Add reusable token balance query method

.changelog/unreleased/improvements/1173-token-balance-query.md

@@ -0,0 +1,2 @@
+- Added a reusable token balance query method.
+  ([\#1173](https://github.com/anoma/namada/pull/1173))

apps/src/lib/client/rpc.rs

@@ -1098,7 +1098,7 @@ pub async fn get_token_balance<C: namada::ledger::queries::Client + Sync>(
     client: &C,
     token: &Address,
     owner: &Address,
-) -> Option<token::Amount> {
+) -> token::Amount {
     namada::ledger::rpc::get_token_balance(client, token, owner).await
 }
 

apps/src/lib/client/tx.rs

@@ -702,8 +702,7 @@ where
 
         let balance =
             rpc::get_token_balance(client, &ctx.native_token, &proposal.author)
-                .await
-                .unwrap_or_default();
+                .await;
         if balance
             < token::Amount::from_uint(
                 governance_parameters.min_proposal_fund,

shared/src/ledger/queries/vp/token.rs

@@ -1,3 +1,5 @@
+//! Token validity predicate queries
+
 use namada_core::ledger::storage::{DBIter, StorageHasher, DB};
 use namada_core::ledger::storage_api;
 use namada_core::ledger::storage_api::token::read_denom;
@@ -41,3 +43,40 @@ where
 {
     read_denom(ctx.wl_storage, &addr, None)
 }
+
+#[cfg(any(test, feature = "async-client"))]
+pub mod client_only_methods {
+    use borsh::BorshDeserialize;
+
+    use super::Token;
+    use crate::ledger::queries::{Client, RPC};
+    use crate::types::address::Address;
+    use crate::types::token;
+
+    impl Token {
+        /// Get the balance of the given `token` belonging to the given `owner`.
+        pub async fn balance<CLIENT>(
+            &self,
+            client: &CLIENT,
+            token: &Address,
+            owner: &Address,
+        ) -> Result<token::Amount, <CLIENT as Client>::Error>
+        where
+            CLIENT: Client + Sync,
+        {
+            let balance_key = token::balance_key(token, owner);
+            let response = RPC
+                .shell()
+                .storage_value(client, None, None, false, &balance_key)
+                .await?;
+
+            let balance = if response.data.is_empty() {
+                token::Amount::default()
+            } else {
+                token::Amount::try_from_slice(&response.data)
+                    .unwrap_or_default()
+            };
+            Ok(balance)
+        }
+    }
+}

shared/src/ledger/rpc.rs

@@ -36,7 +36,6 @@ use crate::types::governance::{ProposalVote, VotePower};
 use crate::types::hash::Hash;
 use crate::types::key::*;
 use crate::types::storage::{BlockHeight, BlockResults, Epoch, PrefixValue};
-use crate::types::token::balance_key;
 use crate::types::{storage, token};
 
 /// Query the status of a given transaction.
@@ -137,9 +136,10 @@ pub async fn get_token_balance<C: crate::ledger::queries::Client + Sync>(
     client: &C,
     token: &Address,
     owner: &Address,
-) -> Option<token::Amount> {
-    let balance_key = balance_key(token, owner);
-    query_storage_value(client, &balance_key).await
+) -> token::Amount {
+    unwrap_client_response::<C, _>(
+        RPC.vp().token().balance(client, token, owner).await,
+    )
 }
 
 /// Get account's public key stored in its storage sub-space