fix(awsutil): carry IsBase64Encoded through ToLambdaFunctionURLResponse

pkg/awsutil/lambda.go

@@ -19,8 +19,12 @@ func ToLambdaFunctionURLResponse(res events.APIGatewayProxyResponse) events.Lamb
 			}
 			return ""
 		}),
-		Body:       res.Body,
-		StatusCode: res.StatusCode,
+		// IsBase64Encoded MUST be carried over. The gin/apigw-proxy response writer
+		// base64-encodes any body that is not valid UTF-8 and sets this flag; dropping
+		// it here delivers a binary body to the client as literal base64 text.
+		Body:            res.Body,
+		IsBase64Encoded: res.IsBase64Encoded,
+		StatusCode:      res.StatusCode,
 	}
 }
 

pkg/awsutil/lambda_test.go

@@ -0,0 +1,62 @@
+package awsutil
+
+import (
+	"encoding/base64"
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/aws/aws-lambda-go/events"
+)
+
+// A BUFFERED-mode Lambda renders its response through aws-lambda-go-api-proxy,
+// whose writer base64-encodes any body that is not valid UTF-8 and reports it
+// via IsBase64Encoded. Dropping the flag during the conversion delivers the
+// base64 text itself to the client instead of the bytes, so the round trip has
+// to preserve it.
+func TestToLambdaFunctionURLResponsePreservesBase64Flag(t *testing.T) {
+	// A 1x1 transparent PNG — genuinely non-UTF-8, which is what trips the
+	// proxy writer's utf8.Valid branch.
+	png := []byte{
+		0x89, 'P', 'N', 'G', 0x0d, 0x0a, 0x1a, 0x0a,
+		0x00, 0x00, 0x00, 0x0d, 'I', 'H', 'D', 'R',
+		0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x01,
+		0x08, 0x06, 0x00, 0x00, 0x00, 0x1f, 0x15, 0xc4,
+		0x89,
+	}
+	encoded := base64.StdEncoding.EncodeToString(png)
+
+	res := ToLambdaFunctionURLResponse(events.APIGatewayProxyResponse{
+		StatusCode:      http.StatusOK,
+		Body:            encoded,
+		IsBase64Encoded: true,
+		MultiValueHeaders: map[string][]string{
+			"Content-Type": {"image/png"},
+		},
+	})
+
+	assert.True(t, res.IsBase64Encoded, "binary response must stay marked as base64")
+	assert.Equal(t, encoded, res.Body)
+	assert.Equal(t, http.StatusOK, res.StatusCode)
+	assert.Equal(t, "image/png", res.Headers["Content-Type"])
+
+	decoded, err := base64.StdEncoding.DecodeString(res.Body)
+	require.NoError(t, err)
+	assert.Equal(t, png, decoded, "the client must be able to recover the exact bytes")
+}
+
+func TestToLambdaFunctionURLResponseLeavesTextUnencoded(t *testing.T) {
+	res := ToLambdaFunctionURLResponse(events.APIGatewayProxyResponse{
+		StatusCode:      http.StatusOK,
+		Body:            `{"ok":true}`,
+		IsBase64Encoded: false,
+		MultiValueHeaders: map[string][]string{
+			"Content-Type": {"application/json"},
+		},
+	})
+
+	assert.False(t, res.IsBase64Encoded)
+	assert.Equal(t, `{"ok":true}`, res.Body)
+}