gomodfs is a virtual filesystem (supporting FUSE, WebDAV, and NFS) that implements a read-only filesystem emulating the GOMODCACHE directory layout that has all Go modules accessible, without making the cmd/go tool ever think it needs to download anything. Instead, gomodfs itself downloads modules on demand to pretend that it was on all disk to begin with.
The motivation of this project is to speed up CI build systems, by sharing the Go module cache between builds (each build in its own ephemeral container or VM) and then sharing the gomodfs filesystem into those containers or VMs (over virtiofs) as a read-only filesystem. The guest builds (potentially running untrusted code from malicious PRs) can usefully share a Go module cache that isn't writable.
Without gomodfs, the alternative is to put GOMODCACHE on a tmpfs and run something like a local Athens server. But then each build ends up downloading tons of zip files at startup and extracting potentially hundreds or gigabytes of dependencies to memory, adding considerable overhead before the build even begins. Alternatively, you make all builds share a writable disk, but then you can't run untrusted code.
gomodfs is accessible either via FUSE, WebDAV, or NFS.
FUSE is best for Linux. It's works on macOS too, but it's tedious, especially on EC2 VMs where you need to configure SIP & MDM to get it kernel extensions working.
gomodfs also includes a WebDAV and NFS server.
The WebDAV support came first and was found a little lacking (the macOS kernel forces noexec on those mounts), so we also added NFS support.
Also, sharing either the WebDAV or FUSE mount over virtio-fs into Tart VMs hits bugs and limitations in Apple's virtio-fs implementation. See containers/podman#23886 and docker/for-mac#7059.
As such, we ended up using NFS. gomodfs runs an NFS server on the host and we mount it in the guest VMs.
The gomodfs storage backend is abstract and can be implemented however you'd like.
The primary implementation uses git, as most Go modules change very little between releases, and git does content-addressable de-duping for free. The use of git is somewhat abnormal: there are no commit objects. Only tree and blob objects. And refs to those.
Future implementations of the storage interface might include:
As of 2025-07-27, this is still all very new. Use with caution. It's starting to work, but it might not. Bug reports welcome.
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