Contracts: delete the canceled context on revoke

revoke left the canceled slate's private context in place when the contract
had not been signed (signing already deletes it). Look up the revoked slate id
from its tx log entry and delete the context after cancelling, if one still
exists. Resolves the stale revoke TODOs about the cancelled context.

Addresses PR #754 review (revoke.rs).

libwallet/src/contract/actions/revoke.rs

@@ -37,8 +37,6 @@ where
 	// FUTURE: we may want to boost fees in case we notice something in the mempool. There
 	// are also race conditions possible. We may not want to label txlogenry as Canceled
 	// until the new tx gets on the chain.
-	// NOTE: We should not care about deleting the context because as soon as we sign
-	// a contract, the context is deleted.
 
 	// If we contributed inputs, we must have locked them at which point we also set the
 	// OutputData.tx_log_entry which is the tx_id.
@@ -62,9 +60,27 @@ where
 		None => w.parent_key_id(),
 	};
 
+	// The revoked transaction's slate id, so we can drop its private context below.
+	let revoked_slate_id = w
+		.get_tx_log_entry(parent_key_id.clone(), tx_id)?
+		.and_then(|e| e.tx_slate_id);
+
 	// 1. Unlock the input by calling cancel_tx
 	tx::cancel_tx(&mut *w, keychain_mask, &parent_key_id, Some(tx_id), None)?;
 
+	// Drop the canceled slate's private context if one still exists (signing a contract
+	// already deletes it).
+	if let Some(slate_id) = revoked_slate_id {
+		if w
+			.get_private_context(keychain_mask, slate_id.as_bytes())
+			.is_ok()
+		{
+			let mut batch = w.batch(keychain_mask)?;
+			batch.delete_private_context(slate_id.as_bytes())?;
+			batch.commit()?;
+		}
+	}
+
 	if my_contributed_inputs.len() == 0 {
 		return Ok(None);
 	}
@@ -106,7 +122,6 @@ where
 			proof_args: Default::default(),
 		},
 	)?;
-	// TODO: Think about what to do with transaction context of the cancelled slate. It should probably get deleted.
 
 	Ok(Some(finished_slate))
 }