Fix invalid write index updates in the persistent queue

.chloggen/fix_persistentstorage_index-updates.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: exporterhelper
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix invalid write index updates in the persistent queue
+
+# One or more tracking issues or pull requests related to the change
+issues: [8115]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

exporter/exporterhelper/internal/persistent_queue.go

@@ -205,20 +205,25 @@ func (pq *persistentQueue[T]) putInternal(ctx context.Context, req T) error {
 	}
 
 	itemKey := getItemKey(pq.writeIndex)
-	pq.writeIndex++
+	newIndex := pq.writeIndex + 1
 
 	reqBuf, err := pq.marshaler(req)
 	if err != nil {
 		return err
 	}
-	err = pq.client.Batch(ctx,
-		storage.SetOperation(writeIndexKey, itemIndexToBytes(pq.writeIndex)),
-		storage.SetOperation(itemKey, reqBuf))
 
+	// Carry out a transaction where we both add the item and update the write index
+	setWriteIndexOp := storage.SetOperation(writeIndexKey, itemIndexToBytes(newIndex))
+	setItemOp := storage.SetOperation(itemKey, reqBuf)
+	if err := pq.client.Batch(ctx, setWriteIndexOp, setItemOp); err != nil {
+		return err
+	}
+
+	pq.writeIndex = newIndex
 	// Inform the loop that there's some data to process
 	pq.putChan <- struct{}{}
 
-	return err
+	return nil
 }
 
 // getNextItem pulls the next available item from the persistent storage; if none is found, returns (nil, false)

exporter/exporterhelper/internal/persistent_queue_test.go

@@ -624,6 +624,9 @@ func TestPersistentQueue_StorageFull(t *testing.T) {
 		reqCount++
 	}
 
+	// Check that the size is correct
+	require.Equal(t, reqCount, ps.Size(), "Size must be equal to the number of items inserted")
+
 	// Manually set the storage to only have a small amount of free space left
 	newMaxSize := client.GetSizeInBytes() + freeSpaceInBytes
 	client.SetMaxSizeInBytes(newMaxSize)
@@ -632,6 +635,9 @@ func TestPersistentQueue_StorageFull(t *testing.T) {
 	require.Error(t, ps.Offer(context.Background(), req))
 
 	// Take out all the items
+	// Getting the first item fails, as we can't update the state in storage, so we just delete it without returning it
+	// Subsequent items succeed, as deleting the first item frees enough space for the state update
+	reqCount--
 	for i := reqCount; i > 0; i-- {
 		request, found := ps.Poll()
 		require.True(t, found)