fix: encode empty Bytes as empty hex string in receipts

Base2String yields "0" for the zero-length case, an odd-length non-hex
value that broke the receipt's XDR rewrite for contracts returning empty
Bytes. Add a dedicated K rule mapping empty Bytes to the empty string.
Also harden _attach_result_xdr: persist the receipt without the internal
returnValue field even when the rewrite fails, so a stored receipt never
leaks K-internal fields to getTransaction.

Author: RaoulSchaffranek

src/komet_node/kdist/node.md

@@ -497,13 +497,16 @@ Values with no JSON encoding (e.g. `Error`) render as `null`, i.e. as "no return
 ```
 
 `#bytesToHex` is the inverse of `HexBytes`: lowercase hex, two digits per byte (zero-padded,
-since Base2String drops leading zeroes).
+since Base2String drops leading zeroes). Empty bytes encode as the empty string — the
+general rule would yield `"0"` (Base2String of 0), which `#padZeros` cannot trim.
 
 ```k
     syntax String ::= #bytesToHex( Bytes )      [function, total, symbol(bytesToHex)]
                     | #padZeros( String, Int )  [function, total, symbol(padZeros)]
  // --------------------------------------------------------------------------------
+    rule #bytesToHex( B ) => "" requires lengthBytes(B) ==Int 0
     rule #bytesToHex( B ) => #padZeros( Base2String( Bytes2Int(B, BE, Unsigned), 16 ), 2 *Int lengthBytes(B) )
+      requires lengthBytes(B) >Int 0
 
     rule #padZeros( S, N ) => #padZeros( "0" +String S, N ) requires lengthString(S) <Int N
     rule #padZeros( S, _ ) => S                              [owise]

src/komet_node/server.py

@@ -258,11 +258,17 @@ def _attach_result_xdr(self, tx_hash: str) -> None:
         receipt_file = self.receipts_dir / f'receipt_{tx_hash}.json'
         receipt = json.loads(receipt_file.read_text())
         return_value_json = receipt.pop('returnValue', None)
-        return_value = scval_from_json(return_value_json) if return_value_json is not None else None
-        tx_envelope = TransactionEnvelope.from_xdr(receipt['envelopeXdr'], self.encoder.network_passphrase)
-        receipt['resultXdr'] = transaction_result_xdr(tx_envelope, return_value, success=True)
-        receipt['resultMetaXdr'] = transaction_meta_xdr(tx_envelope, return_value)
-        receipt_file.write_text(json.dumps(receipt))
+        try:
+            return_value = scval_from_json(return_value_json) if return_value_json is not None else None
+            tx_envelope = TransactionEnvelope.from_xdr(receipt['envelopeXdr'], self.encoder.network_passphrase)
+            receipt['resultXdr'] = transaction_result_xdr(tx_envelope, return_value, success=True)
+            receipt['resultMetaXdr'] = transaction_meta_xdr(tx_envelope, return_value)
+        finally:
+            # Persist the receipt even when the rewrite fails (e.g. a return value this
+            # encoder cannot decode): the transaction has already committed, and the stored
+            # receipt must never keep the K-internal `returnValue` field. A receipt with
+            # `resultXdr` omitted is spec-legal; a leaked internal field is not.
+            receipt_file.write_text(json.dumps(receipt))
 
     def _failure_response(self, rpc_id: Any, envelope: dict[str, Any], now: str) -> dict[str, Any]:
         """Synthesise the sendTransaction response for a transaction that got stuck (failed).