fix(expand): use transform.Bytes to fix interpolation of large environment variables

expand.go

@@ -23,7 +23,6 @@ package config
 import (
 	"bytes"
 	"fmt"
-	"io/ioutil"
 	"strings"
 
 	"golang.org/x/text/transform"
@@ -43,7 +42,10 @@ func expandVariables(f LookupFunc, buf *bytes.Buffer) (*bytes.Buffer, error) {
 	if f == nil {
 		return buf, nil
 	}
-	exp, err := ioutil.ReadAll(transform.NewReader(buf, newExpandTransformer(f)))
+	// when a single ${VAR} expands to a string larger than the reader's fixed dst buffer,
+	// transform.Bytes will grow the dst buffer automatically, unlike f.e.
+	// transform.NewReader + ReadAll, which will return ErrShortDst.
+	exp, _, err := transform.Bytes(newExpandTransformer(f), buf.Bytes())
 	if err != nil {
 		return nil, fmt.Errorf("couldn't expand environment: %v", err)
 	}

expand_test.go

@@ -238,6 +238,22 @@ func TestExpanderLongSrc(t *testing.T) {
 	}
 }
 
+func TestExpandVariablesLargeReplacement(t *testing.T) {
+	// Regression: transform.Reader fails with ErrShortDst when the replacement
+	// is larger than transform's internal destination buffer and sits at the
+	// start of the input (Reader requires nDst!=0 || nSrc!=0 to retry).
+	output := strings.Repeat("a", transformBufSize-1) + "aa"
+	lookup := func(key string) (string, bool) {
+		if key == "a" {
+			return output, true
+		}
+		return "", false
+	}
+	out, err := expandVariables(lookup, bytes.NewBufferString("$a"))
+	require.NoError(t, err)
+	assert.Equal(t, output, out.String())
+}
+
 func TestTransformLimit(t *testing.T) {
 	a := strings.Repeat("a", transformBufSize-1)