Missing access to transaction input Datums in on-chain code

[mputz86]

Area

[X] Plutus Foundation Related to the GHC plugin, Haskell-to-Plutus compiler, on-chain code
[] Plutus Application Framework Related to the Plutus application backend (PAB), emulator, Plutus libraries
[] Marlowe Related to Marlowe
[] Other Any other topic (Playgrounds, etc.)

Summary

When validating a spending transaction, the Datums of the spent inputs are not available via ScriptContext.
Precisely:

• The txOutDatumHash in the txInfoInputs (from ScriptContext) gives back a hash, so Just ...
• But using findDatum in Plutus.V1.Ledger.Contexts returns Nothing
• So the data was not added to txInfoData
  • Can be verified by viewing the Tx returned by submiTx...

Steps to reproduce

(see attachments for an example demonstrating the issue)

• Build a contract which creates two UTxO with a Datum
• Create another transaction which spends both UTxO and
• try to access all Datum of all input UTxO via the ScriptContext

Expected behavior

1. According to a discussion with @brunjlar (pioneer discord), all input Datum should be available per default via the ScriptContext in on-chain validation.
2. Maybe it would be worth to update the documentation on txInfoData in TxInfo (there is none at the moment)

System info (please complete the following information):

• OS: macOS Big Sur
• Version: 11.2.3
• Plutus version or commit hash: master branch, commit, 2087467

Screenshots and attachments

• See InputTxDatumsIssue.hs for an example which
  • Reproduces the error
  • Note: Latest version of this file already applies the workaround from the next section

Additional context

• Workaround: By using mustIncludeDatum, the Datum of the inputs can be made available to the transaction and be validated on-chain (by using the findDatum). See latest InputTxDatumsIssue.hs

[ghost]

@mputz86 Thanks for reporting this! Especially for prodiving the repository with code. :)

You can already test the fix in 965aa32 as it will take some time to land into master.

[mputz86]

@ak3n Thanks for the fast fix, works like a charm 😊 👍 .