mesh-controller/internal/catalogue/declaration.go

package catalogue

// Turning a resolution into the declaration a node is sent.
//
// Separate from resolving because they answer different questions. Resolving asks *what should
// this machine run*; this asks *what does that look like as resources*, and the second is where
// settings are applied, generators are called, contributions are collected and credentials are
// placed. Both lived in one file until it was doing four jobs at once — which is the shape the
// system this replaces failed in, one import at a time.

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

// 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)
}

// Grant is one consumer's credential, on the machine that must create it.
type Grant struct {
	// Provision is what was required.
	Provision string
	// Consumer is the node that will use it, which is also what names the file.
	Consumer string
	// From is the module on that machine which asked, so the provider can name what it creates
	// after the thing using it rather than after the machine.
	From string
	// Values are what that module contributed — the name it wants, and anything else the
	// provision's own vocabulary defines.
	Values map[string]any
	// Sealed is the credential, closed to the providing node.
	Sealed string
}

// Rendering is everything needed to turn a resolution into the declaration a node is sent.
type Rendering struct {
	Settings   SettingsBy
	Generators map[string]Generator
	// Grants are the credentials this node must create, for the provisions it offers. Passed in
	// rather than resolved, because who consumes a node is a fact about the rest of the mesh and
	// resolution answers questions about one machine.
	Grants []Grant
}

// 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) {
	// Where each provision's credentials land, so a contribution can name the file rather than
	// carry a value the mesh does not have.
	directories := map[string]string{}
	for _, m := range r.Modules {
		for provision, where := range m.Grants {
			directories[provision] = where
		}
	}
	given, err := r.contributions(with.Settings, with.Grants, directories)
	if err != nil {
		return nil, err
	}

	var out []map[string]any
	for _, m := range r.Modules {
		resources := m.Resources
		for _, to := range sortedKeys(m.Secrets) {
			var found *Needed
			for i, n := range r.Needs {
				if n.Name == to {
					found = &r.Needs[i]
				}
			}
			if found == nil || found.Sealed == "" {
				// Answered on this machine, or answered by a node the mesh could not seal to.
				// Nothing to write either way, and writing an empty credential file would be
				// worse than none: something would read it and fail authenticating.
				continue
			}
			resources = append(append([]map[string]any{}, resources...), map[string]any{
				"id": SecretID(to), "type": "file", "path": m.Secrets[to],
				"sealed": found.Sealed,
			})
		}
		for _, to := range sortedKeys(m.Grants) {
			for _, g := range with.Grants {
				if g.Provision != to {
					continue
				}
				resources = append(append([]map[string]any{}, resources...), map[string]any{
					"id":     GrantID(to, g.Consumer),
					"type":   "file",
					"path":   grantPath(m.Grants[to], g.Consumer),
					"sealed": g.Sealed,
				})
			}
		}
		for _, to := range sortedKeys(m.Binds) {
			var found *Needed
			for i, n := range r.Needs {
				if n.Name == to {
					found = &r.Needs[i]
				}
			}
			if found == nil {
				// Bound to something answered on this machine rather than from the mesh. Nothing
				// to write: the answer is here, and a file saying "it is on this node" would be
				// a fact nobody needs and one more thing to keep true.
				continue
			}
			file, err := boundFile(*found, m.Binds[to])
			if err != nil {
				return nil, err
			}
			resources = append(append([]map[string]any{}, resources...), file)
		}
		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"`
	// Node is the machine it said it from, empty when that is this one.
	//
	// A provision answered from anywhere in the mesh has consumers on other machines, and the
	// provider has to know who they are — a database told to create a password and not who for
	// cannot do anything with it. Contributions were node-local until this, which meant the one
	// case that most needed them was the one they did not reach.
	Node string `json:"node,omitempty"`
	// Secret is the file on this machine holding that consumer's credential, sealed to it.
	//
	// Named rather than carried, for the same reason the private network's key is: the mesh
	// discarded the value and could not put it here if it wanted to. What is here is where to
	// find it.
	Secret string `json:"secret,omitempty"`
	// 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"`
}

// grantPath is where one consumer's sealed credential lands on the providing machine.
//
// Suffixed, so the directory can also hold whatever the module writing it keeps there and so a
// node named like something else in that directory cannot collide with it.
func grantPath(directory, consumer string) string {
	return strings.TrimRight(directory, "/") + "/" + consumer + ".secret"
}

// 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, grants []Grant,
	directories map[string]string) (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 })

	// What consumers on other machines asked for. Merged in with this machine's own, because from
	// the provider's side they are the same thing — somebody wanting something — and a provider
	// that had to read two lists would be a provider that reads one of them.
	sorted := append([]Grant{}, grants...)
	sort.Slice(sorted, func(i, j int) bool {
		if sorted[i].Provision != sorted[j].Provision {
			return sorted[i].Provision < sorted[j].Provision
		}
		return sorted[i].Consumer < sorted[j].Consumer
	})
	for _, g := range sorted {
		out[g.Provision] = append(out[g.Provision], Contribution{
			From: g.From, Node: g.Consumer, Values: g.Values,
			Secret: grantPath(directories[g.Provision], g.Consumer),
		})
	}
	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
}

// boundFile is what a module is told about something it requires from another machine.
//
// Where it is and what the providing module said about using it. **No credential**, and the file
// says so rather than leaving a reader to wonder whether one was meant to be there — a missing
// field looks like a bug, and a stated absence looks like a boundary.
func boundFile(n Needed, path string) (map[string]any, error) {
	body, err := json.MarshalIndent(map[string]any{
		"binding":   1,
		"provision": n.Name,
		"from":      n.From,
		"at":        n.At,
		"serves":    n.Serves,
		"generated": "by the mesh — do not edit; replaced whenever this changes. " +
			"It carries no credential: the mesh has no way to issue one yet",
	}, "", "  ")
	if err != nil {
		return nil, err
	}
	return map[string]any{
		"id": BoundID(n.Name), "type": "file", "path": path, "mode": "0644",
		"content": string(body) + "\n",
	}, nil
}

// ContributionsTo is what this node's set asked of one requirement, settled.
//
// Exported because a provider's grants are assembled from its consumers' resolutions, one machine
// at a time, and the alternative was for the control plane to reimplement settling.
func (r Resolution) ContributionsTo(requirement string, settings SettingsBy) (
	string, map[string]any, error) {
	all, err := r.contributions(settings, nil, nil)
	if err != nil {
		return "", nil, err
	}
	given := all[requirement]
	if len(given) == 0 {
		return "", nil, nil
	}
	if len(given) > 1 {
		// Two modules on one machine wanting the same provision would share one credential, and
		// the provider would be told to create one thing under two names. Refused rather than
		// resolved by picking, which is the rule everywhere else here.
		var who []string
		for _, g := range given {
			who = append(who, g.From)
		}
		sort.Strings(who)
		return "", nil, fmt.Errorf(
			"%s has %d modules asking for %q and they would share one credential: %s",
			r.Node, len(given), requirement, strings.Join(who, ", "))
	}
	return given[0].From, given[0].Values, nil
}