runc worker does not always report the correct list of platforms

[fiam]

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• I've read the contributing guidelines and wholeheartedly agree. I've also read the issue reporting guide.

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• I have found a bug that the documentation does not mention anything about my problem
• I have found a bug that there are no open or closed issues that are related to my problem
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Description of bug

Bug description

In architectures that support multiple platforms, runc worker might report only the default one. For example, on an arm64 host where the arm checks passes, it reports:

$ ./hacks/grpccurl.sh localhost:443 moby.buildkit.v1.Control.ListWorkers
{
  "record": [
    {
      "ID": "vao24g7dghf8k72gi74ljmarv",
      "Labels": {
        "org.mobyproject.buildkit.worker.executor": "oci",
        "org.mobyproject.buildkit.worker.hostname": "ip-10-130-30-112",
        "org.mobyproject.buildkit.worker.network": "host",
        "org.mobyproject.buildkit.worker.oci.process-mode": "sandbox",
        "org.mobyproject.buildkit.worker.selinux.enabled": "false",
        "org.mobyproject.buildkit.worker.snapshotter": "overlayfs"
      },
      "platforms": [
        {
          "Architecture": "arm64",
          "OS": "linux"
        }
      ],
      "GCPolicy": [
        {
          "all": true,
          "reservedSpace": "20000000000"
        }
      ],
      "BuildkitVersion": {
        "package": "github.com/moby/buildkit",
        "version": "v0.19.0",
        "revision": "49628b9df9b196db3371d745c64ac3d75283c4b4.m"
      }
    }
  ]
}

This should also include arm and arm/v6.

Reproduction

1. Use an arm64 host
2. Enable runc worker
3. Launch BuildKit
4. Call ListWorkers

Version information

v0.19.0

Additional information

This seems to happen because the runc worker initializes the list of supported platforms with just the default one. Later, when we call Worker.Platforms, the matcher for arm64 reports that it also includes arm and arm/v6 (which is not necessarily true), preventing them from being added to the list of platforms.

[tonistiigi]

If the user doesn't set the platform in the config file, then it is initialized with a fully detected list of supported platforms https://github.com/moby/buildkit/blob/master/cmd/buildkitd/main.go#L528 . So this could only exist if some manual platform were set but not all the fallback platforms for the same manual ones. Theoretically, this could be useful to allow defining that you don't want the fallback behavior, but I'm not sure if it really was intentional.

[fiam]

@tonistiigi I see, thanks! So the problem only happens when the platform lists is explicitly declared in the config, but it is also empty.

This checks for a nil slice https://github.com/moby/buildkit/blob/master/cmd/buildkitd/main.go#L528, but the worker checks that the slice has elements

buildkit/cmd/buildkitd/main_oci_worker.go

Line 331 in 4712052

if platformsStr := cfg.Platforms; len(platformsStr) != 0 {

Would it be sensible to add an omitempty in the configuration struct? Otherwise intiializing the config by serializing the structure will require setting the platforms manually.

[tonistiigi]

Would it be sensible to add an omitempty in the configuration struct? Otherwise intiializing the config by serializing the structure will require setting the platforms manually.

ok