validate-pending-transactions

#!/bin/bash

# Applies valid pending transaction to the ledger.
# This applies transactions to the ledger but does not mine a new block.
# Transactions are applied by checking the signature and balance of the sender.
#
# XXX: This file shows the weaknesses of doing all of this in bash and relying
# on files to store data.

set -euo pipefail

source ${BASH_SOURCE%/*}/.helpers.sh

# Extract a transactions data to shell variables.
readonly extract_filter='@sh "
local readonly from=\(.from)
local readonly to=\(.to)
local readonly amount=\(.amount)
local readonly signature=\(.signature)
"'

main() {
  if [ ! -f "$pending" ]; then
    echo "No transactions pending"
    exit 0
  fi

  # XXX: The verified transaction list for every block should only include
  # transactions form that block so at some point we have to clear the old list.
  # We'll clear it now only if it hasn't been changed.
  git diff-index --quiet HEAD "$verified" && : > "$verified"

  # Copy pending transactions to a temporary location so we can add invalid
  # transactions back to pending.
  # N.B. This is dumb.
  working=$(mktemp)
  cleanup() {
    rm "$working"
  }
  trap cleanup EXIT
  mv "$pending" "$working"

  # Try to apply each transaction, putting failed transactions back into pending.
  # A transaction may fail to apply because the sender doesn't have enough coin.
  # We'll put it back in pending in case someone gives them money :).
  # A transaction will fail to apply if the signature is invalid. We really
  # shouldn't re-append those.
  while read transaction; do
    apply "$transaction" || echo "$transaction" >> "$pending"
  done < "$working"

  echo "done"
}

apply() {
  transaction="$1"
  eval "$(echo "$transaction" | jq -r "$extract_filter")"
  echo "$from sent $amount coins to $to (verifing...)"

  # verify that the transaction was created by someone that knew
  # the private key for the from address.
  echo -n "Signature "
  verify "$from" "$signature" "$from $to $amount"

  # check balance
  ledger_balance=$(jq -er --arg acc "$from" '.[$acc] // 0' "$ledger")
  new=$((ledger_balance - amount))

  if [ "$new" -lt 0 ]; then
    echo "$from is overdrawn $new, rejecting transaction"
    return 1
  fi

  # modify ledger
  new_ledger="$(jq -er \
    --arg from "$from" \
    --arg to "$to" \
    --arg amount "$amount" \
    '  .[$from] = (.[$from] // 0) - ($amount | tonumber)
     | .[$to] = (.[$to] // 0) + ($amount | tonumber)
     ' $ledger)"

  echo "$new_ledger" > "$ledger"
  echo "$transaction" >> "$verified"
}

main "$@"