WCOW fails to load custom frontend

[pl4nty]

Official custom frontends don't seem to be published for Windows yet, so I applied the following patch to publish my own at ghcr.io/pl4nty/dockerfile:master using frontend/dockerfile/cmd/dockerfile-frontend/hack/release.

frontend/dockerfile/cmd/dockerfile-frontend/Dockerfile

-ARG TARGETPLATFORM
+ARG TARGETPLATFORM TARGETOS
 RUN --mount=target=. --mount=type=cache,target=/root/.cache \
   --mount=target=/go/pkg/mod,type=cache \
   --mount=source=/tmp/.ldflags,target=/tmp/.ldflags,from=version \
-  CGO_ENABLED=0 xx-go build -o /dockerfile-frontend -ldflags "-d $(cat /tmp/.ldflags)" -tags "$BUILDTAGS netgo static_build osusergo" ./frontend/dockerfile/cmd/dockerfile-frontend && \
+  CGO_ENABLED=0 xx-go build -o /dockerfile-frontend -ldflags '$([ "$TARGETOS" = "windows" ] && "" || "-d ")$(cat /tmp/.ldflags)' -tags "$BUILDTAGS netgo static_build osusergo" ./frontend/dockerfile/cmd/dockerfile-frontend && \
   xx-verify --static /dockerfile-frontend

Attempts to use it fail with the following error:

echo "# syntax=ghcr.io/pl4nty/dockerfile:master" > Dockerfile
docker buildx build .
[+] Building 2.6s (4/4) FINISHED                                                                      remote:nsc-remote
 => [internal] load build definition from Dockerfile                                                               0.0s
 => => transferring dockerfile: 80B                                                                                0.0s
 => resolve image config for docker-image://ghcr.io/pl4nty/dockerfile:master                                       1.8s
 => [auth] pl4nty/dockerfile:pull token for ghcr.io                                                                0.0s
 => docker-image://ghcr.io/pl4nty/dockerfile:master@sha256:d8f195c869e936b277f8af94493f8e0e5d58864de916a7ead70b4b  0.7s
 => => resolve ghcr.io/pl4nty/dockerfile:master@sha256:d8f195c869e936b277f8af94493f8e0e5d58864de916a7ead70b4b0708  0.0s
 => => extracting sha256:6c11bae004a3c15df19ad2d172c417f04ad12fa661e9029c2a424e3f3155705a                          0.6s
WARNING: No output specified with remote driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load
Dockerfile:1
--------------------
   1 | >>> # syntax=ghcr.io/pl4nty/dockerfile:master
   2 |
--------------------
ERROR: failed to solve: hcsshim::ProcessBaseLayer \\?\C:\ProgramData\containerd\root\io.containerd.snapshotter.v1.windows\snapshots\6: The system cannot find the path specified.: unknown

I've followed the WCOW setup guide and replicated locally and on GitHub Actions. C:\ProgramData\containerd\root\io.containerd.snapshotter.v1.windows\snapshots exists locally, but only contains a 1 folder. I noticed the number increments after each build.

[profnandaa]

ACK, taking a look.

[billywr]

Attempting to build an image for WCOW from a custom frontend using BuildKit will result to error you got (ERROR: failed to solve: hcsshim::ProcessBaseLayer ....) if the custom frontend contains no base layer.

I created a new custom frontend with a NanoServer image as the base layer, but ended up encountering the error below.

  PS E:\repro-4892> docker buildx build .
[+] Building 1.3s (4/4) FINISHED                                                                                      remote:buildkit-exp
 => [internal] load build definition from Dockerfile                                                                                 0.1s
 => => transferring dockerfile: 80B                                                                                                  0.0s
 => resolve image config for docker-image://docker.io/100909/dockerfile-4892:latest                                                  0.4s
 => [auth] 100909/dockerfile-4892:pull token for registry-1.docker.io                                                                0.0s
 => CACHED docker-image://docker.io/100909/dockerfile-4892:latest@sha256:483bf8f1796a24323c1ea215016297988326afb84ddc97312e0fd17b7c  0.0s
 => => resolve docker.io/100909/dockerfile-4892:latest@sha256:483bf8f1796a24323c1ea215016297988326afb84ddc97312e0fd17b7caeb5ca       0.0s
WARNING: No output specified with remote driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load
Dockerfile:1
--------------------
   1 | >>> # syntax=docker.io/100909/dockerfile-4892
   2 |
--------------------
**

ERROR: failed to solve: failed to create shim task: invalid OCI spec - Type 'bind' not supported: unknown

**
View build details: docker-desktop://dashboard/build/buildkit-exp/buildkit-exp0/0gzd6kdf2hllfq7ygzllikkm2

------------

Investigations into the containerd and hcsshim source codes indicate that containerd passes 'windows-layer' as the mount type to hscshim. I suspect that hscshim could be generating predefined mount types (yet to ascertain this).

Any idea of the valid Mount Types for this case?

[profnandaa]

/cc. @gabriel-samfira