osbuildexecutor: show full osbuild exector on json decode errors

This is a short term workaround to get better visibility why
the osbuild executor sometimes sends non-json data. When reading
the result from the executor the entire output is now read and
if the json parsing goes wrong it will use the entire body
in the error message for better debug visibility.

internal/osbuildexecutor/runner-impl-aws-ec2.go

@@ -149,14 +149,14 @@ func handleBuild(inputArchive, host string) (*osbuild.Result, error) {
 
 	var osbuildResult osbuild.Result
 
-	err = json.NewDecoder(resp.Body).Decode(&osbuildResult)
+	body, err := io.ReadAll(resp.Body)
 	if err != nil {
-		return nil, fmt.Errorf("Unable to decode response body into osbuild result: %w", err)
+		return nil, fmt.Errorf("Unable to read response body: %w", err)
 	}
 
-	_, err = io.ReadAll(resp.Body)
+	err = json.Unmarshal(body, &osbuildResult)
 	if err != nil {
-		return nil, fmt.Errorf("Unable to wait for executor to close connection: %w", err)
+		return nil, fmt.Errorf("Unable to decode response body %q into osbuild result: %w", body, err)
 	}
 
 	return &osbuildResult, nil

internal/osbuildexecutor/runner-impl-aws-ec2_test.go

@@ -91,6 +91,24 @@ func TestHandleBuild(t *testing.T) {
 	require.True(t, osbuildResult.Success)
 }
 
+func TestHandleBuildNoJSON(t *testing.T) {
+	buildServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, err := io.ReadAll(r.Body)
+		require.NoError(t, err)
+
+		w.WriteHeader(http.StatusCreated)
+		_, err = w.Write([]byte("bad non-json text"))
+		require.NoError(t, err)
+	}))
+
+	cacheDir := t.TempDir()
+	inputArchive := filepath.Join(cacheDir, "test.tar")
+	require.NoError(t, os.WriteFile(inputArchive, []byte("test"), 0600))
+
+	_, err := osbuildexecutor.HandleBuild(inputArchive, buildServer.URL)
+	require.ErrorContains(t, err, `Unable to decode response body "bad non-json text" into osbuild result:`)
+}
+
 func TestHandleOutputArchive(t *testing.T) {
 	serverDir := t.TempDir()
 	serverOutputDir := filepath.Join(serverDir, "output")