mesh-controller/internal/catalogue/resolve.go

package catalogue

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

// Resolving is turning "these modules are assigned here" into "this is what the node runs".
//
// It refuses rather than guesses, everywhere. novox/hq ADR 0009: a requirement with several
// answers is refused and named, because counting candidates has no surprising behaviour and a
// solver that picks has to be understood before its answer can be trusted.

// Node is what resolution needs to know about the machine.
type Node struct {
	Name string
	Site string
	// Capabilities the machine actually has, as its profile reported them. Only the present ones
	// — a capability that was looked for and not found is the same as one nobody looked for, as
	// far as deciding what may run here goes.
	Capabilities map[string]bool
}

// World is what the rest of the mesh already has.
//
// Some requirements cannot be answered on the machine that has them — a database runs somewhere
// and is reached over the network — so resolving one node needs to know what the others offer.
type World struct {
	// Held is the claims already taken, for the scopes wider than one node.
	Held []Held
	// Offered is what other nodes provide at mesh scope: the name, and which nodes provide it.
	Offered map[string][]string
	// Pinned is which node this machine was told to get a provision from, by name. Only consulted
	// when more than one node could answer -- a choice recorded before it was needed should not
	// start meaning something the day a second provider appears, and one recorded and then made
	// unnecessary should not quietly stop applying either.
	Pinned map[string]string
	// Unchecked takes brokered requirements on trust instead of refusing when nothing answers
	// them.
	//
	// For the first of two passes. Working out what a node offers the mesh needs that node
	// resolved, and resolving it may need what the mesh offers — so the first pass answers only
	// *what does each node offer*, and the second pass answers everything with that in hand. A
	// declaration is never built from an unchecked resolution.
	Unchecked bool
}

// Held is a claim somebody already has, used for the scopes wider than one node.
type Held struct {
	Claim  string
	Scope  string
	Node   string
	Module string
	Site   string
}

// Resolution is what a node should run, and why.
type Resolution struct {
	// Node is which machine this was resolved for, so a generator can be asked about it.
	Node string

	// Modules in the order they were resolved: assigned first, then what they pulled in.
	Modules []Manifest
	// Because says why each module is here — assigned, or required by something.
	Because map[string]string
	// Claims is what this node's set holds, so wider scopes can be checked against it.
	Claims []Held
	// Needs is what this node's set gets from other nodes. Recorded rather than resolved away,
	// because it is where a credential will have to be handed back once there is a mechanism for
	// that, and because "what does this machine depend on that is not on it" has no other answer.
	Needs []Needed
}

// Needed is one thing this node's set takes from elsewhere in the mesh.
type Needed struct {
	// Name is the provision, as required.
	Name string
	// From is the node providing it.
	From string
	// For is the module that wanted it.
	For string
}

// Refusal is why a set of assignments cannot become a declaration.
//
// Every reason at once rather than the first, and each says what to do about it. A person
// resolving these fixes them in one pass or in four.
type Refusal struct{ Problems []string }

func (r *Refusal) Error() string {
	return "these assignments cannot be applied:\n  - " + strings.Join(r.Problems, "\n  - ")
}

// ErrAmbiguous is returned inside a Refusal when a requirement has more than one answer.
var ErrAmbiguous = errors.New("more than one module provides that")

// Resolve works out everything a node runs, from what was assigned to it.
//
// The catalogue is every module the mesh knows about; assigned is what a person put on this node.
// What comes back is the closure — assigned modules plus everything they require — or a refusal
// naming every reason it could not be closed.
func Resolve(catalogue map[string]Manifest, assigned []string, node Node, world World) (Resolution, error) {
	elsewhere := world.Held
	var problems []string

	// Which names are answered from elsewhere in the mesh rather than from this machine. A
	// property of the name, not of each provider: two modules disagreeing about whether a
	// database is local would make the same requirement mean different things depending on which
	// one happened to answer it.
	brokered := map[string]bool{}
	local := map[string]bool{}
	for _, m := range catalogue {
		for _, o := range m.Provides {
			if o.At() == ScopeMesh {
				brokered[o.Name] = true
				continue
			}
			local[o.Name] = true
		}
	}
	for want := range brokered {
		if local[want] {
			problems = append(problems, fmt.Sprintf(
				"the catalogue disagrees about %q: some modules provide it here and others from "+
					"anywhere in the mesh, so the same requirement would mean two things", want))
		}
	}

	// What each name can be satisfied by. Built once from the whole catalogue, because "how many
	// modules provide this" is the question the whole rule turns on.
	offers := map[string][]string{}
	for _, m := range catalogue {
		for _, o := range m.Offers() {
			offers[o] = append(offers[o], m.Module)
		}
	}
	for k := range offers {
		sort.Strings(offers[k])
	}

	chosen := map[string]bool{}
	because := map[string]string{}
	var order []string
	var needs []Needed

	// What the set already offers, which is the first thing a requirement is checked against.
	//
	// Without this, assigning zsh does not satisfy something that requires a shell: the
	// requirement is counted against the catalogue, three modules provide it, and the answer is
	// still "choose one" after somebody has chosen one. That makes the remedy useless, and it is
	// how this read when first used.
	satisfied := map[string]bool{}

	// Everything a person assigned goes in first. Those are choices already made, and a
	// requirement one of them answers is not a choice to put back to anybody.
	queue := append([]string{}, assigned...)
	for _, a := range assigned {
		because[a] = "assigned"
		if m, known := catalogue[a]; known {
			for _, o := range m.Offers() {
				satisfied[o] = true
			}
		}
	}

	// What has already been complained about. A requirement can be wanted by several modules at
	// once, and saying the same thing twice makes a person hunt for the difference between two
	// identical lines before realising there is none.
	reported := map[string]bool{}

	for len(queue) > 0 {
		want := queue[0]
		queue = queue[1:]
		if chosen[want] || reported[want] {
			continue
		}
		// Already answered by something in the set. This is the case that makes assigning zsh do
		// what a person meant by it.
		if satisfied[want] && !isModule(catalogue, want) {
			continue
		}

		// Answered from elsewhere in the mesh, if it is that kind of name. **Never installed
		// here.** Choosing a machine to put a database on is a decision with consequences
		// nobody would want made silently by something resolving a web application.
		if brokered[want] {
			reported[want] = true
			where := world.Offered[want]
			switch {
			case world.Unchecked:
				// First pass. Whether anything answers this is exactly the question this pass
				// exists to make answerable, so it is not asked here.
			case len(where) == 0:
				remedy := "and nothing in the catalogue could"
				if answers := offers[want]; len(answers) == 1 {
					remedy = "— assign " + answers[0] + " to a node"
				} else if len(answers) > 1 {
					remedy = "— assign one of these to a node: " + strings.Join(answers, ", ")
				}
				problems = append(problems, fmt.Sprintf(
					"nothing in this mesh provides %q, wanted by %s %s",
					want, because[want], remedy))
			case len(where) == 1:
				if chosenNode, pinned := world.Pinned[want]; pinned && chosenNode != where[0] {
					// One provider, and it is not the one this machine was told to use. Silently
					// using the other would be the mesh overruling a choice somebody made.
					problems = append(problems, fmt.Sprintf(
						"%s was told to get %q from %s, and only %s provides it",
						node.Name, want, chosenNode, where[0]))
					break
				}
				needs = append(needs, Needed{Name: want, From: where[0], For: because[want]})
			default:
				sorted := append([]string{}, where...)
				sort.Strings(sorted)
				chosenNode, pinned := world.Pinned[want]
				if !pinned {
					problems = append(problems, fmt.Sprintf(
						"%d nodes provide %q, wanted by %s — say which with `pin %s %s <node>`: %s",
						len(where), want, because[want], node.Name, want,
						strings.Join(sorted, ", ")))
					break
				}
				var offers bool
				for _, w := range where {
					if w == chosenNode {
						offers = true
					}
				}
				if !offers {
					// Pointed at a machine that does not answer this. Refused rather than
					// falling back to another: a fallback would quietly move somebody's data to
					// a machine they did not choose, which is the whole reason this is asked.
					problems = append(problems, fmt.Sprintf(
						"%s was told to get %q from %s, and %s does not provide it — these do: %s",
						node.Name, want, chosenNode, chosenNode, strings.Join(sorted, ", ")))
					break
				}
				needs = append(needs, Needed{Name: want, From: chosenNode, For: because[want]})
			}
			continue
		}

		candidates := offers[want]
		switch len(candidates) {
		case 0:
			reported[want] = true
			problems = append(problems, fmt.Sprintf(
				"nothing provides %q, wanted by %s", want, because[want]))
			continue
		case 1:
			// No choice to make, so none is made. This is the case that lets `install i3` bring
			// in xorg without anybody being asked anything.
		default:
			reported[want] = true
			problems = append(problems, fmt.Sprintf(
				"%q is wanted by %s and %d modules provide it — choose one and assign it: %s",
				want, because[want], len(candidates), strings.Join(candidates, ", ")))
			continue
		}

		m := catalogue[candidates[0]]
		if chosen[m.Module] {
			continue
		}
		chosen[m.Module] = true
		order = append(order, m.Module)
		for _, o := range m.Offers() {
			satisfied[o] = true
		}
		if _, ok := because[m.Module]; !ok {
			because[m.Module] = fmt.Sprintf("required by %s", because[want])
		}

		for _, r := range m.Wants() {
			if _, ok := because[r]; !ok {
				because[r] = m.Module
			}
			queue = append(queue, r)
		}
	}

	resolution := Resolution{Node: node.Name, Because: because, Needs: needs}
	for _, n := range order {
		resolution.Modules = append(resolution.Modules, catalogue[n])
	}

	problems = append(problems, checkCapabilities(resolution.Modules, node)...)
	claims, claimProblems := checkClaims(resolution.Modules, node, elsewhere)
	problems = append(problems, claimProblems...)
	problems = append(problems, checkResources(resolution.Modules)...)
	resolution.Claims = claims

	if len(problems) > 0 {
		sort.Strings(problems)
		return Resolution{}, &Refusal{Problems: problems}
	}
	return resolution, nil
}

// isModule reports whether a name is a module in its own right rather than only something
// modules provide.
//
// A requirement naming a module is not satisfied by something else providing that name: `i3`
// requires `xorg` and means xorg, not "anything calling itself a display server".
func isModule(catalogue map[string]Manifest, want string) bool {
	_, ok := catalogue[want]
	return ok
}

// checkCapabilities refuses a module the machine cannot run.
//
// Said as a fact about the machine rather than about the module, because that is what it is and
// because nothing can be installed to change it.
func checkCapabilities(modules []Manifest, node Node) []string {
	var problems []string
	for _, m := range modules {
		for _, c := range m.Capabilities {
			if !node.Capabilities[c] {
				problems = append(problems, fmt.Sprintf(
					"%s needs the capability %q and %s does not have it — this is the wrong "+
						"machine, not a missing module", m.Module, c, node.Name))
			}
		}
	}
	return problems
}

// checkClaims refuses two modules holding one singular thing.
//
// Within this node's own set, and against what is already held elsewhere for the wider scopes. A
// claim at mesh scope is the same idea as the mesh's one hub, said once instead of hard-coded.
func checkClaims(modules []Manifest, node Node, elsewhere []Held) ([]Held, []string) {
	var problems []string
	var held []Held

	byScope := map[string]map[string]string{} // scope → claim → module
	for _, m := range modules {
		for _, c := range m.Claims {
			scope := c.At()
			if byScope[scope] == nil {
				byScope[scope] = map[string]string{}
			}
			if other, taken := byScope[scope][c.Name]; taken {
				problems = append(problems, fmt.Sprintf(
					"%s and %s both claim %q, and only one thing may hold it per %s",
					other, m.Module, c.Name, scope))
				continue
			}
			byScope[scope][c.Name] = m.Module
			held = append(held, Held{Claim: c.Name, Scope: scope, Node: node.Name,
				Module: m.Module, Site: node.Site})
		}
	}

	// And against the rest of the mesh, for the scopes that reach past this machine.
	for _, h := range held {
		for _, e := range elsewhere {
			if e.Node == node.Name || e.Claim != h.Claim || e.Scope != h.Scope {
				continue
			}
			switch h.Scope {
			case ScopeMesh:
				problems = append(problems, fmt.Sprintf(
					"%s on %s claims %q, which %s on %s already holds — one per mesh",
					h.Module, node.Name, h.Claim, e.Module, e.Node))
			case ScopeSite:
				if node.Site != "" && node.Site == e.Site {
					problems = append(problems, fmt.Sprintf(
						"%s on %s claims %q, which %s on %s already holds at %s — one per site",
						h.Module, node.Name, h.Claim, e.Module, e.Node, node.Site))
				}
			}
		}
	}
	return held, problems
}

// checkResources refuses two modules writing the same thing.
//
// This costs no manifest field: the mesh already holds every resource of every module, so two
// declaring one path or one unit are visible without either having to know about the other. A
// declared claim is only for the abstract conflicts nothing in the resources reveals.
func checkResources(modules []Manifest) []string {
	var problems []string
	owner := map[string]string{}

	for _, m := range modules {
		for _, r := range m.Resources {
			for _, field := range []string{"path", "unit", "name", "package"} {
				value, ok := r[field].(string)
				if !ok || value == "" {
					continue
				}
				key := field + " " + value
				if other, taken := owner[key]; taken && other != m.Module {
					problems = append(problems, fmt.Sprintf(
						"%s and %s both declare the %s %q", other, m.Module, field, value))
				}
				owner[key] = m.Module
			}
		}
	}
	return problems
}

// SettingsBy is the layers that apply to each module, keyed by module name.
type SettingsBy map[string][]Layer

// Generator works out a module's resources for one node, where they cannot be written in advance.
type Generator interface {
	// Resources for this node. Absent means the node is not part of whatever this generates,
	// which is an ordinary answer rather than a failure — a machine assigned the module before it
	// has an address on the network is in exactly that state.
	Resources(node string) ([]map[string]any, bool, error)
}

// Rendering is everything needed to turn a resolution into the declaration a node is sent.
type Rendering struct {
	Settings   SettingsBy
	Generators map[string]Generator
}

// Declaration is everything the resolved modules put on the node, with settings applied.
//
// Resource identities are prefixed with the module they came from. Two modules may reasonably
// both call something "config", and without this the second would silently replace the first —
// the node applying one of them and reporting success.
func (r Resolution) Declaration(with Rendering) ([]map[string]any, error) {
	given, err := r.contributions(with.Settings)
	if err != nil {
		return nil, err
	}

	var out []map[string]any
	for _, m := range r.Modules {
		resources := m.Resources
		for _, to := range sortedKeys(m.Receives) {
			file, err := receivedFile(to, m.Receives[to], given[to])
			if err != nil {
				return nil, err
			}
			resources = append(append([]map[string]any{}, resources...), file)
		}
		if m.Computed != "" {
			generator, known := with.Generators[m.Computed]
			if !known {
				return nil, fmt.Errorf(
					"%s says its resources are computed by %q, and this control plane has no %q",
					m.Module, m.Computed, m.Computed)
			}
			generated, part, err := generator.Resources(r.Node)
			if err != nil {
				return nil, err
			}
			if !part {
				// Assigned, and not yet part of what this generates. Nothing to put on the
				// machine, which is different from an error: a node given the network module
				// before it has an address is in exactly that state, briefly.
				continue
			}
			resources = generated
		}
		for _, unsettled := range resources {
			resource, err := ApplySettings(unsettled, with.Settings[m.Module])
			if err != nil {
				return nil, err
			}
			copied := map[string]any{}
			for k, v := range resource {
				copied[k] = v
			}
			copied["id"] = m.Module + "." + fmt.Sprint(resource["id"])
			// A service saying what it reflects names resources within its own module, so those
			// are prefixed too or they would point at nothing.
			if reflects, ok := resource["restart-on"].([]any); ok {
				var renamed []any
				for _, id := range reflects {
					renamed = append(renamed, m.Module+"."+fmt.Sprint(id))
				}
				copied["restart-on"] = renamed
			}
			out = append(out, copied)
		}
	}
	return out, nil
}

// Contribution is one module telling the answer to a requirement what it needs from it.
type Contribution struct {
	// From is the module that said it, so the provider and a person reading the file can tell
	// which route belongs to what.
	From string `json:"from"`
	// Values are the module's own, with settings applied. What the keys mean is agreed by the
	// requirement's name — everything providing `reverse-proxy` understands the same shape, which
	// is what makes swapping one for another cost nothing.
	Values map[string]any `json:"values"`
}

// contributions collects what every module in this set contributes, by requirement.
//
// Ordered by contributing module, because the result becomes a file on a machine and a file whose
// lines move about is a file that looks changed when nothing changed.
func (r Resolution) contributions(settings SettingsBy) (map[string][]Contribution, error) {
	out := map[string][]Contribution{}
	modules := append([]Manifest{}, r.Modules...)
	sort.Slice(modules, func(i, j int) bool { return modules[i].Module < modules[j].Module })

	for _, m := range modules {
		for _, to := range sortedKeys(m.Contributes) {
			// Settings reach a contribution the same way they reach a file. A route's hostname is
			// exactly the kind of thing that differs between one mesh and the next, and a module
			// that could not have it set would have to be edited to be reused.
			values, err := settle(m.Contributes[to], settings[m.Module], nil,
				m.Module+" contributing to "+to)
			if err != nil {
				return nil, fmt.Errorf("%s contributing to %s: %w", m.Module, to, err)
			}
			out[to] = append(out[to], Contribution{From: m.Module, Values: values})
		}
	}
	return out, nil
}

// receivedFile is the file a provider is given its consumers' contributions in.
func receivedFile(requirement, path string, given []Contribution) (map[string]any, error) {
	if given == nil {
		// Nobody contributed. The file is still written, empty, rather than left absent: a
		// provider that finds no file cannot tell "nothing asked for me" from "the mesh never
		// wrote it", and the two want completely different responses.
		given = []Contribution{}
	}
	// The note goes *inside* the document, not above it. The first version wrote a `//` header
	// and produced a file that says "do not edit" to a person and fails to parse for the program
	// meant to read it — which is the whole audience.
	body, err := json.MarshalIndent(map[string]any{
		"contributions": 1,
		"requirement":   requirement,
		"generated": "by the mesh — do not edit; replaced whenever a module contributing to " +
			requirement + " arrives or leaves",
		"given": given,
	}, "", "  ")
	if err != nil {
		return nil, err
	}
	return map[string]any{
		"id": ReceivedID(requirement), "type": "file", "path": path, "mode": "0644",
		"content": string(body) + "\n",
	}, nil
}

// sortedKeys is map iteration made repeatable, which everything written to a machine needs.
func sortedKeys[V any](m map[string]V) []string {
	out := make([]string, 0, len(m))
	for k := range m {
		out = append(out, k)
	}
	sort.Strings(out)
	return out
}

// Offers is a list of node-scoped provisions, which is what nearly everything is.
func Offers(names ...string) []Offer {
	out := make([]Offer, 0, len(names))
	for _, n := range names {
		out = append(out, Offer{Name: n})
	}
	return out
}

// FromAnywhere is a provision answered by whichever node in the mesh runs it.
func FromAnywhere(names ...string) []Offer {
	out := make([]Offer, 0, len(names))
	for _, n := range names {
		out = append(out, Offer{Name: n, Scope: ScopeMesh})
	}
	return out
}