Ensure we never try to broadcast a transaction <= 64 bytes

While these are consensus-valid, they have been nonstandard for
quite some time and will not relay nor confirm.

diff --git a/lightning/src/events/bump_transaction.rs b/lightning/src/events/bump_transaction.rs
index 4f4e0c15c0d95cc86900234a78111a713300ef12..d05b8244da85761b37c6879de2c29dc0855444c7 100644 (file)
--- a/lightning/src/events/bump_transaction.rs
+++ b/lightning/src/events/bump_transaction.rs
@@ -582,11 +582,25 @@ where
                        // match, but we still need to have at least one output in the transaction for it to be
                        // considered standard. We choose to go with an empty OP_RETURN as it is the cheapest
                        // way to include a dummy output.
-                       log_debug!(self.logger, "Including dummy OP_RETURN output since an output is needed and a change output was not provided");
-                       tx.output.push(TxOut {
-                               value: Amount::ZERO,
-                               script_pubkey: ScriptBuf::new_op_return(&[]),
-                       });
+                       if tx.input.len() <= 1 {
+                               // Transactions have to be at least 65 bytes in non-witness data, which we can run
+                               // under if we have too few witness inputs.
+                               log_debug!(self.logger, "Including large OP_RETURN output since an output is needed and a change output was not provided and the transaction is small");
+                               debug_assert!(!tx.input.is_empty());
+                               tx.output.push(TxOut {
+                                       value: Amount::ZERO,
+                                       // Minimum transaction size is 60 bytes, so we need a 5-byte script to get a
+                                       // 65 byte transaction. We do that as OP_RETURN <3 0 bytes, plus 1 byte len>.
+                                       script_pubkey: ScriptBuf::new_op_return(&[0, 0, 0]),
+                               });
+                               debug_assert_eq!(tx.base_size(), 65);
+                       } else {
+                               log_debug!(self.logger, "Including dummy OP_RETURN output since an output is needed and a change output was not provided");
+                               tx.output.push(TxOut {
+                                       value: Amount::ZERO,
+                                       script_pubkey: ScriptBuf::new_op_return(&[]),
+                               });
+                       }
                }
        }