feat: Support environment variables for config settings

Enable Viper to work with environment variables. Kuberlr will honour
environment variables with a name matching the key uppercased and
prefixed with `KUBRERLR` (like `KUBERLR_ALLOWDOWNLOAD`) and use them for
the config settings.

These environment variables take precedence over the config file
settings.

Signed-off-by: Víctor Cuadrado Juan <[email protected]>

Author: viccuad

README.md

@@ -131,3 +131,7 @@ Timeout = 1
 # Default "https://dl.k8s.io"
 KubeMirrorUrl = "https://dl.k8s.io"
 ```
+
+The behaviour can also be adjusted by using environment variables matching the
+config file: `KUBERLR_ALLOWDOWNLOAD`, `KUBERLR_SYSTEMPATH`, `KUBERLR_TIMEOUT`,
+`KUBERLR_KUBEMIRRORURL`, and so on.

internal/config/config.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"os"
+	"strings"
 
 	"github.com/spf13/viper"
 
@@ -34,6 +35,11 @@ func (c *Cfg) Load() (*viper.Viper, error) {
 
 	v.SetConfigType("toml")
 
+	// read environment variables, they take precedence
+	v.AutomaticEnv()
+	v.SetEnvPrefix("KUBERLR")
+	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
+
 	if len(c.Paths) == 0 {
 		return v, nil
 	}

internal/config/config_test.go

@@ -4,6 +4,8 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 type testData struct {
@@ -41,7 +43,7 @@ func writeConfig(path, data string) error {
 	return os.WriteFile(
 		filepath.Join(path, "kuberlr.conf"),
 		[]byte(data),
-		0600)
+		0o600)
 }
 
 func TestOnlySystemConfigExists(t *testing.T) {
@@ -106,6 +108,34 @@ func TestHomeConfigOverridesSystemOne(t *testing.T) {
 	}
 }
 
+func TestEnvironmentVariables(t *testing.T) {
+	td, err := setup()
+	if err != nil {
+		t.Error(err)
+	}
+	require.NoError(t, err)
+	defer teardown(td)
+
+	err = writeConfig(td.FakeHome, "AllowDownload = false")
+	if err != nil {
+		t.Error(err)
+	}
+
+	t.Setenv("KUBERLR_ALLOWDOWNLOAD", "true")
+
+	c := Cfg{
+		Paths: []string{},
+	}
+
+	v, err := c.Load()
+	if err != nil {
+		t.Errorf("Unexpected error loading config: %v", err)
+	}
+	if v.GetBool("AllowDownload") != true {
+		t.Error("env var wasn't taken into account")
+	}
+}
+
 func TestMergeConfigs(t *testing.T) {
 	td, err := setup()
 	if err != nil {