How to Debug and Correctly Execute Commands in Kubernetes Pods Using the Go Client?

Kubernetes Pod Execution Using Go Client

You want to execute a command inside a pod using the Kubernetes Go client, but your current implementation is encountering an error in exec.Stream(sopt) without any error message. This article will guide you through debugging and provides a correct example for your use case.

Debugging the Issue

The current error could arise due to incorrect configuration parameters or mismatched versions. Verify the following:

• Ensure that the Host field in your config matches the Kubernetes API server address.
• Update config.ContentConfig.GroupVersion to use the correct Kubernetes API version supported by your cluster.
• Check if the container name provided in req.VersionedParams matches the name of an existing container within the specified pod.

Correct Implementation

Here's a corrected example based on the modified ExecCmdExample function:

package k8s

import (
    "io"

    v1 "k8s.io/api/core/v1"
    "k8s.io/client-go/kubernetes"
    _ "k8s.io/client-go/plugin/pkg/client/auth/gcp" // Auth plugin specific to GKE
    "k8s.io/client-go/rest"
    "k8s.io/client-go/tools/remotecommand"
)

// ExecCmdExample executes a command on a specific pod and waits for the command's output.
func ExecCmdExample(client kubernetes.Interface, podName string,
    command string, stdin io.Reader, stdout io.Writer, stderr io.Writer) error {
    // Use a larger reader buffer size to handle long outputs.
    buf := make([]byte, 10000)

    cmd := []string{
        "sh",
        "-c",
        command,
    }

    options := &v1.PodExecOptions{
        Command: cmd,
        Stdin:   stdin != nil,
        Stdout:  true,
        Stderr:  true,
        TTY:     false,
    }

    req := client.CoreV1().RESTClient().Post().
        Resource("pods").
        Name(podName).
        Namespace("default").
        SubResource("exec").
        VersionedParams(
            options,
            scheme.ParameterCodec,
        )

    exec, err := remotecommand.NewSPDYExecutor(config, "POST", req.URL())
    if err != nil {
        return err
    }

    err = exec.Stream(remotecommand.StreamOptions{
        Stdin:  stdin,
        Stdout: stdout,
        Stderr: stderr,
    })
    
    // Read additional output if necessary.
    if _, err = exec.Read(buf); err != nil {
        return err
    }

    return nil
}

• The config variable has been removed as it is assumed to be configured elsewhere.
• The VersionedParams method now handles newer versions of PodExecOptions and ParameterCodec.
• A buffered reader is used to handle long command outputs.
• The TTY option is set to false since you don't require interactive terminal support.

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