Dynamically bind all environment variables based on a config struct

[mehdy]

Preflight Checklist

• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

When you want to use environment variables as a source of configuration you need to bind each item explicitly or have another source of configuration to populate the store and then be able to use AutomaticEnv to override the values from the environment variables.

So this piece of code is not doing what I'd expect:

package main

import (
        "fmt"
        "strings"

        "github.com/spf13/viper"
)

type Config struct {
        This  string `mapstructure:"this"`
        That  string `mapstructure:"that"`
        Other struct {
                Thing string `mapstructure:"thing"`
        } `mapstructure:"other"`
}

func main() {
        viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
        viper.AutomaticEnv()

        var config Config
        viper.Unmarshal(&config)
        fmt.Printf("This: %+v\n", config)
}

P.S. This feels like an issue that should've been brought up many times but couldn't find another issue like this so I apologize in-advance if it's redundant.

Proposed Solution

To solve the issue I wrote this function and it could be a part of Viper itself.

package main

import (
        "fmt"
        "reflect"
        "strings"

        "github.com/spf13/viper"
)

type Config struct {
        This  string `mapstructure:"this"`
        That  string `mapstructure:"that"`
        Other struct {
                Thing string `mapstructure:"thing"`
        } `mapstructure:"other"`
}

func listStructKeys(s interface{}) ([]string, error) {
        // Recursively get the config struct tag mapstructure
        keys := []string{}
        ct := reflect.TypeOf(s)

        if ct.Kind() != reflect.Struct {
                return nil, fmt.Errorf("listStructKeys: %v is not a struct", ct)
        }

        for i := range ct.NumField() {
                field := ct.Field(i)
                tag := field.Tag.Get("mapstructure")

                if field.Type.Kind() == reflect.Struct {
                        res, err := listStructKeys(reflect.New(field.Type).Elem().Interface())
                        if err != nil {
                                return nil, err
                        }
                        for _, k := range res {
                                keys = append(keys, fmt.Sprintf("%s.%s", tag, k))
                        }
                } else {
                        keys = append(keys, tag)
                }
        }

        return keys, nil
}

func bindAllEnv(s interface{}) error {
        keys, err := listStructKeys(s)
        if err != nil {
                return err
        }

        for _, k := range keys {
                viper.BindEnv(k)
        }

        return nil
}

func main() {
        viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
        viper.AutomaticEnv()
        bindAllEnv(Config{})

        var config Config
        viper.Unmarshal(&config)
        fmt.Printf("This: %+v\n", config)
}

So if Viper had a method like this it'd be so easy to tell it to bind all configurations defined by my Config struct.

viper.BindEnvByStruct(config{})
// or
viper.BindAllEnv(config{})
// or
viper.AutomaticEnv(config{}) // with changing the signature to `func AutomaticEnv(s ...interface{}) error` which I'd assume you wouldn't like since it could be considered a breaking change.

If you're up for it I'd like to make a PR for it.

Alternatives Considered

No response

Additional Information

No response

[github-actions]

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[zyy17]

@mehdy I have the same issue as you. Your method seems similar to the one mentioned in #584. Viper has implemented BindStruct in #1429 but didn't turn on for backward compatibility(#1706). For your case, you can turn on viper_bind_struct as the following command:

THAT=123 go run -tags viper_bind_struct test.go

[mehdy]

That's weird and undocumented! But then it means I should close this issue.