url helpers aren't thread safe #581

.sass

background-image: asset-url("flags.png");

Expected behavior

works like if I disable concurrent compilation

Actual behavior

if concurrent complation is enabled #469 it fails to compile assets sometimes

System configuration

lastest sprockets 4 e25e0f5
ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-linux]