In order to use a directory under "$GOPATH/src/mod/cache/download" as a cache directory accessible from an HTTP proxy, it must include a @v/list file that includes each version. This can currently be done by running ls *.info | sed 's/.info$//' > list in each @v directory, but it would be much more convenient if vgo did that as part of normal operation, so it could be just as simple as copying the directory to S3 or GCS or some such.

@rsc has said this requires vgo to lock directories. I don't know if there's an open issue for that.