rootless: containerd worker: Add fallback when buildkitd cannot access to conainerd

[ktock]

As discussed in #2965, the current rootless containerd worker doesn't allow fallbacks when it can't access to the containerd socket (e.g. by permission denied). But we should perform a fallback to keep the backward compatibility with the previous version of buildkitd.

This commit fixes this issue by adding a fallback logic for the case when buildkitd failed to access to the containerd socket.

main

$ rootlesskit buildkitd --addr=unix:///tmp/buildkit/buildkitd.sock
INFO[2022-07-19T01:49:45Z] auto snapshotter: using overlayfs            
INFO[2022-07-19T01:49:45Z] found worker "8envtv9nxu2808m374pct8b4z", labels=map[org.mobyproject.buildkit.worker.executor:oci org.mobyproject.buildkit.worker.hostname:2f891249d0b4 org.mobyproject.buildkit.worker.network:host org.mobyproject.buildkit.worker.oci.process-mode:sandbox org.mobyproject.buildkit.worker.snapshotter:overlayfs], platforms=[linux/amd64 linux/amd64/v2 linux/amd64/v3 linux/386] 
buildkitd: failed to dial "/run/containerd/containerd.sock": connection error: desc = "transport: error while dialing: dial unix /run/containerd/containerd.sock: connect: permission denied"
failed to connect client to "/run/containerd/containerd.sock" . make sure containerd is running
github.com/moby/buildkit/worker/containerd.NewWorkerOpt
	/go/src/github.com/moby/buildkit/worker/containerd/containerd.go:33
main.containerdWorkerInitializer
	/go/src/github.com/moby/buildkit/cmd/buildkitd/main_containerd_worker.go:262
main.newWorkerController
	/go/src/github.com/moby/buildkit/cmd/buildkitd/main.go:690
main.newController
	/go/src/github.com/moby/buildkit/cmd/buildkitd/main.go:633
main.main.func3
	/go/src/github.com/moby/buildkit/cmd/buildkitd/main.go:263
github.com/urfave/cli.HandleAction
	/go/src/github.com/moby/buildkit/vendor/github.com/urfave/cli/app.go:526
github.com/urfave/cli.(*App).Run
	/go/src/github.com/moby/buildkit/vendor/github.com/urfave/cli/app.go:288
main.main
	/go/src/github.com/moby/buildkit/cmd/buildkitd/main.go:313
runtime.main
	/usr/local/go/src/runtime/proc.go:255
runtime.goexit
	/usr/local/go/src/runtime/asm_amd64.s:1581
[rootlesskit:child ] error: command [buildkitd --addr=unix:///tmp/buildkit/buildkitd.sock] exited: exit status 1
[rootlesskit:parent] error: child exited: exit status 1

PR

$ rootlesskit buildkitd --addr=unix:///tmp/buildkit/buildkitd.sock
INFO[2022-07-19T01:49:15Z] auto snapshotter: using overlayfs            
INFO[2022-07-19T01:49:16Z] found worker "8envtv9nxu2808m374pct8b4z", labels=map[org.mobyproject.buildkit.worker.executor:oci org.mobyproject.buildkit.worker.hostname:2f891249d0b4 org.mobyproject.buildkit.worker.network:host org.mobyproject.buildkit.worker.oci.process-mode:sandbox org.mobyproject.buildkit.worker.snapshotter:overlayfs], platforms=[linux/amd64 linux/amd64/v2 linux/amd64/v3 linux/386] 
WARN[2022-07-19T01:49:16Z] skipping containerd worker, as failed to connect client to "/run/containerd/containerd.sock": failed to dial "/run/containerd/containerd.sock": connection error: desc = "transport: error while dialing: dial unix /run/containerd/containerd.sock: connect: permission denied" 
INFO[2022-07-19T01:49:16Z] found 1 workers, default="8envtv9nxu2808m374pct8b4z" 
WARN[2022-07-19T01:49:16Z] currently, only the default worker can be used. 
INFO[2022-07-19T01:49:16Z] running server on /tmp/buildkit/buildkitd.sock 

[tonistiigi]

closes #2965?

[ktock]

Yes.