zcash_client_sqlite: Read rcm correctly from data DB after Canopy

ZIP 212 alters the note plaintext to store a seed from which rcm is
derived, rather than storing rcm directly. In the mobile SDKs we only
need rcm, so for post-ZIP 212 notes, we derive rcm from the seed and
store rcm in the data DB.

However, when selecting notes to spend, `create_to_address` was using the
transaction's target height to determine if Canopy is active, and parsing
the rcm value as the seed if so. This effectively applied a seed->rcm
derivation to all selected notes' rcms once Canopy activated on the
chain. As a result, the note commitments were incorrect, and thus the
anchors derived from the witness paths were also incorrect. This caused
two kinds of observed failures:

- If more than one note was selected, the builder would fail with
  "anchor mismatch", as the note commitments would be effectively
  randomised, causing the derived anchors to also randomise.
- If a single note was selected, the transaction would be built using
  the randomised anchor, and then rejected when sent to the network.

The fix is to "pretend" in `create_to_address` that all notes are
pre-ZIP 212 notes. This works fine because we never need to serialize
back to the note plaintext while spending a note.

zcash_client_sqlite/src/transact.rs

@@ -7,7 +7,7 @@ use std::convert::TryInto;
 use std::path::Path;
 use zcash_client_backend::encoding::encode_extended_full_viewing_key;
 use zcash_primitives::{
-    consensus::{self, NetworkUpgrade, Parameters},
+    consensus,
     keys::OutgoingViewingKey,
     merkle_tree::{IncrementalWitness, MerklePath},
     note_encryption::Memo,
@@ -227,19 +227,16 @@ pub fn create_to_address<P: AsRef<Path>>(
             let rseed = {
                 let d: Vec<_> = row.get(2)?;
 
-                if Network::is_nu_active(NetworkUpgrade::Canopy, height) {
-                    let mut r = [0u8; 32];
-                    r.copy_from_slice(&d[..]);
-                    Rseed::AfterZip212(r)
-                } else {
-                    let r = jubjub::Fr::from_repr(
-                        d[..]
-                            .try_into()
-                            .map_err(|_| Error(ErrorKind::InvalidNote))?,
-                    )
-                    .ok_or(Error(ErrorKind::InvalidNote))?;
-                    Rseed::BeforeZip212(r)
-                }
+                // We store rcm directly in the data DB, regardless of whether the note
+                // used a v1 or v2 note plaintext, so for the purposes of spending let's
+                // pretend this is a pre-ZIP 212 note.
+                let r = jubjub::Fr::from_repr(
+                    d[..]
+                        .try_into()
+                        .map_err(|_| Error(ErrorKind::InvalidNote))?,
+                )
+                .ok_or(Error(ErrorKind::InvalidNote))?;
+                Rseed::BeforeZip212(r)
             };
 
             let from = extfvk.fvk.vk.to_payment_address(diversifier).unwrap();