Save a transaction secret reported after the file exists

[j0ntz]

CHANGELOG

Does this branch warrant an entry to the CHANGELOG?

• Yes
• No

Dependencies

none

Description

A transaction's secret lives in its metadata file and nowhere else: setupNewTxMetadata writes txFile.secret, and combineTxWithFile reads txSecret back from file.secret alone. That file is written once, the first time the core sees a transaction, so a secret an engine learns any later than that was simply dropped, and a transaction already on file could never gain one.

Monero is where this bites. A Monero wallet only knows a transaction key once it has built the transaction, and it can report keys for transactions the core saved back when the engine reported none. The key is the sender's only proof of a payment, and it cannot be recovered from the seed, so dropping it is permanent.

saveTxSecret saves such a secret onto the existing file:

• It runs ahead of the unchanged-transaction check, because mergeTx does not carry the secret: a transaction that gained one still compares as unchanged and would be skipped. For the same reason it reports the transactions it saved itself, so a listener does not keep holding one with no secret.
• It reads the file from disk when fileNames lists it but files does not. Transaction files only load when somebody asks for the transactions, so the old transactions this exists for normally have no file in memory.
• It writes to disk before reporting the file as changed, so a failed write cannot leave the in-memory copy claiming a secret that never landed, which would make every later report skip the transaction.
• An existing secret is never replaced. Only the wallet that built a transaction can produce its secret, so a disagreement means the stored one is no less trustworthy, and overwriting would let a re-sync of bad data destroy a good key.

withTxFile serializes the load-modify-write cycles over a single transaction file, shared with updateCurrencyWalletTxMetadata. Both writers rewrite the whole file from a snapshot, so interleaved they could each drop the other's change.

Companion fix in the Monero engine: EdgeApp/edge-currency-accountbased#1086

Asana: https://app.asana.com/0/1215088146871429/1216965258637021

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