devtools: add utxo_snapshot.sh

to allow easy (if not time-consuming) generation and verification of
snapshots.

contrib/devtools/utxo_snapshot.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) 2019 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+export LC_ALL=C
+
+set -ueo pipefail
+
+if (( $# < 3 )); then
+  echo 'Usage: utxo_snapshot.sh <generate-at-height> <snapshot-out-path> <bitcoin-cli-call ...>'
+  echo
+  echo "  if <snapshot-out-path> is '-', don't produce a snapshot file but instead print the "
+  echo "  expected assumeutxo hash"
+  echo
+  echo 'Examples:'
+  echo
+  echo "  ./contrib/devtools/utxo_snapshot.sh 570000 utxo.dat ./src/bitcoin-cli -datadir=\$(pwd)/testdata"
+  echo '  ./contrib/devtools/utxo_snapshot.sh 570000 - ./src/bitcoin-cli'
+  exit 1
+fi
+
+GENERATE_AT_HEIGHT="${1}"; shift;
+OUTPUT_PATH="${1}"; shift;
+# Most of the calls we make take a while to run, so pad with a lengthy timeout.
+BITCOIN_CLI_CALL="${*} -rpcclienttimeout=9999999"
+
+# Block we'll invalidate/reconsider to rewind/fast-forward the chain.
+PIVOT_BLOCKHASH=$($BITCOIN_CLI_CALL getblockhash $(( GENERATE_AT_HEIGHT + 1 )) )
+
+(>&2 echo "Rewinding chain back to height ${GENERATE_AT_HEIGHT} (by invalidating ${PIVOT_BLOCKHASH}); this may take a while")
+${BITCOIN_CLI_CALL} invalidateblock "${PIVOT_BLOCKHASH}"
+
+if [[ "${OUTPUT_PATH}" = "-" ]]; then
+  (>&2 echo "Generating txoutset info...")
+  ${BITCOIN_CLI_CALL} gettxoutsetinfo | grep hash_serialized_2 | sed 's/^.*: "\(.\+\)\+",/\1/g'
+else
+  (>&2 echo "Generating UTXO snapshot...")
+  ${BITCOIN_CLI_CALL} dumptxoutset "${OUTPUT_PATH}"
+fi
+
+(>&2 echo "Restoring chain to original height; this may take a while")
+${BITCOIN_CLI_CALL} reconsiderblock "${PIVOT_BLOCKHASH}"