config: support expand env in yaml

cli/agent/command.go

@@ -60,12 +60,27 @@ Examples:
 YAML config file path.`,
 	)
 
+	var configExpandEnv bool
+	cmd.Flags().BoolVar(
+		&configExpandEnv,
+		"config.expand-env",
+		false,
+		`
+Whether to expand environment variables in the config file.
+
+This will replaces references to ${VAR} or $VAR with the corresponding
+environment variable. The replacement is case-sensitive.
+
+References to undefined variables will be replaced with an empty string. A
+default value can be given using form ${VAR:default}.`,
+	)
+
 	// Register flags and set default values.
 	conf.RegisterFlags(cmd.Flags())
 
 	cmd.Run = func(cmd *cobra.Command, args []string) {
 		if configPath != "" {
-			if err := picoconfig.Load(configPath, &conf); err != nil {
+			if err := picoconfig.Load(configPath, &conf, configExpandEnv); err != nil {
 				fmt.Printf("load config: %s\n", err.Error())
 				os.Exit(1)
 			}

cli/server/command.go

@@ -68,12 +68,27 @@ Examples:
 YAML config file path.`,
 	)
 
+	var configExpandEnv bool
+	cmd.Flags().BoolVar(
+		&configExpandEnv,
+		"config.expand-env",
+		false,
+		`
+Whether to expand environment variables in the config file.
+
+This will replaces references to ${VAR} or $VAR with the corresponding
+environment variable. The replacement is case-sensitive.
+
+References to undefined variables will be replaced with an empty string. A
+default value can be given using form ${VAR:default}.`,
+	)
+
 	// Register flags and set default values.
 	conf.RegisterFlags(cmd.Flags())
 
 	cmd.Run = func(cmd *cobra.Command, args []string) {
 		if configPath != "" {
-			if err := picoconfig.Load(configPath, &conf); err != nil {
+			if err := picoconfig.Load(configPath, &conf, configExpandEnv); err != nil {
 				fmt.Printf("load config: %s\n", err.Error())
 				os.Exit(1)
 			}

pkg/config/config.go

@@ -4,16 +4,24 @@ import (
 	"bytes"
 	"fmt"
 	"os"
+	"strings"
 
 	"gopkg.in/yaml.v3"
 )
 
-func Load(path string, conf interface{}) error {
+// Load load the YAML configuration from the file at the given path.
+//
+// This will expand environment VARiables if expand is true.
+func Load(path string, conf interface{}, expand bool) error {
 	buf, err := os.ReadFile(path)
 	if err != nil {
 		return fmt.Errorf("read file: %s: %w", path, err)
 	}
 
+	if expand {
+		buf = []byte(expandEnv(string(buf)))
+	}
+
 	dec := yaml.NewDecoder(bytes.NewReader(buf))
 	dec.KnownFields(true)
 
@@ -23,3 +31,22 @@ func Load(path string, conf interface{}) error {
 
 	return nil
 }
+
+// expandEnv replaces ${VAR} or $VAR in the given string with the corresponding
+// environment variable. The replacement is case-sensitive.
+//
+// References to undefined variables are replaced with an empty string. A
+// default value can be given using form ${VAR:default}
+func expandEnv(s string) string {
+	return os.Expand(s, func(v string) string {
+		elems := strings.SplitN(v, ":", 2)
+		key := elems[0]
+
+		env := os.Getenv(key)
+		if env == "" && len(elems) == 2 {
+			// If no env exists use the default.
+			return elems[1]
+		}
+		return env
+	})
+}

pkg/config/config_test.go

@@ -29,7 +29,28 @@ sub:
 		assert.NoError(t, err)
 
 		var conf fakeConfig
-		assert.NoError(t, Load(f.Name(), &conf))
+		assert.NoError(t, Load(f.Name(), &conf, false))
+
+		assert.Equal(t, "val1", conf.Foo)
+		assert.Equal(t, "val2", conf.Bar)
+		assert.Equal(t, 5, conf.Sub.Car)
+	})
+
+	t.Run("expand env", func(t *testing.T) {
+		f, err := os.CreateTemp("", "pico")
+		assert.NoError(t, err)
+
+		_ = os.Setenv("PICO_VAL1", "val1")
+		_ = os.Setenv("PICO_VAL2", "val2")
+
+		_, err = f.WriteString(`foo: $PICO_VAL1
+bar: ${PICO_VAL2}
+sub:
+  car: ${PICO_VAL3:5}`)
+		assert.NoError(t, err)
+
+		var conf fakeConfig
+		assert.NoError(t, Load(f.Name(), &conf, true))
 
 		assert.Equal(t, "val1", conf.Foo)
 		assert.Equal(t, "val2", conf.Bar)
@@ -44,11 +65,11 @@ sub:
 		assert.NoError(t, err)
 
 		var conf fakeConfig
-		assert.Error(t, Load(f.Name(), &conf))
+		assert.Error(t, Load(f.Name(), &conf, false))
 	})
 
 	t.Run("not found", func(t *testing.T) {
 		var conf fakeConfig
-		assert.Error(t, Load("notfound", &conf))
+		assert.Error(t, Load("notfound", &conf, false))
 	})
 }