sftp: Limit concurrent backend operations

internal/backend/sftp/config.go

@@ -15,6 +15,15 @@ type Config struct {
 
 	Layout  string `option:"layout" help:"use this backend directory layout (default: auto-detect)"`
 	Command string `option:"command" help:"specify command to create sftp connection"`
+
+	Connections uint `option:"connections" help:"set a limit for the number of concurrent connections (default: 5)"`
+}
+
+// NewConfig returns a new config with default options applied.
+func NewConfig() Config {
+	return Config{
+		Connections: 5,
+	}
 }
 
 func init() {
@@ -75,10 +84,11 @@ func ParseConfig(s string) (interface{}, error) {
 		return nil, errors.Fatal("sftp path starts with the tilde (~) character, that fails for most sftp servers.\nUse a relative directory, most servers interpret this as relative to the user's home directory.")
 	}
 
-	return Config{
-		User: user,
-		Host: host,
-		Port: port,
-		Path: p,
-	}, nil
+	cfg := NewConfig()
+	cfg.User = user
+	cfg.Host = host
+	cfg.Port = port
+	cfg.Path = p
+
+	return cfg, nil
 }