Handle special case

Author: fraillt

opentelemetry-sdk/src/metrics/internal/mod.rs

@@ -111,7 +111,10 @@ where
 
         let sorted_count = sorted_trackers.len();
         let new_tracker = match sorted_trackers.entry(sorted_attrs) {
-            Entry::Occupied(occupied_entry) => occupied_entry.get().clone(),
+            Entry::Occupied(occupied_entry) => {
+                // do not return early, because collection phase might clear `all_trackers` multiple times
+                occupied_entry.get().clone()
+            }
             Entry::Vacant(vacant_entry) => {
                 if !is_under_cardinality_limit(sorted_count) {
                     sorted_trackers.entry(STREAM_OVERFLOW_ATTRIBUTES.clone())
@@ -197,8 +200,28 @@ where
             ));
         }
 
-        for (attrs, tracker) in to_collect.drain() {
-            let tracker = Arc::into_inner(tracker).expect("the only instance");
+        for (attrs, mut tracker) in to_collect.drain() {
+            // Handles special case:
+            // measure-thread: get inserted tracker from `sorted_attribs` (holds tracker)
+            // collect-thread: replace sorted_attribs (clears sorted_attribs)
+            // collect-thread: clear all_attribs
+            // collect_thread: THIS-LOOP: loop until measure-thread still holds a tracker
+            // measure-thread: insert tracker into `all_attribs``
+            // collect_thread: exits this loop after clearing trackers
+            let tracker = loop {
+                match Arc::try_unwrap(tracker) {
+                    Ok(inner) => {
+                        break inner;
+                    }
+                    Err(reinserted) => {
+                        tracker = reinserted;
+                        match self.all_attribs.write() {
+                            Ok(mut all_trackers) => all_trackers.clear(),
+                            Err(_) => return,
+                        };
+                    }
+                };
+            };
             dest.push(map_fn(attrs, tracker));
         }
     }