Refactor for Maintainability

Docker's registry service validates and resolves image tags before fetching their manifests. A Backend Engineer candidate is shown this Go function and told a single bad request from a client recently took down the whole registry process.

func ProcessImageTag(registry string, image string, tag string) *Manifest {
    if registry != "" {
        if image != "" {
            if tag != "" {
                if len(tag) < 128 {
                    if tag != "latest" {
                        fullRef := registry + "/" + image + ":" + tag
                        manifest, _ := fetchManifest(fullRef)
                        log.Printf("fetched manifest for %s", fullRef)
                        return manifest
                    } else {
                        fullRef := registry + "/" + image + ":latest"
                        manifest, _ := fetchManifest(fullRef)
                        log.Printf("fetched manifest for %s", fullRef)
                        return manifest
                    }
                } else {
                    panic("tag too long")
                }
            } else {
                panic("tag required")
            }
        } else {
            panic("image required")
        }
    } else {
        panic("registry required")
    }
    return nil
}

Refactor this to improve its readability and maintainability. Explain what's wrong with the current version, including whatever caused the outage, and what you'd change.

Implement the validation portion as process_image_reference(registry, image, tag). Return a dictionary containing either reference or error; fetching the manifest is outside this function.