feat: implement simulateTransaction#36
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Cover the spec-required response shape: latestLedger as a JSON number,
minResourceFee as a stringified number, results[0].xdr carrying the host
function return value as base64 SCVal XDR, results[0].auth as an array of
base64 strings, and transactionData as valid SorobanTransactionData XDR.
Optional fields must be omitted rather than null, matching the omitempty
serialization of real stellar-rpc.
Also assert that simulation never commits (no ledger bump, no receipt, no
trace), that failures are reported as a result-level {error, latestLedger}
object without the success-only fields, that non-InvokeHostFunction
transactions are rejected with a result-level error, and that missing or
malformed transaction params return JSON-RPC error -32602.
All six tests currently fail with -32601 because the method is not
implemented yet.
Dispatch a simulateTransaction request through the K semantics like a
regular invocation, but discard the resulting configuration
(interpreter.run gains a commit flag) and write no receipt, trace, or
ledger bump. The semantics report the invocation's return value as a
JSON-encoded ScVal; the server serializes it to base64 SCVal XDR
(json_to_scval) and fills in a synthetic minResourceFee and an
empty-footprint SorobanTransactionData, since K cannot build XDR.
Failures (trapped call, unknown contract, non-invoke transaction) are
reported in the result body as {error, latestLedger}, matching real
stellar-rpc; only undecodable transaction XDR is a -32602 error.
Both errors predate this branch: komet's SCVec.val annotation is tuple[SCValue] (a 1-tuple) rather than tuple[SCValue, ...], so the call in scvalue_from_xdr needs a targeted ignore, and the server test fixture was missing its return type annotation. make check now passes.
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What
Implements the
simulateTransactionRPC method as a non-committing dry run of a contract invocation, per the Stellar RPC specification.The server decodes the base64 transaction envelope (which may be unsigned) and requires exactly one
InvokeHostFunctionoperation with an invoke-contract host function. The invocation is dispatched through the K semantics like a regularsendTransaction, but nothing is committed:interpreter.rungains acommitflag that discards the resulting configuration, and no receipt, trace, or ledger bump is written.On success the response contains:
latestLedgeras a JSON number,minResourceFeeas a stringified constant (the semantics do not meter resources),results[0].xdr— the host function's return value as base64 SCVal XDR, withauth: [],transactionData— a minimal validSorobanTransactionData(empty footprint, zero resources).Failures (trapped call, unknown contract, non-invoke transaction, upload/deploy host functions) are reported in the result body as
{error, latestLedger}, matching real stellar-rpc; only an undecodabletransactionparameter is a JSON-RPC-32602error. Optional fields are omitted rather than set tonull, matching theomitemptyserialization of the Go structs.How
Following the repo architecture, dispatch and response content live in the K semantics (
node.md): asimulateTransactionrule runs the invocation and reports the return value from the<hostStack>as a JSON-encoded ScVal, or a result-level error when the call trapped. The final XDR serialization is the one part done in Python, since K has no XDR encoder or base64 hook:json_to_scval(scval.py) converts the JSON ScVal to base64 SCVal XDR, andserver.pyfills inminResourceFeeandtransactionData. Docs (server.md,architecture.md,notes.md) describe the method and its current limitations (no auth recording, no resource metering, noevents/restorePreamble/stateChanges,ScMapreturn values not yet encoded).Testing
src/tests/integration/test_server.pycover the response shape (number/string types, base64 XDR round-trips), the no-commit guarantee (no ledger bump, no receipt, no trace), result-level errors for failed simulations and non-invoke transactions, and-32602for missing/malformed params.test_server.pysuite passes (32 tests), andmake checkis clean (including two mypy findings that predate this branch).