Support AWS Signature V2 for Mattermost for S3 storage. (#6462)

Certain S3 compatible servers only use Legacy Signature (AWS
Signature V2), current code only supports signature v4.

This PR adds facility to click a button on the UI to enable
legacy signature with S3 compatible servers.

app/diagnostics.go

@@ -244,6 +244,7 @@ func trackConfig() {
 		"enable_public_links": utils.Cfg.FileSettings.EnablePublicLink,
 		"driver_name":         utils.Cfg.FileSettings.DriverName,
 		"amazon_s3_ssl":       *utils.Cfg.FileSettings.AmazonS3SSL,
+		"amazon_s3_signv2":    *utils.Cfg.FileSettings.AmazonS3SignV2,
 		"thumbnail_width":     utils.Cfg.FileSettings.ThumbnailWidth,
 		"thumbnail_height":    utils.Cfg.FileSettings.ThumbnailHeight,
 		"preview_width":       utils.Cfg.FileSettings.PreviewWidth,

app/file.go

@@ -56,13 +56,23 @@ const (
 	MaxImageSize = 6048 * 4032 // 24 megapixels, roughly 36MB as a raw image
 )
 
+// Similar to s3.New() but allows initialization of signature v2 or signature v4 client.
+// If signV2 input is false, function always returns signature v4.
+func s3New(endpoint, accessKey, secretKey string, secure bool, signV2 bool) (*s3.Client, error) {
+	if signV2 {
+		return s3.NewV2(endpoint, accessKey, secretKey, secure)
+	}
+	return s3.NewV4(endpoint, accessKey, secretKey, secure)
+}
+
 func ReadFile(path string) ([]byte, *model.AppError) {
 	if utils.Cfg.FileSettings.DriverName == model.IMAGE_DRIVER_S3 {
 		endpoint := utils.Cfg.FileSettings.AmazonS3Endpoint
 		accessKey := utils.Cfg.FileSettings.AmazonS3AccessKeyId
 		secretKey := utils.Cfg.FileSettings.AmazonS3SecretAccessKey
 		secure := *utils.Cfg.FileSettings.AmazonS3SSL
-		s3Clnt, err := s3.New(endpoint, accessKey, secretKey, secure)
+		signV2 := *utils.Cfg.FileSettings.AmazonS3SignV2
+		s3Clnt, err := s3New(endpoint, accessKey, secretKey, secure, signV2)
 		if err != nil {
 			return nil, model.NewLocAppError("ReadFile", "api.file.read_file.s3.app_error", nil, err.Error())
 		}
@@ -94,7 +104,8 @@ func MoveFile(oldPath, newPath string) *model.AppError {
 		accessKey := utils.Cfg.FileSettings.AmazonS3AccessKeyId
 		secretKey := utils.Cfg.FileSettings.AmazonS3SecretAccessKey
 		secure := *utils.Cfg.FileSettings.AmazonS3SSL
-		s3Clnt, err := s3.New(endpoint, accessKey, secretKey, secure)
+		signV2 := *utils.Cfg.FileSettings.AmazonS3SignV2
+		s3Clnt, err := s3New(endpoint, accessKey, secretKey, secure, signV2)
 		if err != nil {
 			return model.NewLocAppError("moveFile", "api.file.write_file.s3.app_error", nil, err.Error())
 		}
@@ -128,10 +139,12 @@ func WriteFile(f []byte, path string) *model.AppError {
 		accessKey := utils.Cfg.FileSettings.AmazonS3AccessKeyId
 		secretKey := utils.Cfg.FileSettings.AmazonS3SecretAccessKey
 		secure := *utils.Cfg.FileSettings.AmazonS3SSL
-		s3Clnt, err := s3.New(endpoint, accessKey, secretKey, secure)
+		signV2 := *utils.Cfg.FileSettings.AmazonS3SignV2
+		s3Clnt, err := s3New(endpoint, accessKey, secretKey, secure, signV2)
 		if err != nil {
 			return model.NewLocAppError("WriteFile", "api.file.write_file.s3.app_error", nil, err.Error())
 		}
+
 		bucket := utils.Cfg.FileSettings.AmazonS3Bucket
 		ext := filepath.Ext(path)