From 0177d349df77cc51571ce9e860b329631e8cd63f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Krist=C3=B3f=20T=C3=B3th?= <mrtoth@strongds.hu>
Date: Fri, 15 Oct 2021 22:14:59 +0200
Subject: [PATCH] Add poc to parse config from env. Should refactor (famous
 last words)

---
 config/config.go      | 115 ++++++++++++++++++++++++++++++++++++++++++
 config/config_test.go |  49 ++++++++++++++++++
 2 files changed, 164 insertions(+)
 create mode 100644 config/config.go
 create mode 100644 config/config_test.go

diff --git a/config/config.go b/config/config.go
new file mode 100644
index 0000000..773eaca
--- /dev/null
+++ b/config/config.go
@@ -0,0 +1,115 @@
+package config
+
+import (
+	"os"
+	"strings"
+	"fmt"
+	"reflect"
+	"errors"
+	"strconv"
+)
+
+
+var ConfigFetchMethod = os.Environ
+
+
+func Build(prefix string, configStruct interface{}) (err error) {
+	defer func() {
+		if e := recover(); e != nil {
+			switch r := e.(type) {
+			case string:
+				err = errors.New(r)
+			case error:
+				err = r
+			default:
+				err = errors.New("Undefined panic")
+			}
+		}
+	}()
+	assertIsPtr(configStruct)
+
+	fieldNames := getConfigStructFieldNames(configStruct)
+	config := loadConfig(prefix)
+
+	structRef := reflect.ValueOf(configStruct)
+	for _, fieldName := range fieldNames {
+		value, ok := config[fieldName]
+		if !ok {
+			// no such field loaded, avoid overwriting default
+			continue
+		}
+		field := reflect.Indirect(structRef).FieldByName(fieldName)
+		typeRef := field.Type()
+		ret, err := tryParseString(value, typeRef.Kind())
+		if err != nil {
+			return err
+		}
+		field.Set(reflect.ValueOf(ret).Convert(typeRef))
+	}
+
+	return nil
+}
+
+func tryParseString(what string, toType reflect.Kind) (interface{}, error) {
+	switch toType {
+	case reflect.String:
+		return what, nil
+	case reflect.Int:
+		i, err := strconv.Atoi(what)
+		if err != nil {
+			return nil, err
+		}
+		return i, nil
+	case reflect.Bool:
+		b, err := strconv.ParseBool(what)
+		if err != nil {
+			return nil, err
+		}
+		return b, nil
+	default:
+		return nil, fmt.Errorf("Failed to parse value %v", what)
+	}
+}
+
+func assertIsPtr(what interface{}) {
+	r := reflect.ValueOf(what)
+	if r.Kind() != reflect.Ptr {
+		panic(fmt.Errorf("Supplied value is not a pointer to a struct"))
+	}
+}
+
+func getConfigStructFieldNames(configStruct interface{}) []string {
+	configFields := []string{}
+
+	val := reflect.ValueOf(configStruct).Elem()
+	for i := 0; i < val.NumField(); i++ {
+		configFields = append(configFields, val.Type().Field(i).Name)
+	}
+
+	return configFields
+}
+
+func loadConfig(prefix string) map[string]string {
+	prefix = fmt.Sprintf("%s_", prefix)
+
+	config := map[string]string{}
+	for _, env := range ConfigFetchMethod() {
+		parts := strings.SplitN(env, "=", 2)
+		key, value := parts[0], parts[1]
+		if strings.HasPrefix(key, prefix) {
+			key = strings.TrimPrefix(key, prefix)
+			config[key] = value
+		}
+	}
+	return config
+}
+
+// CheckKeysExist returns an error if not all keys are present in config map
+func CheckKeysExist(config map[string]string, keys ...string) error {
+	for _, key := range keys {
+		if _, ok := config[key]; ok {
+			return fmt.Errorf("Config key '%s' is not set", key)
+		}
+	}
+	return nil
+}
\ No newline at end of file
diff --git a/config/config_test.go b/config/config_test.go
new file mode 100644
index 0000000..d703771
--- /dev/null
+++ b/config/config_test.go
@@ -0,0 +1,49 @@
+package config_test
+
+import (
+	"testing"
+
+	"kdelsd/config"
+)
+
+
+type TestConfig struct {
+	INT int
+	BOOL bool
+	STR string
+	OTHER_STR string
+}
+
+var testConfigInstance = TestConfig{
+	INT: 42,
+	BOOL: true,
+	STR: "sajtok",
+	OTHER_STR: "default-value",
+}
+
+var testEnv = []string {
+	"TEST_INT=42",
+	"TEST_BOOL=true",
+	"TEST_STR=sajtok",
+}
+
+func setup() {
+	config.ConfigFetchMethod = func() []string {
+		return testEnv
+	}
+}
+
+func TestBuildConfig(t *testing.T) {
+	setup()
+	c := TestConfig{}
+	c.OTHER_STR = testConfigInstance.OTHER_STR
+
+	err := config.Build("TEST", &c)
+	if err != nil {
+		panic(err)
+	}
+
+	if c != testConfigInstance {
+		t.Errorf("%v != %v", c, testConfigInstance)
+	}
+}
\ No newline at end of file