Nil pointer dereference in `pullImage` when using `--private-registry` without credentials

[ddjain]

krknctl run panics with a nil pointer dereference when --private-registry and --private-registry-scenarios are provided without authentication flags (--private-registry-username, --private-registry-token). This makes it impossible to use a public custom registry.

Steps to Reproduce

krknctl run network-chaos \
    --private-registry quay.io \
    --private-registry-scenarios rh-ee-darjain/krkn-chaos \
    --traffic-type egress \
    --kubeconfig ~/.kube/config

Expected Behavior

The image should be pulled without authentication (the repository is public) and the scenario should execute normally.

Actual Behavior

[🔐 Private Registry] quay.io

container runtime: Docker
🌍pulling scenario image...
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x10099cca0]

goroutine 1 [running]:
github.com/krkn-chaos/krknctl/pkg/scenarioorchestrator/docker.pullImage(...)
    pkg/scenarioorchestrator/docker/scenario_orchestrator.go:246

Root Cause

In pkg/scenarioorchestrator/docker/scenario_orchestrator.go, pullImage calls registry.ToDockerV2AuthString() when registry is non-nil. However, ToDockerV2AuthString() returns (nil, nil) when neither Token nor Username is set (i.e., no credentials were provided). The return value is dereferenced without a nil check:

// scenario_orchestrator.go:240-247
pullOptions := dockerimage.PullOptions{}
if registry != nil {
    registryAuth, err := registry.ToDockerV2AuthString()
    if err != nil {
        return err
    }
    pullOptions.RegistryAuth = *registryAuth // panic: registryAuth is nil
}

ToDockerV2AuthString() in pkg/provider/models/models.go:104-124:

func (r *RegistryV2) ToDockerV2AuthString() (*string, error) {
    authConfig := registry.AuthConfig{}
    if r.Token != nil {
        authConfig.RegistryToken = *r.Token
    } else {
        if r.Username != nil {
            authConfig.Username = *r.Username
            authConfig.Password = *r.Password
        } else {
            return nil, nil // no credentials → nil return
        }
    }
    // ...
}

Fix

Add a nil check on registryAuth before dereferencing:

if registry != nil {
    registryAuth, err := registry.ToDockerV2AuthString()
    if err != nil {
        return err
    }
    if registryAuth != nil {
        pullOptions.RegistryAuth = *registryAuth
    }
}