Move cron scheduling inside application (#338)

* Move cron scheduling inside application

* Make envvar a fallback and check for errors

* Panic significantly less

* propagate error out of runBackup

* Add structured logging

* FIx error propagation to exit

* Enable the new scheduler by default

* Review fixes

* Added docs and better error propagation

cmd/backup/config_provider.go

@@ -0,0 +1,81 @@
+// Copyright 2021-2022 - Offen Authors <hioffen@posteo.de>
+// SPDX-License-Identifier: MPL-2.0
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/joho/godotenv"
+	"github.com/offen/envconfig"
+)
+
+// envProxy is a function that mimics os.LookupEnv but can read values from any other source
+type envProxy func(string) (string, bool)
+
+func loadConfig(lookup envProxy) (*Config, error) {
+	envconfig.Lookup = func(key string) (string, bool) {
+		value, okValue := lookup(key)
+		location, okFile := lookup(key + "_FILE")
+
+		switch {
+		case okValue && !okFile: // only value
+			return value, true
+		case !okValue && okFile: // only file
+			contents, err := os.ReadFile(location)
+			if err != nil {
+				return "", false
+			}
+			return string(contents), true
+		case okValue && okFile: // both
+			return "", false
+		default: // neither, ignore
+			return "", false
+		}
+	}
+
+	var c = &Config{}
+	if err := envconfig.Process("", c); err != nil {
+		return nil, fmt.Errorf("failed to process configuration values, error: %w", err)
+	}
+
+	return c, nil
+}
+
+func loadEnvVars() (*Config, error) {
+	return loadConfig(os.LookupEnv)
+}
+
+func loadEnvFiles(directory string) ([]*Config, error) {
+	items, err := os.ReadDir(directory)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, err
+		}
+		return nil, fmt.Errorf("failed to read files from env directory, error: %w", err)
+	}
+
+	var cs = make([]*Config, 0)
+	for _, item := range items {
+		if !item.IsDir() {
+			p := filepath.Join(directory, item.Name())
+			envFile, err := godotenv.Read(p)
+			if err != nil {
+				return nil, fmt.Errorf("error reading config file %s, error: %w", p, err)
+			}
+			lookup := func(key string) (string, bool) {
+				val, ok := envFile[key]
+				return val, ok
+			}
+			c, err := loadConfig(lookup)
+			if err != nil {
+				return nil, fmt.Errorf("error loading config from file %s, error: %w", p, err)
+			}
+			cs = append(cs, c)
+		}
+	}
+
+	return cs, nil
+}