mesh-controller/internal/broker/agreement.go

package broker

import (
	"fmt"
	"sort"
	"strings"
)

// Do the emitters and the consumers of a catalogue agree?
//
// **The check that was missing** (novox/hq 04-ISSUES/127). Every manifest was individually
// well-formed and every derivation individually correct, and no cross-module subscription in the
// mesh matched anything: a consumer's declaration derived into a namespace nobody publishes to.
// Nothing failed, because a subscription that matches nothing is not an error — it is silence.
//
// The comparison has to be over the whole catalogue, because the two halves live in different
// manifests, and it cannot simply demand that every consumed event have a live emitter: a module
// may be installed long before the one whose events it wants. So the rule is narrower and still
// catches this: **where the emitter is present, it must emit what the consumer asked for.**

// AConsumer is one module's interest in another's events, as this check needs it.
type AConsumer struct {
	Module   string
	Consumes []string
}

// AnEmitter is one module's events.
type AnEmitter struct {
	Module string
	Emits  []string
}

// Disagreements are the consumed events whose emitter is in the catalogue and does not emit them.
//
// Returned as sentences rather than as structs: every one of them is read by a person deciding
// whether a manifest or a catalogue is wrong, and a pair of names without the reason is a puzzle.
func Disagreements(emitters []AnEmitter, consumers []AConsumer, seats []DeclaredSeat) []string {
	emits := map[string]map[string]bool{}
	for _, e := range emitters {
		if emits[e.Module] == nil {
			emits[e.Module] = map[string]bool{}
		}
		for _, name := range e.Emits {
			emits[e.Module][name] = true
		}
	}
	// A seat's events are published by its holder under the seat's name, so a consumer naming the
	// seat is naming something real even though no module declares it as its own.
	for _, s := range seats {
		if len(s.Emits) == 0 {
			continue
		}
		if emits[s.Name] == nil {
			emits[s.Name] = map[string]bool{}
		}
		for _, name := range s.Emits {
			emits[s.Name][name] = true
		}
	}

	var out []string
	for _, c := range consumers {
		for _, pattern := range c.Consumes {
			emitter, event, named := strings.Cut(pattern, ".")
			// Every event from everyone, or every event from one module: both are deliberate and
			// neither names a particular event to check.
			if !named || emitter == "*" || emitter == catalogueTheRest || event == catalogueTheRest {
				continue
			}
			known, present := emits[emitter]
			if !present {
				// Not installed here, which is ordinary: a module lives in its own repository and
				// may be registered later. Nothing to compare, so nothing to say.
				continue
			}
			if matchesAny(event, known) {
				continue
			}
			out = append(out, fmt.Sprintf(
				"%s consumes %q and %s emits %s — so that subscription would match nothing, and "+
					"nothing would report it",
				c.Module, pattern, emitter, listOf(known)))
		}
	}
	sort.Strings(out)
	return out
}

// matchesAny says whether one of an emitter's event names satisfies a consumer's pattern.
func matchesAny(pattern string, emitted map[string]bool) bool {
	want := strings.Split(pattern, ".")
	for name := range emitted {
		if matches(want, strings.Split(name, ".")) {
			return true
		}
	}
	return false
}

func matches(pattern, name []string) bool {
	for i, part := range pattern {
		if part == catalogueTheRest {
			return i < len(name)
		}
		if i >= len(name) {
			return false
		}
		if part != "*" && part != name[i] {
			return false
		}
	}
	return len(pattern) == len(name)
}

func listOf(names map[string]bool) string {
	if len(names) == 0 {
		return "nothing"
	}
	out := make([]string, 0, len(names))
	for n := range names {
		out = append(out, n)
	}
	sort.Strings(out)
	return strings.Join(out, ", ")
}