mesh-controller/internal/catalogue/settings.go

package catalogue

import (
	"encoding/json"
	"fmt"
	"sort"
	"strings"
)

// Settings are how somebody changes a config file a module owns, without editing it.
//
// Managed files are generated and never edited (novox/hq ADR 0011), so a person's intention has
// to be expressed somewhere the generator can see it. Then the file is produced from the module's
// defaults and that intention together, and upstream can rewrite its half freely.
//
// **An override beats a default, so there is nothing to resolve.** When the module changes a key
// somebody has set, the setting wins — it was a statement about that key, made deliberately, and
// the default was only ever what to do in the absence of one. This is the same shape as the rest
// of the mesh refusing to guess: there is no guess to make.

// Merge is how a file's content is combined with settings. A module names one per resource.
const (
	// MergeJSON treats the content as a JSON document and merges settings into it by key.
	MergeJSON = "json"
)

// Layer is one source of settings, in the order they are applied.
type Layer struct {
	// Where these came from, for saying which layer set a value.
	From   string
	Values map[string]any
}

// ApplySettings produces a resource's final content from the module's own and the layers over it.
//
// Layers are applied in order, so a later one beats an earlier one — the mesh's settings for a
// module first, then this node's. A node that differs from the rest is expressed by differing,
// rather than by repeating everything the rest already say.
func ApplySettings(resource map[string]any, layers []Layer) (map[string]any, error) {
	how, _ := resource["merge"].(string)
	if how == "" {
		// Not a mergeable file. Settings for it are ignored rather than silently doing nothing
		// somewhere else — see UnusedSettings, which is what says so.
		return resource, nil
	}
	if how != MergeJSON {
		return nil, fmt.Errorf(
			"%v says it merges as %q, and this control plane knows how to merge %q",
			resource["id"], how, MergeJSON)
	}

	content, _ := resource["content"].(string)
	var base map[string]any
	if strings.TrimSpace(content) == "" {
		base = map[string]any{}
	} else if err := json.Unmarshal([]byte(content), &base); err != nil {
		return nil, fmt.Errorf("%v says it merges as JSON and its content is not JSON: %w",
			resource["id"], err)
	}

	protected := map[string]bool{}
	for _, k := range stringsOf(resource["protected"]) {
		protected[k] = true
	}

	merged := deepCopy(base)
	for _, layer := range layers {
		for key, value := range layer.Values {
			if protected[key] {
				// The module said it must own this one. Refused rather than ignored: a setting
				// that is quietly dropped is somebody believing they changed something.
				return nil, fmt.Errorf(
					"%s sets %q on %v, and that module keeps %q for itself — it is not settable",
					layer.From, key, resource["id"], key)
			}
			merged[key] = mergeValue(merged[key], value)
		}
	}

	out := map[string]any{}
	for k, v := range resource {
		out[k] = v
	}
	// Keys sorted by the encoder, so the same settings always produce the same bytes. A file
	// whose lines move for no reason makes every reconcile look like a change, and anything
	// reflecting it would restart for ever.
	rendered, err := json.MarshalIndent(merged, "", "  ")
	if err != nil {
		return nil, err
	}
	out["content"] = string(rendered) + "\n"
	delete(out, "merge")
	delete(out, "protected")
	return out, nil
}

// mergeValue combines one value with the one over it.
//
// Two objects merge key by key, so setting one field of a nested block does not delete its
// siblings. Anything else is replaced whole: a list that merged element-wise could neither be
// shortened nor reordered, and there is no correct guess about which element is "the same one".
func mergeValue(under, over any) any {
	underMap, isUnderMap := under.(map[string]any)
	overMap, isOverMap := over.(map[string]any)
	if !isUnderMap || !isOverMap {
		return over
	}
	merged := deepCopy(underMap)
	for k, v := range overMap {
		merged[k] = mergeValue(merged[k], v)
	}
	return merged
}

func deepCopy(in map[string]any) map[string]any {
	out := map[string]any{}
	for k, v := range in {
		if nested, ok := v.(map[string]any); ok {
			out[k] = deepCopy(nested)
			continue
		}
		out[k] = v
	}
	return out
}

// UnusedSettings names settings that reached no file.
//
// Somebody who sets a key on a module with nothing mergeable, or misspells one, has changed
// nothing — and would find out by the machine not behaving differently, which is the slowest
// way there is. This is what makes that visible at the moment they set it.
func UnusedSettings(m Manifest, layers []Layer) []string {
	mergeable := false
	for _, r := range m.Resources {
		if how, _ := r["merge"].(string); how != "" {
			mergeable = true
		}
	}
	if mergeable {
		return nil
	}

	var unused []string
	for _, layer := range layers {
		for key := range layer.Values {
			unused = append(unused, fmt.Sprintf("%s sets %q, and %s has no file to merge it into",
				layer.From, key, m.Module))
		}
	}
	sort.Strings(unused)
	return unused
}

func stringsOf(v any) []string {
	raw, ok := v.([]any)
	if !ok {
		return nil
	}
	var out []string
	for _, item := range raw {
		if s, ok := item.(string); ok {
			out = append(out, s)
		}
	}
	return out
}