This logic doesn’t seem correct.
The right way would be to check filepath.IsAbs in ctr.

My reason for making this change here is to my understanding, the value passed to --runtime is evaluated in ShimManager's resolveRuntimePath method. This method uses filepath.IsAbs to check if the runtime provided is an absolute path to an existing file.

In resolveRuntimePath's fallthrough logic for when the provided value is not an absolute filepath, the BinaryName util is used to check for a value like io.containerd.runc.v1 and return containerd-shim-runc-v1 in that case. A string like ./containerd-shim-runc-v2 passed to BinaryName results in a parts slice ["" "/containerd-shim-runc-v2"] of length 2 and will miss this error check on the parts length.

I'm not too familiar with all the possible runtime name formats - if a format like runc.v1 is acceptable then these changes could be updated to add an additional check for if the first element of parts is ""

if a format like runc.v1 is acceptable

Yes. Actually --runtime aws.firecracker appears in https://github.com/firecracker-microvm/firecracker-containerd/blob/main/docs/getting-started.md#usage

Still not robust to catch a case like foo/bar.

Probably, we should just do :

runtime := clicontext.String("runtime") 
if strings.Contains(runtime, "/") && !filepath.IsAbs(runtime) {
  return fmt.Errorf("runtime must be a runtime name (e.g., \"io.containerd.runc.v2\") or absolute path to the runtime binary (e.g., \"/usr/local/bin/containerd-shim-runc-v2\"), got %q", runtime)
}

From my testing, the existing check len(parts) < 2 already covers this case as strings.Split("foo/bar", ".") will result in a slice of length 1 : ["foo/bar"]

It doesn't however cover foo/bar.baz.foo - I'll go ahead with this suggestion 🙂

Would you like this to go in runtime/v2/manager.go here ? or would it belong better in cmd/ctr/commands/run/run_unix.go ?

I prefer to have that under runtime/ rather than cmd/

It is bit scary to set the PATH here since it will affect all tests in the same process. How about using t.Setenv?

• You need to remove the directory
• If it is okay to take testing.T, you can use t.TempDir