From 28f727de1f85a047881653391ba901c667ee87bf Mon Sep 17 00:00:00 2001
From: Guido Zuidhof <me@guido.io>
Date: Thu, 21 Nov 2024 11:33:20 +0100
Subject: [PATCH 1/2] Never flush without items

---
 batcher.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/batcher.go b/batcher.go
index 3f6adf8..cf050dd 100644
--- a/batcher.go
+++ b/batcher.go
@@ -84,7 +84,7 @@ func (b *Batcher[T]) start(ctx context.Context,
 
 		// If the batcher is cancelled and the buffer is not empty, we want to flush the
 		// remaining items with the maximum batch size, so we skip until we reach max size or the buffer is empty.
-		skipFlush := isCancelled && len(b.buffer) > 0 && !isMaxSize
+		skipFlush := (isCancelled && len(b.buffer) > 0 && !isMaxSize) || len(items) == 0
 
 		if !skipFlush {
 			// We need to copy the slice to make sure that the slice that is passed is valid even if asynchronously

From b9c6205461f483e87826772e89dd31758fd4a272 Mon Sep 17 00:00:00 2001
From: Guido Zuidhof <me@guido.io>
Date: Thu, 21 Nov 2024 11:39:07 +0100
Subject: [PATCH 2/2] Add test to reproduce issue

---
 batcher_test.go | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/batcher_test.go b/batcher_test.go
index 4fa5af5..bac4bcb 100644
--- a/batcher_test.go
+++ b/batcher_test.go
@@ -282,4 +282,17 @@ func TestBatcherCancellation(t *testing.T) {
 		assert.ErrorIs(t, batcher.Push(0), ErrBatcherStopped)
 		assert.Equal(t, 0, batcher.CurrentBufferSize())
 	})
+
+	t.Run("no empty flush after cancellation", func(t *testing.T) {
+		t.Parallel()
+		ctx, cancel := context.WithCancel(context.Background())
+
+		_, flusher := startWithMockFlusher(ctx, t, New[int]())
+
+		cancel()
+
+		time.Sleep(time.Millisecond * 5)
+
+		assert.Equal(t, 0, flusher.StartCount())
+	})
 }