Allow templating when loading configs. (stripe#1004)

cmd/veneur-proxy/main.go

@@ -51,7 +51,7 @@ func main() {
 	}
 
 	config, err :=
-		utilConfig.ReadConfig[proxy.Config](*configFile, "veneur_proxy")
+		utilConfig.ReadConfig[proxy.Config](*configFile, nil, "veneur_proxy")
 	if err != nil {
 		logger.WithError(err).Fatal("failed to load config file")
 	}

util/config/config.go

@@ -3,34 +3,56 @@ package config
 import (
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"os"
+	"text/template"
 
 	"github.com/kelseyhightower/envconfig"
 	"gopkg.in/yaml.v2"
 )
 
 func ReadConfig[Config interface{}](
-	path string, envBase string,
+	path string, templateData interface{}, envBase string,
 ) (*Config, error) {
 	file, err := os.Open(path)
 	if err != nil {
 		return nil, fmt.Errorf("failed to open config file: %v", err)
 	}
 	defer file.Close()
 
-	fileData, err := ioutil.ReadAll(file)
+	fileData, err := io.ReadAll(file)
 	if err != nil {
 		return nil, fmt.Errorf("failed to read config file: %v", err)
 	}
 
+	configTemplate, err := template.New("config").Parse(string(fileData))
+	if err != nil {
+		return nil, err
+	}
+
+	configReader, configWriter := io.Pipe()
+	templateErr := make(chan error)
+	go func() {
+		err := configTemplate.Execute(configWriter, templateData)
+		configWriter.Close()
+		templateErr <- err
+	}()
+
+	decoder := yaml.NewDecoder(configReader)
+	decoder.SetStrict(true)
+
 	config := new(Config)
-	err = yaml.UnmarshalStrict(fileData, config)
+	err = decoder.Decode(config)
 	if err != nil {
 		return nil, fmt.Errorf("failed to unmarshal config file: %v", err)
 	}
 
+	err = <-templateErr
+	if err != nil {
+		return nil, err
+	}
+
 	err = envconfig.Process(envBase, config)
 	if err != nil {
 		return nil, fmt.Errorf("failed to process environment variables: %v", err)

util/config/config_test.go

@@ -0,0 +1,49 @@
+package config_test
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/stripe/veneur/v14/util/config"
+)
+
+type Config struct {
+	Key1 string
+}
+
+func TestReadConfigWithoutTemplate(t *testing.T) {
+	file, err := ioutil.TempFile("", "config.yaml")
+	require.NoError(t, err)
+	defer os.Remove(file.Name())
+
+	file.Write([]byte("key1: value1"))
+
+	parsedConfig, err :=
+		config.ReadConfig[Config](file.Name(), nil, "")
+	require.NoError(t, err)
+
+	assert.Equal(t, "value1", parsedConfig.Key1)
+}
+
+type ConfigParams struct {
+	ConfigValue string
+}
+
+func TestReadConfigWithTemplate(t *testing.T) {
+	file, err := ioutil.TempFile("", "config.yaml")
+	require.NoError(t, err)
+	defer os.Remove(file.Name())
+
+	file.Write([]byte("key1: {{.ConfigValue}}"))
+
+	parsedConfig, err :=
+		config.ReadConfig[Config](file.Name(), ConfigParams{
+			ConfigValue: "value1",
+		}, "")
+	require.NoError(t, err)
+
+	assert.Equal(t, "value1", parsedConfig.Key1)
+}