Fix non-canonical inline datum JSON round-trip

[v0d1ch]

● Fix non-canonical inline datum JSON round-trip

Problem

FromJSON (TxOut) crashes when parsing a TxOut whose inline datum was encoded with non-canonical CBOR bytes (e.g. definite-length arrays instead of the indefinite-length form Plutus normally emits).

The error:
"Inline datum not equivalent to inline datum hash"

This happens because ToJSON and FromJSON disagree on how to reconstruct HashableScriptData:

• ToJSON serialises the inline datum as three fields:
  • inlineDatum — the human-readable JSON representation
  • inlineDatumhash — blake2b_256(originalCborBytes)
  • inlineDatumRaw — the original CBOR bytes as hex
• FromJSON ignored inlineDatumRaw entirely and reconstructed HashableScriptData via scriptDataFromJson, which always re-serialises to canonical CBOR. For non-canonical original bytes:
  • H(canonical) ≠ H(original) = inlineDatumhash
  • → parse fails with the above error

Non-canonical Plutus CBOR is valid and accepted by the ledger. It arises when scripts use definite-length array encoding for constructor fields instead of the canonical indefinite-length form. Any node that stores such a UTxO entry and later replays it from a JSON snapshot (e.g. from a SQLite event log) would crash on
deserialisation.

Fix

inlineDatumRaw is always present in JSON produced by ToJSON (as null for non-inline outputs, as hex for inline ones). FromJSON now reads this field first and uses deserialiseFromCBOR AsHashableScriptData to reconstruct HashableScriptData with the original bytes preserved. The scriptDataFromJson path is kept as a fallback for JSON
produced by external tools or older versions that may not include inlineDatumRaw.

The fix is applied to all six affected sites: Babbage, Conway, and Dijkstra eras in both FromJSON (TxOut CtxTx era) and FromJSON (TxOut CtxUTxO era). The repeated logic is extracted into a parseInlineDatum helper.

Testing

Added prop_json_roundtrip_txout_noncanonical_inline_datum — a property test that generates a TxOut with a non-canonical inline datum (definite-length CBOR constructor) and asserts the JSON round-trip is identity. The test fails without the fix and passes with it.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff
• Changelog fragment added in .changes/

[Copilot]

Pull request overview

Fixes a JSON round-trip bug for TxOut values containing inline datums whose underlying CBOR bytes are non-canonical (yet ledger-valid), by ensuring FromJSON preserves the original datum bytes when inlineDatumRaw is available.

Changes:

• Add parseInlineDatum helper to prefer decoding HashableScriptData from inlineDatumRaw CBOR bytes, with a JSON-based fallback for older/external JSON.
• Apply the inline-datum parsing fix across Babbage/Conway/Dijkstra for both CtxTx and CtxUTxO FromJSON (TxOut ...) instances.
• Add a property test (plus generator) that constructs a non-canonical inline datum and asserts JSON round-trip identity.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
cardano-api/test/cardano-api-test/Test/Cardano/Api/Json.hs	Adds a property test covering JSON round-tripping of TxOut with non-canonical inline datum CBOR.
cardano-api/src/Cardano/Api/Tx/Internal/Output.hs	Introduces parseInlineDatum and updates FromJSON (TxOut ...) to preserve original inline datum CBOR via inlineDatumRaw.
cardano-api/gen/Test/Gen/Cardano/Api/Typed.hs	Adds a generator for non-canonical HashableScriptData used by the new property test.

[Crypto2099]

Thanks for getting on this so quickly @v0d1ch! This is currently causing my hydra heads to fail to restart when a non-canonical datum is present in the ledger and the hydra-node crashes for any reason and attempts to validate its snapshot state from the state database.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.