[azblob] Fix file offset post sequential write in downloadFile (#22357)

* fix file offset post download

* changelog update

* lint error fix

* error verifications add

* file pointer check in test case

* check error from seek function call

sdk/storage/azblob/CHANGELOG.md

@@ -8,6 +8,7 @@
 
 ### Bugs Fixed
 * Fix concurrency issue while Downloading File. Fixes [#22156](https://github.com/Azure/azure-sdk-for-go/issues/22156). 
+* Fix file offset update after Download file. Fixes [#22297](https://github.com/Azure/azure-sdk-for-go/issues/22297).
 
 ### Other Changes
 

sdk/storage/azblob/blob/client.go

@@ -596,6 +596,11 @@ func (b *Client) DownloadFile(ctx context.Context, file *os.File, o *DownloadFil
 	}
 	do := (*downloadOptions)(o)
 
+	filePointer, err := file.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return 0, err
+	}
+
 	// 1. Calculate the size of the destination file
 	var size int64
 
@@ -624,7 +629,15 @@ func (b *Client) DownloadFile(ctx context.Context, file *os.File, o *DownloadFil
 	}
 
 	if size > 0 {
-		return b.downloadFile(ctx, file, *do)
+		writeSize, err := b.downloadFile(ctx, file, *do)
+		if err != nil {
+			return 0, err
+		}
+		_, err = file.Seek(filePointer, io.SeekStart)
+		if err != nil {
+			return 0, err
+		}
+		return writeSize, nil
 	} else { // if the blob's size is 0, there is no need in downloading it
 		return 0, nil
 	}

sdk/storage/azblob/client_test.go

@@ -396,7 +396,6 @@ func performUploadAndDownloadFileTest(t *testing.T, _require *require.Assertions
 		})
 	assert.NoError(t, errTransferred)
 	_require.NoError(err)
-	//_require.Equal(response.StatusCode, 201)
 
 	// Set up file to download the blob to
 	destFileName := "BigFile-downloaded.bin"
@@ -411,6 +410,9 @@ func performUploadAndDownloadFileTest(t *testing.T, _require *require.Assertions
 
 	}(destFileName)
 
+	filePointer, err := file.Seek(0, io.SeekCurrent)
+	_require.NoError(err)
+
 	// Perform download
 	_, err = client.DownloadFile(context.Background(),
 		containerName,
@@ -435,6 +437,10 @@ func performUploadAndDownloadFileTest(t *testing.T, _require *require.Assertions
 	assert.NoError(t, errTransferred)
 	_require.NoError(err)
 
+	currPointer, err := destFile.Seek(0, io.SeekCurrent)
+	_require.Equal(filePointer, currPointer)
+	_require.NoError(err)
+
 	// Assert downloaded data is consistent
 	var destBuffer []byte
 	if downloadCount == blob.CountToEnd {
@@ -443,7 +449,6 @@ func performUploadAndDownloadFileTest(t *testing.T, _require *require.Assertions
 		destBuffer = make([]byte, downloadCount)
 	}
 
-	_, err = destFile.Seek(0, 0)
 	_require.NoError(err)
 	n, err := destFile.Read(destBuffer)
 	_require.NoError(err)