test: verify .Open can be used concurrently

client.go

@@ -222,10 +222,10 @@ func (cc *client) Open(path string) (*File, error) {
 }
 
 // Reader will open the file at path and provide a reader to access its contents.
-// Callers need to close the returned Contents
+// Callers need to close the returned Contents.
 //
 // Callers should be aware that network errors while reading can occur since contents
-// are streamed from the FTP server. Having multiple open readers may not be supported.
+// are streamed from the FTP server. Having multiple open readers is not supported.
 func (cc *client) Reader(path string) (*File, error) {
 	cc.mu.Lock()
 	defer cc.mu.Unlock()

client_test.go

@@ -13,6 +13,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -47,17 +48,25 @@ func TestClient(t *testing.T) {
 	t.Run("open larger files", func(t *testing.T) {
 		largerFileSize := size(t, filepath.Join("testdata", "ftp-server", "bigdata", "large.txt"))
 
-		for i := 0; i < 10; i++ {
-			file, err := client.Open("/bigdata/large.txt")
-			require.NoError(t, err)
+		const iterations = 10
+		var wg sync.WaitGroup
+		wg.Add(iterations)
+		for i := 0; i < iterations; i++ {
+			go func(wg *sync.WaitGroup) {
+				defer wg.Done()
 
-			var buf bytes.Buffer
-			_, err = io.Copy(&buf, file)
-			require.NoError(t, err)
+				file, err := client.Open("/bigdata/large.txt")
+				require.NoError(t, err)
 
-			require.NoError(t, file.Close())
-			require.Equal(t, largerFileSize, len(buf.Bytes()))
+				var buf bytes.Buffer
+				_, err = io.Copy(&buf, file)
+				require.NoError(t, err)
+
+				require.NoError(t, file.Close())
+				require.Equal(t, largerFileSize, len(buf.Bytes()))
+			}(&wg)
 		}
+		wg.Wait()
 	})
 
 	t.Run("reader", func(t *testing.T) {
@@ -73,6 +82,7 @@ func TestClient(t *testing.T) {
 	t.Run("read larger files", func(t *testing.T) {
 		largerFileSize := size(t, filepath.Join("testdata", "ftp-server", "bigdata", "large.txt"))
 
+		// reader must process files in sequence
 		for i := 0; i < 10; i++ {
 			file, err := client.Reader("/bigdata/large.txt")
 			require.NoError(t, err)

file.go

@@ -32,14 +32,16 @@ func (f *File) Close() error {
 	if f == nil {
 		return nil
 	}
+	if f.Contents != nil {
+		if err := f.Contents.Close(); err != nil {
+			return err
+		}
+	}
 	if f.cleanup != nil {
 		if err := f.cleanup(); err != nil {
 			return err
 		}
 	}
-	if f.Contents != nil {
-		return f.Contents.Close()
-	}
 	return nil
 }