fix(router): emit Content-Length on intercepted GetObject responses

CHANGELOG.md

@@ -14,6 +14,13 @@ Two version streams move independently:
 
 ## [Unreleased]
 
+### Fixed
+- **s3cmd `get` compatibility**: intercepted `GetObject` responses now
+  carry a `Content-Length` header reflecting the decrypted body size.
+  Previously the proxy streamed decrypted objects without a length, so
+  Go fell back to chunked transfer encoding and s3cmd 2.4.0 downloaded
+  an empty file. Downloads now complete with the correct size.
+
 ### Chart
 
 - **`chart/1.9.3`** — Dashboard usability + Go runtime panels.

s3proxy/internal/router/object.go

@@ -16,6 +16,7 @@ import (
 	"net/http"
 	"net/url"
 	"runtime/debug"
+	"strconv"
 	"strings"
 	"syscall"
 	"time"
@@ -157,6 +158,11 @@ func (o object) get(w http.ResponseWriter, r *http.Request) {
 		releaseLargeBuffer(&plaintext)
 		return
 	default:
+		// The full plaintext is already buffered, so emit an explicit Content-Length
+		// (the upstream value describes the ciphertext, not the decrypted body) so the
+		// server uses identity encoding instead of chunked. Some clients (e.g. s3cmd)
+		// require a Content-Length header on the response.
+		w.Header().Set("Content-Length", strconv.Itoa(len(plaintext)))
 		w.WriteHeader(http.StatusOK)
 		_, writeErr := w.Write(plaintext)
 		releaseLargeBuffer(&plaintext)

s3proxy/internal/router/router_test.go

@@ -13,6 +13,7 @@ import (
 	"log/slog"
 	"net/http"
 	"net/http/httptest"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -198,6 +199,35 @@ func TestGetObjectUsesRouterKEK(t *testing.T) {
 	assert.Equal(t, "secret payload", rec.Body.String())
 }
 
+func TestGetObjectSetsPlaintextContentLength(t *testing.T) {
+	keks := newTestKEKs(t, "expected encryption key")
+	version, curKEK := keks.Current()
+	client := newEncryptedGetObjectClient(t, curKEK, version, []byte("secret payload"))
+	router := Router{keks: keks, log: testLogger()}
+	req := httptest.NewRequest(http.MethodGet, "/bucket/key", nil)
+	rec := httptest.NewRecorder()
+
+	router.getHandler(req, client, true, "key", "bucket").ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+	// The header must reflect the plaintext length, not the +28-byte ciphertext.
+	assert.Equal(t, strconv.Itoa(len("secret payload")), rec.Result().Header.Get("Content-Length"))
+}
+
+func TestGetObjectSetsZeroContentLengthForEmptyObject(t *testing.T) {
+	keks := newTestKEKs(t, "expected encryption key")
+	version, curKEK := keks.Current()
+	client := newEncryptedGetObjectClient(t, curKEK, version, []byte{})
+	router := Router{keks: keks, log: testLogger()}
+	req := httptest.NewRequest(http.MethodGet, "/bucket/key", nil)
+	rec := httptest.NewRecorder()
+
+	router.getHandler(req, client, true, "key", "bucket").ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+	assert.Equal(t, "0", rec.Result().Header.Get("Content-Length"))
+}
+
 func TestGetObjectFailsWithWrongRouterKEK(t *testing.T) {
 	storedKeks := newTestKEKs(t, "old encryption key")
 	routerKeks := newTestKEKs(t, "new encryption key")