Fix EvictedQueue bug with zero capacity

The EvictedQueue was checking for the length _before_ inserting, and
popping extra items, then doing the insertion. In the case where the
capacity is set to zero, it caused the pop operation to be a no-op on
the first insert, and then insert an item anyway.

This commit fixes the issue by moving the length check after the insert
and popping any extra items.

Fixes open-telemetry#1148

Author: wperron

opentelemetry-sdk/CHANGELOG.md

@@ -9,6 +9,11 @@
 - Propagate shutdown calls from `PeriodicReader` to metrics exporter
   [#1138](https://github.com/open-telemetry/opentelemetry-rust/pull/1138).
 
+### Changed
+
+- Fix EvictedQueue bug when capacity is set to 0
+  [#1151](https://github.com/open-telemetry/opentelemetry-rust/pull/1151).
+
 ### Removed
 
 - Samplers no longer has access to `InstrumentationLibrary` as one of parameters

opentelemetry-sdk/src/trace/evicted_queue.rs

@@ -26,11 +26,12 @@ impl<T> EvictedQueue<T> {
     /// recording dropped count if over capacity.
     pub(crate) fn push_back(&mut self, value: T) {
         let queue = self.queue.get_or_insert_with(Default::default);
-        if queue.len() as u32 == self.max_len {
+        queue.push_back(value);
+
+        if queue.len() as u32 > self.max_len {
             queue.pop_front();
             self.dropped_count += 1;
         }
-        queue.push_back(value);
     }
 
     /// Moves all the elements of other into self, leaving other empty.
@@ -119,4 +120,19 @@ mod tests {
             (1..=capacity).collect::<VecDeque<_>>()
         );
     }
+
+    #[test]
+    fn zero_capacity_test() {
+        let capacity = 0;
+        let mut queue = EvictedQueue::new(capacity);
+
+        queue.push_back(1);
+
+        assert_eq!(queue.dropped_count, 1);
+        assert_eq!(queue.len(), capacity as usize);
+        assert_eq!(
+            queue.queue.unwrap(),
+            (1..=capacity).collect::<VecDeque<_>>()
+        );
+    }
 }