mesh-controller/internal/broker/writers.go

package broker

import (
	"fmt"
	"slices"
	"strings"
)

// The writers table (novox/hq to-be 45 §1, ADR 0227 rule 1), compiled in.
//
// **Every kind of state the core keeps has one writer; anyone else asks it.** The table is the design's,
// row for row, with what each row writes on the bus where it writes there. Two things read it: the
// composition of every principal's grants (PermissionsFor), which **refuses a grant that lets a
// principal publish on a subject another writes** — so a second writer cannot be granted by accident,
// and the composition says which state and whose — and the test beside it, which walks the rows
// against the design's list and the composition of a whole mesh. Changing a writer is a change to
// this table, through a decision.

// WriterRow is one row of the writers table.
type WriterRow struct {
	State  string
	Writer string
	KeptIn string
	Others string
	// Subjects are the bus subjects a write of this state is a publish to; none for state kept off the
	// bus (the controller's store, a module's own) or not built yet.
	Subjects []string
	// Writes says a principal granted a publish pattern overlapping Subjects is this state's writer —
	// given the pattern, because a machine writes its own report and no other's.
	Writes func(p Principal, pattern string) bool
	// Shared says why more than one principal may publish here; empty for one writer.
	Shared string
}

// isController, and the other writers' tests, are who a row's writer is as a principal.
func isController(p Principal, _ string) bool { return p.Kind == KindController }

// ownMachine is a node writing a subject whose third token is its own name, and no wildcard there.
func ownMachine(p Principal, pattern string) bool {
	tokens := strings.Split(pattern, ".")
	return p.Kind == KindNode && len(tokens) > 2 && tokens[2] == p.Node
}

// holdsSeatOf is a principal holding the seat a `mesh.seat.<seat>.…` pattern names: its module's own
// principal, or the node tools carrying a module that holds it (ADR 0175, the runtime is its modules).
func holdsSeatOf(p Principal, pattern string) bool {
	tokens := strings.Split(pattern, ".")
	if len(tokens) < 3 {
		return false
	}
	seat := tokens[2]
	holds := func(seats []Seat) bool {
		return slices.ContainsFunc(seats, func(s Seat) bool { return s.Name == seat })
	}
	switch p.Kind {
	case KindModule:
		return holds(p.Holds)
	case KindNodeTools:
		return slices.ContainsFunc(p.Carries, func(d Declared) bool { return holds(d.Holds) })
	}
	return false
}

// ownModule is a module publishing under its own name, or the node tools carrying it.
func ownModule(p Principal, pattern string) bool {
	tokens := strings.Split(pattern, ".")
	if len(tokens) < 3 {
		return false
	}
	module := tokens[2]
	switch p.Kind {
	case KindModule:
		return p.Module == module
	case KindNodeTools:
		return slices.ContainsFunc(p.Carries, func(d Declared) bool { return d.Module == module })
	}
	return false
}

// kvOf is a bucket's write subjects.
func kvOf(bucket string) []string { return []string{"$KV." + bucket + ".>"} }

// WritersTable is to-be 45 §1, in its order.
var WritersTable = []WriterRow{
	{State: "a machine's declaration", Writer: "controller (lease holder)", KeptIn: "the bus, last per subject",
		Others: "read", Subjects: []string{"mesh.node.*.declare"}, Writes: isController},
	{State: "a machine's applied state and its report", Writer: "the node-engine's apply queue",
		KeptIn: "the machine; the report on the bus", Others: "the reconcile and a delivery enqueue, never apply",
		Subjects: []string{"mesh.control.*.report"}, Writes: ownMachine},
	{State: "the controller lease", Writer: "the controller instance holding it", KeptIn: "key-value " + LeaseBucket,
		Others: "a candidate waits", Subjects: kvOf(LeaseBucket), Writes: isController},
	{State: "plans and their tiers", Writer: "controller (lease holder), compare-and-set on the plan's revision",
		KeptIn: "the controller's store", Others: "read through plans"},
	{State: "conditions", Writer: "controller", KeptIn: "key-value " + ConditionsBucket + " (and its history, " +
		ConditionHistoryBucket + ")", Others: "raise or clear only through observations the controller reads",
		Subjects: append(kvOf(ConditionsBucket), kvOf(ConditionHistoryBucket)...), Writes: isController},
	{State: "calls and their outcomes", Writer: "controller", KeptIn: "key-value " + CallsBucket,
		Others: "read by id", Subjects: kvOf(CallsBucket), Writes: isController},
	{State: "the hand-act log", Writer: "controller, through the verbs that act", KeptIn: "key-value " + HandActsBucket,
		Others: "—", Subjects: kvOf(HandActsBucket), Writes: isController},
	{State: "stream definitions and bus permissions", Writer: "controller", KeptIn: "the bus", Others: "—",
		// A stream's definition, and a durable consumer's by the API that names it so. Not every
		// consumer create: a module watching its own bucket makes and deletes an ordered consumer on the
		// bucket's stream (ADR 0201), which defines nothing the mesh keeps.
		Subjects: []string{"$JS.API.STREAM.CREATE.>", "$JS.API.STREAM.UPDATE.>", "$JS.API.STREAM.DELETE.>",
			"$JS.API.CONSUMER.DURABLE.CREATE.>"},
		Writes: isController},
	{State: "builds and their outcomes", Writer: "the build seat's holder", KeptIn: "its own state",
		Others:   "the controller asks",
		Subjects: []string{"mesh.seat.node-build-agent.event.built", "mesh.seat.mesh-build-machine.event.built"},
		Writes:   holdsSeatOf,
		Shared:   "every machine holding the build seat answers the asks it took; each outcome names its ask"},
	{State: "a merge announced", Writer: "one announcer per forge (the hook, or the poll when the hook is absent — never both)",
		KeptIn: "the bus", Others: "—", Subjects: []string{"mesh.mod.*.event.pull.merged"}, Writes: ownModule},
	{State: "a provider's standing", Writer: "the provider", KeptIn: "the provider's events",
		Others:   "the controller keeps the newest word as a condition",
		Subjects: []string{"mesh.mod.*.event.provisioner.failing", "mesh.mod.*.event.provisioner.recovered"},
		Writes:   ownModule},
	{State: "the operator-channel's open messages", Writer: "the seat's holder", KeptIn: "its own key-value state",
		Others: "—"},
	{State: "the facts snapshot", Writer: "controller", KeptIn: "the artifact store, facts/latest",
		Others: "the build seat reads"},
}

// CheckWriters refuses a grant that lets a principal publish on a subject the writers table gives
// another writer (to-be 45 §1): which state, whose, and the pattern that would make a second writer.
func CheckWriters(p Principal, publish []string) error {
	var problems []string
	for _, pattern := range publish {
		for _, row := range WritersTable {
			if row.Writes == nil {
				continue
			}
			for _, subject := range row.Subjects {
				if !SubjectsOverlap(pattern, subject) || row.Writes(p, pattern) {
					continue
				}
				problems = append(problems, fmt.Sprintf("%s may publish %s, which writes %s (%s), whose writer is %s",
					p.Username(), pattern, row.State, subject, row.Writer))
			}
		}
	}
	if len(problems) == 0 {
		return nil
	}
	return fmt.Errorf("a second writer would be granted (novox/hq to-be 45 §1, one writer per piece of state):\n  %s",
		strings.Join(problems, "\n  "))
}

// SubjectsOverlap says some subject matches both patterns: `*` is one token, `>` one or more to the end.
func SubjectsOverlap(a, b string) bool {
	x, y := strings.Split(a, "."), strings.Split(b, ".")
	for i := 0; ; i++ {
		switch {
		case i == len(x) && i == len(y):
			return true
		case i == len(x) || i == len(y):
			return false
		case x[i] == ">" || y[i] == ">":
			return true
		case x[i] == "*" || y[i] == "*" || x[i] == y[i]:
			continue
		default:
			return false
		}
	}
}