Taker Client Fails to Recover from Transaction Broadcast Failure

[DavidRajnoha]

Summary

When a taker client fails to broadcast a transaction due to spent inputs (bad-txns-inputs-missingorspent), the retry mechanism never triggers, leaving the client in a zombie state that polls indefinitely without recovery.

Disclaimer

The issue has been encountered within an emulated environment (https://github.com/DavidRajnoha/coinjoin-simulator) – the conditions under which the bug occurs (availability of makers, frequency of taker coinjoin requests) might be different from real environment.

Steps to reproduce

1. Start tumbler/RPC coinjoin
2. Maker's UTXO gets spent between negotiation and broadcast
3. Broadcast fails: error pushing = -25 bad-txns-inputs-missingorspent
4. RPC Client becomes unresponsive to further rpc commands

Probable root cause

Missing Callback in handle_unbroadcast_transaction()

Location: src/jmclient/taker.py:932-934

  if not self.push_ourselves():
      jlog.error("Failed to broadcast transaction: ")
      jlog.info(btc.human_readable_transaction(tx))
      # MISSING: self.on_finished_callback(False, fromtx=True)

When self-broadcast fails, the error is logged but on_finished_callback() is never called to trigger retry logic.

Compare to push() method (lines 981-982) which correctly handles failure:

  if not pushed:
      self.on_finished_callback(False, fromtx=True)  # Triggers retry

Issue 2: Flawed Stall Monitor Logic

Location: src/jmclient/client_protocol.py:645-652

  if not self.client.txid:
      jlog.info("Stall detected. Retrying transaction if possible ...")
      self.client.on_finished_callback(False, True, 0.0)
  else:
      jlog.info("Tx was already pushed; ignoring")

Problem: Assumes if txid is set, broadcast succeeded. However, txid is calculated and set before broadcast attempt (line 941 in push()), so it's set regardless of success/failure.

Logs

  06:13:01 [INFO]  all makers have sent their signatures
  06:13:02 [INFO]  txid = 8062310600b7db8f...  # txid set BEFORE broadcast
  06:16:02 [WARN]  error pushing = -25 bad-txns-inputs-missingorspent
  06:16:02 [ERROR] Failed to broadcast transaction:  # No callback triggered!
  06:30:00 [INFO]  STALL MONITOR: Tx was already pushed; ignoring  # Wrong!
  06:31:24 [DEBUG] rpc: getwalletinfo []  # Called by external client, polls forever...