[confmap] log a warning when using $VAR in config

confmap/converter/expandconverter/expand.go

@@ -5,51 +5,71 @@ package expandconverter // import "go.opentelemetry.io/collector/confmap/convert
 
 import (
 	"context"
+	"fmt"
 	"os"
+	"regexp"
 
 	"go.opentelemetry.io/collector/confmap"
+	"go.uber.org/zap"
 )
 
-type converter struct{}
+type converter struct {
+	logger *zap.Logger
+
+	// Record of which env vars we have logged a warning for
+	loggedDeprecations map[string]struct{}
+}
 
 // New returns a confmap.Converter, that expands all environment variables for a given confmap.Conf.
 //
 // Notice: This API is experimental.
 func New(_ confmap.ConverterSettings) confmap.Converter {
-	return converter{}
+	return converter{
+		loggedDeprecations: make(map[string]struct{}),
+		logger:             zap.NewNop(), // TODO: pass logger in ConverterSettings
+	}
 }
 
-func (converter) Convert(_ context.Context, conf *confmap.Conf) error {
+func (c converter) Convert(_ context.Context, conf *confmap.Conf) error {
 	out := make(map[string]any)
 	for _, k := range conf.AllKeys() {
-		out[k] = expandStringValues(conf.Get(k))
+		out[k] = c.expandStringValues(conf.Get(k))
 	}
 	return conf.Merge(confmap.NewFromStringMap(out))
 }
 
-func expandStringValues(value any) any {
+func (c converter) expandStringValues(value any) any {
 	switch v := value.(type) {
 	case string:
-		return expandEnv(v)
+		return c.expandEnv(v)
 	case []any:
 		nslice := make([]any, 0, len(v))
 		for _, vint := range v {
-			nslice = append(nslice, expandStringValues(vint))
+			nslice = append(nslice, c.expandStringValues(vint))
 		}
 		return nslice
 	case map[string]any:
 		nmap := map[string]any{}
 		for mk, mv := range v {
-			nmap[mk] = expandStringValues(mv)
+			nmap[mk] = c.expandStringValues(mv)
 		}
 		return nmap
 	default:
 		return v
 	}
 }
 
-func expandEnv(s string) string {
+func (c converter) expandEnv(s string) string {
 	return os.Expand(s, func(str string) string {
+		// Matches on $VAR style environment variables
+		// in order to make sure we don't log a warning for ${VAR}
+		var regex = regexp.MustCompile(fmt.Sprintf(`\$%s`, str))
+		if _, exists := c.loggedDeprecations[str]; !exists && regex.MatchString(s) {
+			msg := fmt.Sprintf("Variable substitution using $VAR will be deprecated in favor of ${VAR} and ${env:VAR}, please update $%s", str)
+			c.logger.Warn(msg, zap.String("variable", str))
+			c.loggedDeprecations[str] = struct{}{}
+		}
+
 		// This allows escaping environment variable substitution via $$, e.g.
 		// - $FOO will be substituted with env var FOO
 		// - $$FOO will be replaced with $FOO

confmap/go.mod

@@ -10,13 +10,13 @@ require (
 	github.com/stretchr/testify v1.8.4
 	go.uber.org/goleak v1.3.0
 	go.uber.org/multierr v1.11.0
+	go.uber.org/zap v1.26.0
 	gopkg.in/yaml.v3 v3.0.1
 )
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/go-viper/mapstructure/v2 v2.0.0-alpha.1 // indirect
-	github.com/kr/text v0.2.0 // indirect
 	github.com/mitchellh/copystructure v1.2.0 // indirect
 	github.com/mitchellh/reflectwalk v1.0.2 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect