Correctly decode empty bar responses

As it turns out the Alpaca Data API does not strictly adhere to always
returning an array of bars in the response to a bars request. Rather, if
no data is returned it will just set the member to null. This value
currently causes a decode error, as serde_json does not treat that as a
valid variant for an array.
With this change we fix the problem by explicitly using the vec_from_str
function for decoding purposes. This function correctly handles such
upstream values.

CHANGELOG.md

@@ -1,3 +1,9 @@
+Unreleased
+----------
+- Fixed JSON decoding error when no bars are returned in response to
+  `data::v2::bars::Get` request
+
+
 0.22.1
 ------
 - Added support for realtime market data streaming via

src/data/v2/bars.rs

@@ -11,6 +11,7 @@ use serde::Serialize;
 use serde_urlencoded::to_string as to_query;
 
 use crate::data::DATA_BASE_URL;
+use crate::util::vec_from_str;
 use crate::Str;
 
 
@@ -107,6 +108,7 @@ pub struct Bar {
 #[non_exhaustive]
 pub struct Bars {
   /// The list of returned bars.
+  #[serde(deserialize_with = "vec_from_str")]
   pub bars: Vec<Bar>,
   /// The symbol the bars correspond to.
   pub symbol: String,
@@ -198,6 +200,27 @@ mod tests {
     assert!(res.next_page_token.is_some())
   }
 
+  /// Check that we can decode a response containing no bars correctly.
+  #[test(tokio::test)]
+  async fn no_bars() {
+    let api_info = ApiInfo::from_env().unwrap();
+    let client = Client::new(api_info);
+    let start = Utc.ymd(2021, 11, 5).and_hms_milli(0, 0, 0, 0);
+    let end = Utc.ymd(2021, 11, 5).and_hms_milli(0, 0, 0, 0);
+
+    let request = BarReq {
+      symbol: "SPY".to_string(),
+      limit: None,
+      start,
+      end,
+      timeframe: TimeFrame::OneDay,
+      page_token: None,
+      adjustment: None,
+    };
+    let res = client.issue::<Get>(&request).await.unwrap();
+    assert_eq!(res.bars, Vec::new())
+  }
+
   /// Check that we can request historic bar data for a stock.
   #[test(tokio::test)]
   async fn request_bars() {