keep batch locked until emit

src/main/scala/util/Batcher.scala

@@ -19,18 +19,25 @@ final class Batcher[Key, Elem, Batch](
   private val buffers = ConcurrentHashMap[Key, Buffer](initialCapacity)
 
   def add(key: Key, elem: Elem): Unit =
-    val newBuffer = buffers.compute(
+    buffers.compute(
       key,
       (_, buffer) =>
         val prev = Option(buffer)
         if prev.isEmpty then scheduler.scheduleOnce(timeout, () => emitAndRemove(key))
-        Buffer(
+        val newBuffer = Buffer(
           append(prev.map(_.batch), elem),
           prev.fold(1)(_.counter + 1)
         )
+        if newBuffer.counter >= maxBatchSize then
+          emit(key, newBuffer.batch)
+          null
+        else newBuffer
     )
-    if newBuffer.counter >= maxBatchSize then emitAndRemove(key)
 
   private def emitAndRemove(key: Key): Unit =
-    Option(buffers.remove(key)).foreach: buffer =>
-      emit(key, buffer.batch)
+    buffers.computeIfPresent(
+      key,
+      (_, buffer) =>
+        emit(key, buffer.batch)
+        null
+    )