Fix possible call to nil finishFn in asyncbuffer

asyncbuffer/buffer.go

@@ -89,6 +89,17 @@ func New(r io.ReadCloser, finishFn ...context.CancelFunc) *AsyncBuffer {
 	return ab
 }
 
+// callFinishFn calls the finish functions registered with the AsyncBuffer.
+func (ab *AsyncBuffer) callFinishFn() {
+	ab.finishOnce.Do(func() {
+		for _, fn := range ab.finishFn {
+			if fn != nil {
+				fn()
+			}
+		}
+	})
+}
+
 // addChunk adds a new chunk to the AsyncBuffer, increments len and signals that a chunk is ready
 func (ab *AsyncBuffer) addChunk(chunk *byteChunk) {
 	ab.mu.Lock()
@@ -119,11 +130,7 @@ func (ab *AsyncBuffer) readChunks() {
 			logrus.WithField("source", "asyncbuffer.AsyncBuffer.readChunks").Warningf("error closing upstream reader: %s", err)
 		}
 
-		ab.finishOnce.Do(func() {
-			for _, fn := range ab.finishFn {
-				fn()
-			}
-		})
+		ab.callFinishFn()
 	}()
 
 	// Stop reading if the reader is closed
@@ -403,11 +410,7 @@ func (ab *AsyncBuffer) Close() error {
 	ab.paused.Release()
 
 	// Finish downloading
-	ab.finishOnce.Do(func() {
-		for _, fn := range ab.finishFn {
-			fn()
-		}
-	})
+	ab.callFinishFn()
 
 	return nil
 }