Fix decoding events via .as_root_event() and add test (#767)

* fix decoding events via as_root_event and add test

* fmt and clippy

* Update subxt/src/events/events_type.rs

Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

subxt/src/events/events_type.rs

@@ -201,10 +201,12 @@ pub struct EventDetails {
     all_bytes: Arc<[u8]>,
     // start of the bytes (phase, pallet/variant index and then fields and then topic to follow).
     start_idx: usize,
-    // start of the fields (ie after phase nad pallet/variant index).
-    fields_start_idx: usize,
+    // start of the event (ie pallet/variant index and then the fields and topic after).
+    event_start_idx: usize,
+    // start of the fields (ie after phase and pallet/variant index).
+    event_fields_start_idx: usize,
     // end of the fields.
-    fields_end_idx: usize,
+    event_fields_end_idx: usize,
     // end of everything (fields + topics)
     end_idx: usize,
     metadata: Metadata,
@@ -221,10 +223,13 @@ impl EventDetails {
         let input = &mut &all_bytes[start_idx..];
 
         let phase = Phase::decode(input)?;
+
+        let event_start_idx = all_bytes.len() - input.len();
+
         let pallet_index = u8::decode(input)?;
         let variant_index = u8::decode(input)?;
 
-        let fields_start_idx = all_bytes.len() - input.len();
+        let event_fields_start_idx = all_bytes.len() - input.len();
 
         // Get metadata for the event:
         let event_metadata = metadata.event(pallet_index, variant_index)?;
@@ -246,7 +251,7 @@ impl EventDetails {
         }
 
         // the end of the field bytes.
-        let fields_end_idx = all_bytes.len() - input.len();
+        let event_fields_end_idx = all_bytes.len() - input.len();
 
         // topics come after the event data in EventRecord. They aren't used for
         // anything at the moment, so just decode and throw them away.
@@ -259,8 +264,9 @@ impl EventDetails {
             phase,
             index,
             start_idx,
-            fields_start_idx,
-            fields_end_idx,
+            event_start_idx,
+            event_fields_start_idx,
+            event_fields_end_idx,
             end_idx,
             all_bytes,
             metadata,
@@ -281,14 +287,14 @@ impl EventDetails {
     pub fn pallet_index(&self) -> u8 {
         // Note: never panics; we expect these bytes to exist
         // in order that the EventDetails could be created.
-        self.all_bytes[self.fields_start_idx - 2]
+        self.all_bytes[self.event_fields_start_idx - 2]
     }
 
     /// The index of the event variant that the event originated from.
     pub fn variant_index(&self) -> u8 {
         // Note: never panics; we expect these bytes to exist
         // in order that the EventDetails could be created.
-        self.all_bytes[self.fields_start_idx - 1]
+        self.all_bytes[self.event_fields_start_idx - 1]
     }
 
     /// The name of the pallet from whence the Event originated.
@@ -319,7 +325,7 @@ impl EventDetails {
 
     /// Return the bytes representing the fields stored in this event.
     pub fn field_bytes(&self) -> &[u8] {
-        &self.all_bytes[self.fields_start_idx..self.fields_end_idx]
+        &self.all_bytes[self.event_fields_start_idx..self.event_fields_end_idx]
     }
 
     /// Decode and provide the event fields back in the form of a [`scale_value::Composite`]
@@ -384,7 +390,7 @@ impl EventDetails {
     /// the pallet and event enum variants as well as the event fields). A compatible
     /// type for this is exposed via static codegen as a root level `Event` type.
     pub fn as_root_event<E: Decode>(&self) -> Result<E, CodecError> {
-        E::decode(&mut self.bytes())
+        E::decode(&mut &self.all_bytes[self.event_start_idx..self.event_fields_end_idx])
     }
 }
 
@@ -503,6 +509,7 @@ mod tests {
             event_record,
             events,
             events_raw,
+            AllEvents,
         },
         *,
     };
@@ -572,6 +579,39 @@ mod tests {
         assert_eq!(actual_fields_no_context, expected.fields);
     }
 
+    #[test]
+    fn statically_decode_single_root_event() {
+        #[derive(Clone, Debug, PartialEq, Decode, Encode, TypeInfo)]
+        enum Event {
+            A(u8, bool, Vec<String>),
+        }
+
+        // Create fake metadata that knows about our single event, above:
+        let metadata = metadata::<Event>();
+
+        // Encode our events in the format we expect back from a node, and
+        // construst an Events object to iterate them:
+        let event = Event::A(1, true, vec!["Hi".into()]);
+        let events = events::<Event>(
+            metadata,
+            vec![event_record(Phase::ApplyExtrinsic(123), event.clone())],
+        );
+
+        let ev = events
+            .iter()
+            .next()
+            .expect("one event expected")
+            .expect("event should be extracted OK");
+
+        // This is the line we're testing:
+        let decoded_event = ev
+            .as_root_event::<AllEvents<Event>>()
+            .expect("can decode event into root enum again");
+
+        // It should equal the event we put in:
+        assert_eq!(decoded_event, AllEvents::Test(event));
+    }
+
     #[test]
     fn dynamically_decode_single_event() {
         #[derive(Clone, Debug, PartialEq, Decode, Encode, TypeInfo)]