mempool.

  Note that we unfortunately can't use MiniWallet here, as the only available UTXO to spend from the snapshot chainstate is at height 200, where a P2PKH created from the test framework's deterministic private key is used (see `TestNode.generate(...)` and the `PRIV_KEYS` array). Coinbase outputs with smaller heights (<= 199) would be part of the pre-generated chain and hence not qualify for the "UTXO is only in snapshot chainstate and has never been seen in a block" scenario, coinbase outputs with larger heights (>= 201) can't be spent due to immaturity, as the snapshot chainstate block height is 299.

  One could of course mine a different chain with outputs that MiniWallet supports (e.g. taproot anyone-can-spend), but this would change the hardcoded AssumeUTXO hash, colliding with other PRs like #28838, so I wanted to avoid that.

ACKs for top commit:
  maflcko:
    lgtm ACK 931575418e
  jamesob:
    ACK 931575418e

Tree-SHA512: 0665868e1e91fe74f408d0a239cc264bbbc11a6b55bcc0e86cc8b4b2ec1f44977884b817dbe9065a7c768332cab464636656858bc8b9c8e7d7810498e0a17d78

test/functional/feature_assumeutxo.py

@@ -11,7 +11,6 @@ The assumeutxo value generated and used here is committed to in
 
 ## Possible test improvements
 
-- TODO: test submitting a transaction and verifying it appears in mempool
 - TODO: test what happens with -reindex and -reindex-chainstate before the
       snapshot is validated, and make sure it's deleted successfully.
 
@@ -35,11 +34,14 @@ Interesting starting states could be loading a snapshot when the current chain t
 """
 from shutil import rmtree
 
+from test_framework.messages import tx_from_hex
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
     assert_raises_rpc_error,
 )
+from test_framework.wallet import getnewdestination
+
 
 START_HEIGHT = 199
 SNAPSHOT_BASE_HEIGHT = 299
@@ -207,6 +209,22 @@ class AssumeutxoTest(BitcoinTestFramework):
 
         assert_equal(n1.getblockchaininfo()["blocks"], SNAPSHOT_BASE_HEIGHT)
 
+        self.log.info("Submit a spending transaction for a snapshot chainstate coin to the mempool")
+        # spend the coinbase output of the first block that is not available on node1
+        spend_coin_blockhash = n1.getblockhash(START_HEIGHT + 1)
+        assert_raises_rpc_error(-1, "Block not found on disk", n1.getblock, spend_coin_blockhash)
+        prev_tx = n0.getblock(spend_coin_blockhash, 3)['tx'][0]
+        prevout = {"txid": prev_tx['txid'], "vout": 0, "scriptPubKey": prev_tx['vout'][0]['scriptPubKey']['hex']}
+        privkey = n0.get_deterministic_priv_key().key
+        raw_tx = n1.createrawtransaction([prevout], {getnewdestination()[2]: 24.99})
+        signed_tx = n1.signrawtransactionwithkey(raw_tx, [privkey], [prevout])['hex']
+        signed_txid = tx_from_hex(signed_tx).rehash()
+
+        assert n1.gettxout(prev_tx['txid'], 0) is not None
+        n1.sendrawtransaction(signed_tx)
+        assert signed_txid in n1.getrawmempool()
+        assert not n1.gettxout(prev_tx['txid'], 0)
+
         PAUSE_HEIGHT = FINAL_HEIGHT - 40
 
         self.log.info("Restarting node to stop at height %d", PAUSE_HEIGHT)