it works!

config.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/BurntSushi/toml"
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/endpoints"
+)
+
+type Upstream struct {
+	Addr    string   `toml:"addr"`
+	Program string   `toml:"program"`
+	Args    []string `toml:"args"`
+}
+
+type OAuthProvider struct {
+	Kind         string `toml:"kind"`
+	ClientID     string `toml:"client_id"`
+	ClientSecret string `toml:"client_secret"`
+	RedirectURL  string `toml:"redirect_url"`
+}
+
+type Config struct {
+	ListenURL     string        `toml:"listen_url"`
+	GuardedPaths  []string      `toml:"guarded_paths"`
+	AllowedUsers  []string      `toml:"allowed_users"`
+	Upstream      Upstream      `toml:"upstream"`
+	OAuthProvider OAuthProvider `toml:"provider"`
+}
+
+func LoadConfig() (Config, oauth2.Config, error) {
+	config := Config{}
+	oa2 := oauth2.Config{}
+	_, err := toml.DecodeFile("config.toml", &config)
+	if err != nil {
+		return config, oa2, fmt.Errorf("unable to parse 'config.toml' tompl decoding error: %w", err)
+	}
+
+	oa2.ClientID = config.OAuthProvider.ClientID
+	oa2.ClientSecret = config.OAuthProvider.ClientSecret
+	oa2.Endpoint = oauth2.Endpoint{}
+	oa2.RedirectURL = config.OAuthProvider.RedirectURL
+	oa2.Scopes = []string{}
+
+	switch config.OAuthProvider.Kind {
+	case "github":
+		oa2.Endpoint = endpoints.GitHub
+		oa2.Scopes = []string{"read:user"}
+	case "google":
+		oa2.Endpoint = endpoints.Google
+		oa2.Scopes = []string{"https://www.googleapis.com/auth/userinfo.email"}
+	}
+
+	return config, oa2, err
+}