Make ephemeral_addresses.address unique

This also provides additional documentation for why it's necessary
to store ephemeral_addresses table entries at indicies that do not
correspond to valid addresses.

zcash_client_sqlite/src/wallet/db.rs

@@ -106,8 +106,8 @@ CREATE INDEX "addresses_accounts" ON "addresses" (
 ///
 /// All but the last `GAP_LIMIT` addresses are defined to be "reserved" addresses. Since the next
 /// index to reserve is determined by dead reckoning from the last stored address, we use dummy
-/// entries after the maximum valid index in order to allow the last `GAP_LIMIT` addresses at the
-/// end of the index range to be used.
+/// entries having `NULL` for the value of the `address` column after the maximum valid index in
+/// order to allow the last `GAP_LIMIT` addresses at the end of the index range to be used.
 ///
 /// Note that the fact that `used_in_tx` references a specific transaction is just a debugging aid.
 /// The same is mostly true of `seen_in_tx`, but we also take into account whether the referenced
@@ -116,13 +116,15 @@ pub(super) const TABLE_EPHEMERAL_ADDRESSES: &str = r#"
 CREATE TABLE ephemeral_addresses (
     account_id INTEGER NOT NULL,
     address_index INTEGER NOT NULL,
+    -- nullability of this column is controlled by the index_range_and_address_nullity check
     address TEXT,
     used_in_tx INTEGER,
     seen_in_tx INTEGER,
     FOREIGN KEY (account_id) REFERENCES accounts(id),
     FOREIGN KEY (used_in_tx) REFERENCES transactions(id_tx),
     FOREIGN KEY (seen_in_tx) REFERENCES transactions(id_tx),
     PRIMARY KEY (account_id, address_index),
+    CONSTRAINT ephemeral_addr_uniq UNIQUE (address),
     CONSTRAINT used_implies_seen CHECK (
         used_in_tx IS NULL OR seen_in_tx IS NOT NULL
     ),
@@ -135,10 +137,6 @@ CREATE TABLE ephemeral_addresses (
 // libsqlite3-sys requires at least version 3.14.0.
 // "WITHOUT ROWID" tells SQLite to use a clustered index on the (composite) primary key.
 const_assert_eq!(GAP_LIMIT, 20);
-pub(super) const INDEX_EPHEMERAL_ADDRESSES_ADDRESS: &str = r#"
-CREATE INDEX ephemeral_addresses_address ON ephemeral_addresses (
-    address ASC
-)"#;
 
 /// Stores information about every block that the wallet has scanned.
 ///

zcash_client_sqlite/src/wallet/init.rs

@@ -422,7 +422,6 @@ mod tests {
             db::INDEX_ACCOUNTS_UIVK,
             db::INDEX_HD_ACCOUNT,
             db::INDEX_ADDRESSES_ACCOUNTS,
-            db::INDEX_EPHEMERAL_ADDRESSES_ADDRESS,
             db::INDEX_NF_MAP_LOCATOR_IDX,
             db::INDEX_ORCHARD_RECEIVED_NOTES_ACCOUNT,
             db::INDEX_ORCHARD_RECEIVED_NOTES_TX,

zcash_client_sqlite/src/wallet/init/migrations/ephemeral_addresses.rs

@@ -45,24 +45,23 @@ impl<P: consensus::Parameters> RusqliteMigration for Migration<P> {
             "CREATE TABLE ephemeral_addresses (
                 account_id INTEGER NOT NULL,
                 address_index INTEGER NOT NULL,
+                -- nullability of this column is controlled by the index_range_and_address_nullity check
                 address TEXT,
                 used_in_tx INTEGER,
                 seen_in_tx INTEGER,
                 FOREIGN KEY (account_id) REFERENCES accounts(id),
                 FOREIGN KEY (used_in_tx) REFERENCES transactions(id_tx),
                 FOREIGN KEY (seen_in_tx) REFERENCES transactions(id_tx),
                 PRIMARY KEY (account_id, address_index),
+                CONSTRAINT ephemeral_addr_uniq UNIQUE (address),
                 CONSTRAINT used_implies_seen CHECK (
                     used_in_tx IS NULL OR seen_in_tx IS NOT NULL
                 ),
                 CONSTRAINT index_range_and_address_nullity CHECK (
                     (address_index BETWEEN 0 AND 0x7FFFFFFF AND address IS NOT NULL) OR
                     (address_index BETWEEN 0x80000000 AND 0x7FFFFFFF + 20 AND address IS NULL AND used_in_tx IS NULL AND seen_in_tx IS NULL)
                 )
-            ) WITHOUT ROWID;
-            CREATE INDEX ephemeral_addresses_address ON ephemeral_addresses (
-                address ASC
-            );",
+            ) WITHOUT ROWID;"
         )?;
 
         // Make sure that at least `GAP_LIMIT` ephemeral transparent addresses are

zcash_client_sqlite/src/wallet/transparent/ephemeral.rs

@@ -282,14 +282,18 @@ fn reserve_until<P: consensus::Parameters>(
     )?;
 
     for raw_index in range_to_store {
-        let address_str_opt = match NonHardenedChildIndex::from_index(raw_index) {
-            Some(address_index) => Some(
+        // The range to store may contain indicies that are out of the valid range of non hardened
+        // child indices; we still store explicit rows in the ephemeral_addresses table for these
+        // so that it's possible to find the first unused address using dead reckoning with the gap
+        // limit.
+        let address_str_opt = NonHardenedChildIndex::from_index(raw_index)
+            .map(|address_index| {
                 ephemeral_ivk
-                    .derive_ephemeral_address(address_index)?
-                    .encode(params),
-            ),
-            None => None,
-        };
+                    .derive_ephemeral_address(address_index)
+                    .map(|addr| addr.encode(params))
+            })
+            .transpose()?;
+
         stmt_insert_ephemeral_address.execute(named_params![
             ":account_id": account_id.0,
             ":address_index": raw_index,