[feature] Basic config validation (#562)

* add optional config validation

* clarify that host and protocol are required

* add validation for host and protocol

* pass prerunArgs as a struct (validate by default)

internal/config/validate.go

@@ -0,0 +1,62 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package config
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/sirupsen/logrus"
+	"github.com/spf13/viper"
+)
+
+// Validate validates global config settings which don't have defaults, to make sure they are set sensibly.
+func Validate() error {
+	errs := []error{}
+
+	// host
+	if viper.GetString(Keys.Host) == "" {
+		errs = append(errs, fmt.Errorf("%s must be set", Keys.Host))
+	}
+
+	// protocol
+	protocol := viper.GetString(Keys.Protocol)
+	switch protocol {
+	case "https":
+		// no problem
+		break
+	case "http":
+		logrus.Warnf("%s was set to 'http'; this should *only* be used for debugging and tests!", Keys.Protocol)
+	case "":
+		errs = append(errs, fmt.Errorf("%s must be set", Keys.Protocol))
+	default:
+		errs = append(errs, fmt.Errorf("%s must be set to either http or https, provided value was %s", Keys.Protocol, protocol))
+	}
+
+	if len(errs) > 0 {
+		errStrings := []string{}
+		for _, err := range errs {
+			errStrings = append(errStrings, err.Error())
+		}
+		return errors.New(strings.Join(errStrings, "; "))
+	}
+
+	return nil
+}